Biology

Build from cells and biomolecules to metabolism, inheritance, and evolution. Then study biological diversity, plants, animals, and ecological systems, with experiments and named organisms connected to the concepts they explain.

Cells and Biological Molecules

Build from biological organization to cells, membranes, and molecules.

Life and Biological Organization

• biology — Biology is the study of life and includes fields such as anatomy, physiology, botany, zoology, genetics, and evolution. The term has also been used in broader theoretical contexts, including sociobiology and debates over its relation to social behavior.

• Characteristics of life

  • life — Life is the biological state characterized by metabolism, growth, reproduction, and homeostasis, often contrasted with viruses at the edge of that condition. In science clues it appears in discussions of abiogenesis, the Miller-Urey experiment, Schrödinger’s What Is Life?, and NASA’s later working definition. It is also the final word in titles such as Terrence Malick’s The Tree of Life and Fellini’s La dolce vita.

Cell Biology

• Cell biology
  • autophagy — Autophagy is the cellular process in which damaged components are enclosed in a double membrane and delivered to lysosomes for degradation. It is strongly linked to starvation responses, mTOR signaling, and studies of disease and tumor suppression.
    • mitophagy — Mitophagy is the selective autophagic destruction of damaged mitochondria. It is promoted in responses to mitochondrial depolarization and damage, with proteins such as PINK1 and Parkin helping target defective mitochondria to lysosomes.
  • cell adhesion — Cell adhesion is the process by which cells bind to each other or to the extracellular matrix. It is mediated by molecules such as cadherins, integrins, selectins, and related junctional proteins like catenins, including beta-catenin-linked adhesion complexes. It underlies structures like desmosomes and tight junctions.
    • cadherins — Cadherins are calcium-dependent cell adhesion proteins that help cells stick together in adherens junctions and desmosomes. Important examples include E-cadherin, N-cadherin, and the GPI-anchored T-cadherin.
      • E-cadherin — E-cadherin is an epithelial cell-adhesion protein that helps form adherens junctions by binding beta-catenin. Loss of E-cadherin expression is a key step in epithelial-mesenchymal transition and can promote metastasis.

  • cell counting — Cell counting is the process of quantifying the number of cells in a sample. Common approaches include hemocytometers, Trypan blue staining, flow cytometry, image analysis, and Coulter-counter-based impedance measurements.

  • Cell differentiation — Cell differentiation is the process by which unspecialized cells become specialized cell types. It is driven by developmental signals and transcriptional programs, and can be reversed in some cases by reprogramming factors.
    • cellular differentiation — Cellular differentiation is the process by which cells specialize into distinct types and functions. It is central to development and stem cell biology, where reprogramming factors such as Oct4 and Sox2 can reverse it.
  • Cell junctions — Cell junctions are structures that connect neighboring cells and help tissues maintain adhesion and communication. Examples include tight junctions, gap junctions, and desmosomes, with connexins and cadherins playing key roles.

    • desmosomes — Desmosomes are anchoring cell junctions that fasten epithelial and cardiac cells together through cadherin-based connections. They link intermediate filaments, and autoimmune attack on them is associated with pemphigus vulgaris.

    • gap junction — A gap junction is a cell-to-cell connection made of paired connexons that lets small molecules pass directly between adjacent cells. In animal tissue, it is the functional equivalent of plasmodesmata in plants.

    • Tight junctions — Tight junctions are epithelial cell-cell adhesions built from claudins and occludins that seal the space between cells. They preserve apical-basolateral polarity and form a watertight barrier in tissues such as the blood-brain barrier.

  • cell lysis — Cell lysis is the bursting and breakdown of a cell membrane, often caused by hypotonic conditions, detergents, or enzymatic attack. It also names the destructive phase in some bacteriophages and is associated with sonication and lab extraction buffers.
    • lysis — Lysis is the rupture or bursting of a cell membrane, whether by osmotic stress, enzymes like lysozyme, or laboratory disruption with detergents and sonication. It is also the defining process of the lytic cycle and a central outcome of complement-mediated membrane attack.

  • Cell proliferation — Cell proliferation is the process by which cells increase in number through growth and division. Common clues include assays and markers such as CFSE dilution, BrdU incorporation, Ki67, PCNA, and MTT-based metabolic readouts.

  • cells — Cells are the basic units of life and the basic living units that make up tissues. They contain organelles such as mitochondria and Golgi bodies, and are the objects counted, cultured, passaged, and sorted in many laboratory techniques. Cells can be adherent or suspension types; examples such as HeLa and Jurkat are common in biology. The HeLa line is a famous immortal cell line derived from Henrietta Lacks.

    • adipocytes — Adipocytes are fat-storing cells that include the white and brown types, with the white form predominating in adults and the brown form rich in mitochondria. They are regulated by PPAR-gamma in differentiation from mesenchymal stem cells and secrete signals such as leptin, resistin, and estrogen.

    • Bone cells
      • osteoclasts — Osteoclasts are the multinucleated bone-resorbing cells that break down bone and raise blood calcium levels. They are derived from macrophage lineage cells and are inhibited by OPG, calcitonin, and bisphosphonates.
    • Cardiac pacemaker cells
      • pacemaker cells — Pacemaker cells regulate the pattern and duration of the heartbeat. They generate rhythmic electrical activity that helps control contraction of cardiac muscle.

    • coenocyte — A coenocyte is a multinucleate mass of cytoplasm that is not subdivided into separate cells. The term is used for certain fungi, algae, and slime molds such as Physarum.

    • endothelial cells — Endothelial cells line the interior of blood and lymphatic vessels and make up much of the tunica intima. They contain Weibel-Palade bodies with von Willebrand factor and P-selectin, and they release nitric oxide to regulate vascular tone.

    • ependymal cells — Ependymal cells line the ventricles of the brain and the cavities of the central nervous system. They help circulate cerebrospinal fluid with their cilia and are thought to have stem-cell-like properties.

    • Granular cells — Granular cells are renin-secreting cells in the wall of the afferent arteriole, where they function as juxtaglomerular cells. The term can also refer to widespread neurons in the brain, including cerebellar granular cells.

    • hepatocytes — Hepatocytes are the main functional cells of the liver, arranged in plates around sinusoids and separated from them by the Space of Disse. They synthesize albumin, clotting factors, bile acids, and urea-cycle enzymes, and they are major sites of gluconeogenesis and detoxification.
      • hepatocyte — A hepatocyte is a liver cell, the basic epithelial cell that makes up most of the liver. It is a standard example of a differentiated animal cell.
    • multinucleated cells — Multinucleated cells contain more than one nucleus, often because cell division occurs without cytokinesis or because cells fuse together. They are seen in structures such as syncytia, fungal hyphae that fail to undergo karyogamy, and certain apicomplexan stages.

      • multinucleated — Multinucleated describes cells or tissues that contain more than one nucleus, including syncytia, coenocytes, skeletal muscle cells, and certain fungal hyphae. It is the property produced when cytokinesis fails or when cells fuse, as in placental syncytiotrophoblasts and giant cells in granulomas.

      • syncytium — A syncytium is a multinucleated mass of cytoplasm formed when cell division occurs without cytokinesis. Examples include skeletal muscle fibers, osteoclasts, placental trophoblasts, and cells infected by HIV.

    • prokaryotes — Prokaryotes are organisms whose cells lack a nucleus and other membrane-bound organelles. They include bacteria and archaea, and are often contrasted with eukaryotes in cell biology and molecular biology. Prokaryotic cells are typically described as having a single circular chromosome and may also contain plasmids; Woese separated them into bacteria and archaea. Many bacteria have peptidoglycan cell walls, and prokaryotes were central to the early evolution of photosynthesis.
      • Kingdom Monera — Monera was the old kingdom used for prokaryotic organisms, including true bacteria and related forms. Later classifications split it into Eubacteria and Archaebacteria.
    • stem cells — Undifferentiated, self-renewing cells that can differentiate into specialized cell types. Major forms include embryonic, adult, hematopoietic, and induced pluripotent stem cells. They are central to regenerative medicine and were first demonstrated experimentally in classic bone marrow studies.

      • embryonic stem cells — Embryonic stem cells are pluripotent cells usually taken from the inner cell mass of a blastocyst, and induced pluripotent stem cells are a related lab-made form. They are maintained with factors like LIF and marked by proteins such as Oct4, Sox2, and NANOG, and they can form teratomas and embryoid bodies in culture.

      • induced pluripotent stem cells — Induced pluripotent stem cells are reprogrammed somatic cells that regain pluripotency after exposure to factors such as Oct4, Sox2, Klf4, and Myc. The Yamanaka lab helped make them famous as a proposed alternative to embryonic stem cells.

      • mesenchymal stem cells — Mesenchymal stem cells are multipotent cells that can differentiate into connective tissues such as adipocytes, chondrocytes, cardiomyocytes, and osteocytes. Their name comes from mesenchyme, the embryonic connective tissue derived from the middle germ layer.

      • Pluripotency
        • pluripotent — Pluripotent cells can form any embryonic tissue from the three germ layers but not extra-embryonic tissue. The clue set points to stem cells and induced pluripotent cells, a property associated with Shinya Yamanaka’s reprogramming work.

      • pluripotent stem cells — Pluripotent stem cells can differentiate into any body tissue except extraembryonic structures like the placenta. They are associated with the Yamanaka factors, NANOG, and reprogramming mature cells into an embryonic-like state.

      • Potency states
        • totipotent — Totipotent describes a stem cell that can differentiate into every cell type in an organism. Because of that breadth, a totipotent cell can in principle generate an entire organism on its own.

  • cytology — Cytology is the branch of biology concerned with the study of cells. It deals with cell structure, function, multiplication, pathology, and life history.

  • cytolysis — Cytolysis is the bursting of a cell after its membrane is damaged or its environment is hypotonic, so water rushes in and the cell swells until it breaks. In immunology, the membrane attack complex can trigger this same outcome by punching holes in a target cell.

  • cytoplasm — The cytoplasm is the gel-like interior of the cell outside the nucleus, where organelles float and glycolysis occurs. In plants it streams to move nutrients, and the term is sometimes used loosely for the cytosol.

  • endocytosis — Endocytosis is the cellular process by which materials are taken into the cell in vesicles. Its forms include receptor-mediated endocytosis, phagocytosis, and pinocytosis, with clathrin-coated pits and endosomes commonly involved.

    • Pinocytosis — Pinocytosis is a form of endocytosis in which a cell takes in extracellular fluid or dissolved substances in small vesicles. It is often described as nonselective uptake of liquid material by the cell membrane.

    • Receptor-mediated pinocytosis — Receptor-mediated pinocytosis is a form of endocytosis in which a nutrient binds to a coated pit on the cell membrane. The membrane then pinches off to bring the material into the cytoplasm.

  • epithelial-mesenchymal transition — Epithelial-mesenchymal transition is the process in which epithelial cells lose adhesion and polarity and take on a more motile, spindle-shaped form. It is important in gastrulation, wound healing, and cancer metastasis, and it is associated with factors such as Snail, Slug, Twist, and TGF-beta.

  • eukaryotic cells — Eukaryotic cells have a membrane-bound nucleus and organelles, unlike prokaryotes. Plants, animals, fungi, and many protists are eukaryotes, and their cells include structures such as mitochondria.

  • gap junctions — Gap junctions are low-resistance intercellular channels between adjacent animal cells that allow ions and small molecules to pass directly from one cell to another. They are formed by paired connexons built from connexin proteins, and function as electrical synapses in tissues such as cardiac muscle.

  • Granules — Granules are small particulate structures or bodies found in biological and physical contexts. In botany they can refer to chloroplast-related structures in the photosphere, while in cell biology the term also appears in platelet granules and related white blood cell terminology.

  • liquid-liquid phase separation — Liquid-liquid phase separation is the process by which proteins or RNAs form droplets or membrane-less organelles such as stress granules and Cajal bodies. It is also called biomolecular condensation and is common in low-complexity proteins that undergo reversible demixing.

  • microglia — Microglia are the resident macrophages of the central nervous system and the smallest non-neuronal cells in the brain. They prune synapses, shift between ramified and reactive states after injury, and are implicated in neuroinflammation and diseases such as multiple sclerosis, Parkinson’s, and Alzheimer’s.

  • microvilli — Microvilli are tiny finger-like projections on the apical surface of intestinal cells. They increase the surface area for absorption and form the brush border of the small intestine.

  • Muscle cells — Muscle cells, or myocytes, are contractile cells whose sarcomeres contain actin and myosin organized into myofibrils. Their excitation-contraction machinery includes T-tubules, the sarcoplasmic reticulum, and the sliding-filament mechanism that underlies force generation.
    • t-tubules — T-tubules are invaginations of the muscle cell membrane that carry electrical signals deep into striated muscle fibers. They couple the sarcolemma to the sarcoplasmic reticulum at triads and diads, helping trigger calcium release for contraction.

  • nuclear lamins — Lamins are intermediate filament proteins that line the inner surface of the nuclear envelope and help form the nuclear lamina beneath the inner nuclear membrane. They are phosphorylated during mitosis, when they disassemble. Mutations in lamins can cause laminopathies such as Hutchinson-Gilford progeria and Emery-Dreifuss muscular dystrophy, and defects in ZMPSTE24 processing are also linked to lamin-related disease.

  • Peroxisomes — Peroxisomes are membrane-bound organelles that carry out fatty-acid oxidation and detoxify reactive oxygen species using enzymes such as catalase. They are implicated in disorders such as Zellweger syndrome, Refsum disease, adrenoleukodystrophy, and related PEX-gene import defects.
    • peroxisome(s) — Peroxisomes are small membrane-bound organelles rich in catalase and involved in breaking down hydrogen peroxide. They also carry out beta-oxidation of long-chain fatty acids, and defects in them cause disorders such as Zellweger syndrome and Refsum disease.

  • phagocytosis — Phagocytosis is the endocytic process by which cells engulf large particles, microbes, or whole cells. It is performed by macrophages, neutrophils, and microglia, and is helped by opsonization through complement and antibodies; the resulting vesicle fuses with a lysosome. Related clues include the oxidative burst, C5a, IL-8, and the zipper or sinking models.

  • protein sorting — Protein sorting is the cellular process of directing proteins to the proper organelle or compartment. It includes pathways mediated by signal peptides, SRP, TOM and TIM complexes, and mannose-6-phosphate targeting.

    • mannose-6-phosphate — Mannose-6-phosphate is the Golgi-made tag that directs proteins, especially lysosomal hydrolases, to lysosomes. Defects in its synthesis can cause I-cell disease, since the lysosomal enzymes fail to receive the proper trafficking signal.

    • retention sequence — A retention sequence is a signal that keeps a protein in a particular cellular compartment, especially the endoplasmic reticulum. The classic example is the C-terminal KDEL motif, which helps prevent ER-resident proteins from being exported.

    • signal recognition particle — The signal recognition particle is a ribonucleoprotein complex that targets ribosomes translating secretory proteins to the endoplasmic reticulum. It recognizes an N-terminal signal peptide and interacts with the SRP receptor in a GTP-dependent handoff step.

  • protoplasm — Protoplasm is an older term for the living contents of a cell, roughly overlapping with cytoplasm but excluding the nucleus in modern usage. The term also appears in older biological descriptions of fungal hyphae and their nutrient-transporting contents.

  • Receptors
    • G-protein coupled receptors — G-protein coupled receptors are seven-transmembrane cell-surface receptors that signal by activating heterotrimeric G proteins, triggering GDP-to-GTP exchange and dissociation of the alpha and beta-gamma subunits. Their signaling can lead to second-messenger cascades such as PLC cleavage of PIP2. They include rhodopsin, opsins, many olfactory, adrenergic, and muscarinic receptors, Frizzled, Smoothened, and secretin-family receptors, and they are a major drug-target class; beta-2 adrenergic receptor is a representative example.

  • secretion — Secretion is the cellular process of exporting proteins or other materials out of a cell. In bacteria it includes systems such as type III and type IV secretion, and in eukaryotes it underlies hormone release and other transport across membranes.

  • somatic cells — Somatic cells are the non-germline cells of the body, the ones that divide by mitosis rather than meiosis. They are the source cells used in cloning by nuclear transfer and can be reprogrammed into induced pluripotent stem cells with the Yamanaka factors.
    • Somatic cell — A somatic cell is any body cell that is not a sperm or egg cell. In stem-cell work, the nucleus of a somatic cell can be transferred into an enucleated ovum.
  • vesicle — A small membrane-bound sac or pocket used for transport and storage inside cells, including moving substances to their digestion site. In neurons, vesicles dock at axon terminals and can release their contents by exocytosis.
• Cell theory — organisms consist of one or more cells; the cell is the basic unit of living structure and function; new cells arise from existing cells. Schleiden and Schwann developed the nineteenth-century theory from plant and animal observations. The theory concerns cellular life; viruses depend on host cells to reproduce.¹

  • Matthias Schleiden — Matthias Schleiden was a botanist who helped found cell theory by arguing that plant tissues are made of cells. His work helped establish the cell as the basic unit of plant life.

  • Matthias Schleiden and Theodor Schwann — Matthias J. Schleiden and Theodor Schwann were German scientists whose work in the 1830s and 1840s helped establish the cellular basis of cell theory. Schleiden argued that plants are made of cells, while Schwann extended the idea to animals. Their associated works include Phytogenesis and Microscopical Researches.

Cell types and structures

• Prokaryote — no membrane-bound nucleus; circular DNA in a nucleoid; no membrane-bound organelles; smaller (1–10 µm); includes Bacteria and Archaea. Ribosomes are 70S (50S + 30S subunits).

• Eukaryote — membrane-bound nucleus; linear chromosomes; membrane-bound organelles; generally larger (10–100 µm); includes protists, fungi, plants, and animals. Ribosomes are 80S (60S + 40S subunits).

• Cell wall — present in bacteria (peptidoglycan), plants (cellulose), and fungi (chitin); absent in animal cells. The rigid outer layer is made of cellulose, chitin, or peptidoglycan depending on the organism. It helps maintain shape and prevent osmotic rupture, is broken down by lysozyme, and is targeted by some antibiotics and antifungals. In plants, it is involved in plasmodesmata formation and cell expansion.

  • bacterial cell walls — Bacterial cell walls are the peptidoglycan-rich structures that surround the bacterial membrane. They contain NAG and NAM cross-linked by peptide chains, are targeted by lysozymes and beta-lactam antibiotics, and determine whether a bacterium stains Gram-positive or Gram-negative.

  • plant cell wall — The plant cell wall is a cellulose-rich outer layer that supports plant cells and helps form structures such as xylem and tracheids. Its primary and secondary layers are linked by the pectin-rich middle lamella, and its loosening is involved in auxin-driven acid growth.

  • secondary wall — The secondary wall is the rigid inner layer added after cell expansion in plant cells. It is enriched with lignin and, in some tissues, suberin, which make it stronger and more waterproof than the primary wall.

• Plasma membrane — phospholipid bilayer with embedded proteins; selectively permeable; fluid mosaic model (Singer and Nicolson, 1972). It contains transport proteins such as aquaporins and helps maintain membrane asymmetry through flippases, floppases, and scramblases; embedded receptors also participate in signaling.

  • fluid mosaic model — A standard model of membrane structure describing the cell membrane as a flexible two-dimensional phospholipid bilayer in which proteins and lipids can move laterally. It distinguishes integral and peripheral membrane proteins and explains the membrane’s dynamic, patchwork character.

  • Glycocalyx — The glycocalyx is the carbohydrate-rich coat on the outside of cells, formed from glycosylated membrane components. It helps in cell recognition, adhesion, and, in some bacteria, surface attachment.

  • Microdomains
    • lipid rafts — Lipid rafts are cholesterol-rich microdomains in the plasma membrane that are more tightly packed than surrounding regions. They are linked to membrane trafficking and signaling and are often described as detergent-resistant domains.
  • the fluid mosaic model — The fluid mosaic model is the description of cell membranes proposed by Singer and Nicolson. It treats the lipid bilayer as a dynamic structure in which proteins and lipids can move laterally.
• Nucleus — contains DNA on chromosomes; surrounded by the double nuclear envelope with nuclear pores; contains the nucleolus (site of rRNA synthesis and ribosome assembly). Its membrane is continuous with the rough endoplasmic reticulum and nuclear pores regulate transport via importins, exportins, and RanGTP. The envelope breaks down during mitosis, and the nucleus can appear in unusual forms in muscle cells.
  • nuclear envelope — The nuclear envelope is the double lipid bilayer surrounding the nucleus. It contains nuclear pore complexes, is continuous with the endoplasmic reticulum, and is supported by a nuclear lamina with lamins and related envelope proteins. It disassembles during prophase or cell division and re-forms in telophase, with transport across it mediated by importins, exportins, and the Ran-GTP cycle.

    • nuclear lamina — The nuclear lamina is a meshwork of type V intermediate filaments lining the inner face of the nuclear envelope. It helps organize chromatin, anchors nuclear pores, and contains proteins such as lamin that are important in disorders like Emery-Dreifuss muscular dystrophy.

    • nuclear pores — Nuclear pores are gated openings in the nuclear envelope that control traffic between the nucleus and the cytosol. They use importins, exportins, and Ran-GTP to move cargo bearing nuclear localization signals through nuclear pore complexes built from nucleoporins.

  • nuclear pore complex — The nuclear pore complex is the gated protein assembly in the nuclear envelope that controls transport between nucleus and cytoplasm. It relies on FG-repeat nucleoporins, importins and exportins, and the Ran-GTP gradient, and it has an octagonal ring with an internal basket.

• Endoplasmic reticulum (ER) — rough ER (studded with ribosomes, synthesizes secretory/membrane proteins); smooth ER (lipid synthesis, detoxification, Ca²⁺ storage).

• Golgi apparatus — modifies, sorts, and packages proteins from the rough ER; “cis” face receives vesicles, “trans” face ships them. It consists of stacked cisternae and is also involved in glycosylation, including adding mannose-6-phosphate to lysosomal proteins, as well as routing cargo toward secretion or lysosomal use. It is named for the Italian scientist Camillo Golgi, who first described it in 1898.

  • Camillo Golgi — Italian cytologist and neurologist who developed the black reaction stain and won the 1906 Nobel Prize in Medicine, which he shared with Santiago Ramón y Cajal. His name is attached to the Golgi apparatus, a eukaryotic organelle involved in packaging proteins into vesicles and processing glycosylation, as well as other cellular structures identified in his studies.

  • trans-Golgi network — The trans-Golgi network is the exit face of the Golgi apparatus, where cargo is sorted for constitutive and regulated secretion. The prefix trans here also contrasts with cis in stereochemistry.

• Mitochondria — site of aerobic respiration; double membrane (inner membrane folded into cristae); own circular DNA and 70S ribosomes; endosymbiotic origin. They generate ATP via the electron transport chain, and in eukaryotes the Krebs cycle occurs in the mitochondrial matrix. In animals, mitochondrial DNA is usually maternally inherited, and mitochondria also participate in intrinsic apoptosis.
  • mitochondrial matrix — The mitochondrial matrix is the inner compartment of the mitochondrion where the Krebs cycle takes place. It is the site commonly referenced in clues about succinate dehydrogenase, the citric acid cycle, and proton pumping into the intermembrane space.

• Chloroplast — site of photosynthesis in plants and algae; double membrane plus internal thylakoid membranes (stacked into grana) and stroma; own circular DNA; endosymbiotic origin. It contains chlorophyll and hosts the light reactions and carbon-fixation pathways of photosynthesis. Chloroplasts also use TIC and TOC protein-import complexes.

• Lysosome — contains hydrolytic enzymes (acid pH); digests cellular debris, food particles, and foreign material; absent in plant cells (vacuoles serve analogous functions). Membrane-bound; receives mannose-6-phosphate-tagged enzymes from the Golgi and fuses with late endosomes and phagosomes. Defects in lysosomal function or targeting cause storage diseases such as Tay-Sachs, Gaucher, Fabry, Hurler, Wolman, and Niemann-Pick disease.

• Vacuole — storage organelle; central vacuole in plant cells maintains turgor pressure. Its membrane is called the tonoplast, and contractile vacuoles in some protists expel excess water to prevent lysis.
  • Central — Central can refer to a sports division, as in the NHL’s Central Division. It is also the name of the large fluid-filled organelle in plant cells that stores water and other materials.
• Cytoskeleton — three types: microtubules (tubulin, 25 nm; form spindle fibers, cilia, flagella), intermediate filaments (structural support), microfilaments (actin, 7 nm; cell movement, cytokinesis). The cytoskeleton is the internal network of microtubules, microfilaments, and intermediate filaments that maintains cell shape and enables movement. It also serves as a scaffold for intracellular transport and can be remodeled to form structures such as lamellipodia and pseudopodia.
  • Actin filaments
    • microfilament — Microfilaments are the thinnest cytoskeletal fibers, usually built from actin. They help maintain microvilli, support cell shape changes, and participate in cytoplasmic streaming and muscle contraction.
  • Cytoskeletal filaments
    • microtubules — Microtubules are hollow tubulin-based polymers that form part of the cytoskeleton. They make up the core of cilia and flagella, help form mitotic spindle fibers, and exhibit dynamic instability during cell division, with growth and shrinkage governed by catastrophe and rescue. They are moved upon by kinesin and dynein.

      • Astral microtubules — Astral microtubules are the microtubules present during mitosis that radiate from the centrosome rather than attaching to kinetochores. They help orient the mitotic spindle and can interact with the cell membrane to generate force.

      • Dynamic instability — Dynamic instability is the hallmark behavior of microtubules, in which they rapidly switch between growth and shrinkage. Its alternating catastrophe and rescue phases depend on the changing GTP- and GDP-tubulin state at the microtubule ends.

  • intermediate filaments — Intermediate filaments are a major class of cytoskeletal proteins, distinct from microtubules and actin filaments. They include keratins, vimentin, GFAP, and lamins, and they help provide structural support in cells and the nuclear lamina.
    • keratins — Keratins are structural intermediate-filament proteins found in hair, nails, and the epidermis. They are important in cornification and desquamation, and defects in them can cause blistering skin disorders such as epidermolysis bullosa simplex.

  • microfilaments — Microfilaments are thin actin-based components of the cytoskeleton. They help with cell movement, muscle contraction, and cleavage furrow formation during cell division.

  • Microtubule — Microtubules are cytoskeletal structures built from tubulin that help form the mitotic spindle. They are the targets of drugs such as colchicine, nocodazole, and taxol, which disrupt or stabilize their dynamics.

  • motor proteins — Motor proteins are ATP-driven cellular machines such as kinesin and dynein that move cargo along microtubules. They power intracellular transport and other cell processes by converting chemical energy into motion.

    • Dynein — Dynein is a motor protein that moves toward the minus end of microtubules and is associated with cilia and flagella, where it helps generate movement. Its arms act on the microtubule doublets in the axoneme, helping form the axonemal structure alongside tubulin and nexin.

    • Dyneins — Dyneins are minus-end microtubule motor proteins that power retrograde transport and movement in cilia and flagella. They are the counterpart of kinesins and are built around AAA ATPase modules that help them convert chemical energy into motion.

    • kinesin — Kinesin is a plus-end-directed microtubule motor protein that carries cargo through cells and down axons. It moves in a hand-over-hand manner with two ATP-driven heads, and its family includes variants studied by optical trap experiments.

    • Kinesin and dynein — Kinesin and dynein are ATP-powered motor proteins that move cellular cargo along microtubules. They are central to axoplasmic transport and other intracellular transport processes.

    • kinesins — Kinesins are ATP-driven motor proteins that move cargo along microtubules, usually toward the plus end in anterograde transport. They are important in processes such as spindle dynamics and intracellular transport.

    • myosin — Myosin is the motor protein that binds actin and powers muscle contraction by forming the thick filaments of sarcomeres. Different myosin classes also appear in processes such as transcription, cytokinesis, and hearing-related cellular function.

  • tubulin — Tubulin is the protein that polymerizes into microtubules, usually as alpha-beta heterodimers. Quizbowl clues often tie it to colchicine, taxol, gamma-tubulin ring complexes, and the centrosome or mitotic spindle.

• Cytosol — the aqueous, gel-like fluid component of the cytoplasm that surrounds organelles; composed mainly of water, dissolved ions, ATP, enzymes, and metabolites; distinct from the cytoplasm (which includes organelles); site of glycolysis, fatty acid synthesis, and many other metabolic reactions.

• Centrioles — paired structures forming the centrosome; organize the mitotic spindle; absent in most plant cells. They are microtubule structures found in centrosomes and are closely related to basal bodies and cilia formation.

• Peroxisome — contains catalase; breaks down H₂O₂ and fatty acids via beta-oxidation. It is a membrane-bound organelle that contains enzymes involved in hydrogen peroxide metabolism and other oxidative reactions. It participates in fatty acid metabolism, including beta-oxidation, and is associated with Zellweger syndrome and X-linked adrenoleukodystrophy.

• Catalase — an enzyme that protects cells by rapidly decomposing hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂. Human catalase is a tetramer whose subunits contain iron-bearing heme active sites; other organisms also have different catalase families. Superoxide dismutase converts superoxide into peroxide, which catalase can then remove. Catalase helps explain the oxygen bubbles produced when peroxide encounters catalase-containing tissue; peroxisomes are a major cellular location of this chemistry.² It is also classically used in microbiology to distinguish staphylococci from streptococci.

• Cell morphology
  • rod — A rod is a cylindrical bacterial shape, contrasted with cocci, and it also describes the light-sensing cells in the retina used for dim vision. The term also appears in biology for several structures and organisms, including Bacillus and other rod-shaped forms.
• Prokaryotic cells
  • nucleoid — The nucleoid is the non-membrane-bound region of a prokaryotic cell that contains the main bacterial chromosome. It is the DNA-containing area seen in bacteria in place of a true nucleus.
• Cell structures

  • cell walls — Cell walls are rigid outer structures outside the cell membrane. In plants they are made mainly of cellulose, help maintain turgor pressure and structural rigidity, and help define cell shape; in fungi they are made of chitin. In plant cells, they are pierced by plasmodesmata and associated with the middle lamella.

  • Cytoplasmic projections

    • pseudopodia — Temporary footlike cytoplasmic projections used by amoebas and other cells for movement and feeding. They are formed by actin-driven membrane extension and are important in phagocytosis.

      • pseudopod — A pseudopod is a temporary foot-like projection of cytoplasm used by an amoeba for movement and feeding. The term literally means “false foot.”

      • pseudopods or pseudopodia — Pseudopods, or pseudopodia, are cytoplasmic projections used by amoebae and other eukaryotes for motility and phagocytosis. They are important in organisms such as Dictyostelium and come in forms like filose and reticulose projections.

• Plant cell biology
  • Apoplast — The apoplast is the network of cell walls and intercellular spaces outside plant cell membranes. It provides a transport route for water and solutes and is contrasted with the symplast.
• Plant cells
  • Intercellular connections
    • plasmodesmata — Plasmodesmata are microscopic channels in plant cell walls that link adjacent cells, allowing cytoplasm, solutes, and direct cell-to-cell communication through the symplast. They contain a desmotubule, a specialized strand of endoplasmic reticulum, and are analogous to animal gap junctions. They are central to symplastic flow and movement-protein transport.
• Biological membranes
  • membranes — Membranes are thin selectively permeable structures such as cell membranes and phospholipid bilayers. They appear in biology as plasma membranes and in physics as higher-dimensional branes in M-theory.

    • semi-permeable membrane — A semi-permeable membrane allows some molecules to pass through while blocking others. Cell membranes are commonly described as semi-permeable and are important in osmosis.

    • semi-permeable membranes — Semi-permeable membranes allow some substances to pass while blocking others, making osmosis possible. They appear in dialysis and fuel-cell applications, including proton-exchange membranes and filters with controlled pore size.

• Cell adhesion molecules

  • integrins — Integrins are heterodimeric transmembrane adhesion receptors that link cells to the extracellular matrix and to other cells. They bind ligands through motifs such as RGD or LDV, connect to talin and filamin at the cytoplasmic side, and mediate key adhesion complexes like focal adhesions and hemidesmosomes.

  • selectins — Selectins are cell-adhesion proteins that bind sialyl-Lewis X and help leukocytes roll along blood vessel walls. They are commonly identified as E-, L-, and P-selectin, reflecting their expression in epithelial cells, lymphocytes, and platelets.

• Cell adhesion proteins
  • cadherin — Cadherins are calcium-dependent cell adhesion proteins that help cells stick to one another, especially through desmosomes and other junctions. Different types such as E-cadherin, N-cadherin, and P-cadherin are important in development, tissue sorting, and disease.
• Cell membranes
  • Lipid bilayers
    • phospholipid bilayer — A basic membrane structure formed by amphipathic phospholipids arranged with hydrophobic tails inward and hydrophilic heads outward, creating the double-layered barrier of cell membranes.
  • membrane pores — Membrane pores are openings through lipid bilayers that allow transport across cellular membranes. In pyroptosis, gasdermin D forms pores, and nuclear pore complexes provide the gated channel for importins and exportins.
• Membrane proteins

  • Aquaporins — Aquaporins are membrane proteins that form water channels and allow rapid, selective movement of water across cell membranes. They are important in kidney physiology and water reabsorption, especially in response to vasopressin. They were identified by Peter Agre and are associated with disorders such as nephrogenic diabetes insipidus and neuromyelitis optica.

  • flippases — Flippases are membrane enzymes that translocate phospholipids from one leaflet of the bilayer to the other. They help maintain membrane asymmetry and work alongside related transport proteins such as scramblases.
    • Flippase — A flippase is a membrane transporter that moves phospholipids between the two leaflets of the cell membrane. The name also refers to a recombinase used to delete DNA between two FRT sites.
  • ion channels — Ion channels are membrane proteins that let charged particles cross cell membranes. They include voltage-gated, ligand-gated, and mechanically gated types, and they are studied with patch-clamp recordings.

    • CFTR — CFTR is an ATP-binding cassette chloride channel/ion transporter that moves chloride and thiocyanate, especially in sweat glands. It is activated downstream of cAMP, and its most famous mutation is the F508 deletion (deltaF508), which causes cystic fibrosis. CFTR malfunction is associated with abnormal transepithelial potential differences and defective fluid transport in tissues such as the airway and pancreas.

    • potassium channels — Potassium channels are membrane proteins that selectively conduct potassium ions across cell membranes. They include voltage-gated, inward-rectifying, calcium-activated, and ATP-sensitive varieties important in action potentials and insulin release.
      • potassium ion channels — Potassium ion channels are membrane proteins that let potassium leave cells after action potentials and help set the resting potential. Examples include Shaker, BK, SK, and inward-rectifying channels, which are important in excitable tissues and are implicated in arrhythmias and epilepsy.
    • Sodium channels
      • Voltage-gated sodium channels — Voltage-gated sodium channels are membrane proteins that open in response to changes in voltage and drive neuronal depolarization. Their alpha subunit includes four domains with six transmembrane helices each, and the S4 segment acts as a voltage sensor.

  • porins — Porins are beta-barrel membrane proteins that form channels for passive transport. They are especially important in the outer mitochondrial membrane and are related to aquaporin-like transport questions.

  • transmembrane proteins — Transmembrane proteins span a phospholipid bilayer, with parts exposed on both sides of the membrane. They include single-pass and multi-pass proteins such as GPCRs and aquaporins, and they are often analyzed by topology-prediction methods like TMHMM.
• Cell organelles

  • centrosome — The centrosome is the main microtubule-organizing center of animal cells, containing a pair of orthogonally arranged centrioles and surrounding pericentriolar material. It duplicates and moves to opposite poles during cell division, where it helps organize the aster.

  • cilium — A cilium is a short, hairlike eukaryotic organelle built from microtubules, usually in a 9+2 arrangement, and generally shorter than a flagellum. Cilia beat with coordinated power and recovery strokes to propel single-celled organisms and move fluid across tissues, such as mucus in the respiratory tract, fluid in clam siphons, and eggs through the fallopian tubes; they also help establish left-right asymmetry during development. Motile cilia are involved in movement, while primary cilia are nonmotile and function in signaling. Defects in cilia can cause ciliopathies such as Kartagener syndrome, situs inversus, and infertility. Related forms include stereocilia and kinocilia.

  • cisternae — Cisternae are flattened membrane sacs found in the Golgi apparatus and endoplasmic reticulum. They are the stacked compartments that move through cis-to-trans maturation and help sort proteins in eukaryotic cells.

  • organelles — Organelles are specialized structures within cells that perform particular functions. Examples include mitochondria, chloroplasts, lysosomes, ribosomes, centrioles, and the nucleus.

    • Berkeley bodies — Berkeley bodies are yeast organelles involved in trafficking material toward the vacuole. They are associated with S. cerevisiae and are structurally similar to autophagosomes.

    • Centriole — A centriole is a cylindrical microtubule structure found in centrosomes, usually in eukaryotic and especially animal cells. It typically consists of nine microtubule triplets, occurs as a perpendicular pair in a diplosome, and helps organize the mitotic spindle during mitosis. Centrioles are also linked to cilia and flagella formation; a basal body is a specialized centriole that anchors a cilium or flagellum. They can replicate themselves.

    • centrosomes — Centrosomes are the microtubule-organizing centers that help form the mitotic spindle in animal cells. They contain a pair of centrioles and are important for cell division and polarity.

    • Endosomes
      • endosome — An endosome is a membrane-bound vesicle that acts as an intermediate station between the trans-Golgi network and lysosomes. Early endosomes receive phagocytosed material, and late endosomes mature into compartments that help form lysosomes.
    • Endosymbiotic organelles
      • mitochondria and chloroplasts — Mitochondria and chloroplasts are energy-producing organelles thought to descend from bacteria that began living inside larger cells. Mitochondria generate energy from glucose, while chloroplasts capture sunlight, and both are classic examples of endosymbiotic organelles.
    • Glyoxysomes — Glyoxysomes are plant cell organelles, specialized peroxisomes that help convert stored lipids into transportable compounds. They oxidize fatty acids to acetyl-CoA and are important in seedlings and growing tissues.
      • glyoxysome — A glyoxysome is a specialized peroxisome found in germinating seedlings and oil-rich storage tissue. It converts stored fatty acids or lipids into sugars for growing cells.
  • vacuoles — Vacuoles are membrane-bound sacs used for storage in cells, especially for water and solutes in plants. In protists they may be contractile or food vacuoles, helping regulate water balance and digest ingested material.
• Endomembrane system
  • endoplasmic reticulum — The endoplasmic reticulum is the cell’s internal membrane network in eukaryotes, continuous with the nuclear envelope and organized into rough and smooth regions. Rough ER is studded with ribosomes and is involved in protein synthesis, folding, glycosylation, and processing, while smooth ER functions in lipid synthesis and metabolism, detoxification, calcium storage, and vesicle transport to the Golgi apparatus.

    • rough endoplasmic reticulum — The rough endoplasmic reticulum is the ribosome-studded part of the endomembrane system, attached to the nuclear envelope, that helps synthesize secreted and membrane proteins as well as phospholipids and other membrane components. It is the site of signal recognition particle targeting, disulfide-bond formation, and the unfolded protein response.

    • sarcoplasmic reticulum — The sarcoplasmic reticulum is the muscle-cell form of endoplasmic reticulum that stores and releases calcium to trigger contraction. It works with T-tubules, ryanodine receptors, and SERCA pumps, and is especially important in skeletal and cardiac muscle physiology.
      • sarcoplasmic reticulum or SR — The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells. It stores calcium and works with T-tubules at triads to trigger muscle contraction.
    • smooth endoplasmic reticulum — The smooth endoplasmic reticulum is a ribosome-free membrane network found in eukaryotic cells. It synthesizes lipids such as phospholipids and steroids, helps metabolize carbohydrates, and is associated with detoxification by enzymes such as cytochrome P450. In muscle, a specialized form stores calcium for contraction and enables sarcoplasmic calcium release.
• Protein sorting signals
  • KDEL — KDEL is the C-terminal amino-acid sequence that retains soluble ER proteins in the endoplasmic reticulum by allowing retrieval from the Golgi. It works through KDEL receptors and is analogous to other organelle-targeting signals such as KKXX.
• Mitochondrion
  • inner mitochondrial membrane — The inner mitochondrial membrane is the phospholipid bilayer that contains the electron transport chain and ATP synthase. It is folded into cristae and is also the site of the antiporters that shuttle metabolites during oxidative phosphorylation.
    • cristae — Cristae are the folds of the inner mitochondrial membrane, where proteins involved in respiration and ATP synthesis are concentrated. They increase membrane surface area and are often mentioned in connection with mitochondria’s endosymbiotic origin and disorders like Kearns-Sayre syndrome.
• plastids — Plastids are endosymbiotic organelles descended from cyanobacteria and include chloroplasts and other pigment- or nutrient-related forms. They have their own DNA and multiple membranes, and protein targeting to them can involve transit peptides and the Toc/Tic complex.

  • Amyloplasts — Amyloplasts are starch-filled plastids often found in root-cap cells. They can function as statoliths that help plants sense gravity and contribute to gravitropism.

  • chloroplast(s) — Chloroplasts are the green plant organelles where photosynthesis occurs, containing thylakoids, grana, and RuBisCo. They are plastids whose membranes use Tic and Toc protein complexes and whose light reactions release oxygen in the Hill reaction.

  • Chromoplasts — Chromoplasts are plastids that synthesize or store non-green pigments such as carotenoids and xanthophylls. They are commonly found in fruits and flower petals.

  • leucoplasts — Leucoplasts are colorless plastids found in plants and algae that lack pigments. They are associated with storage and synthesis of starch, oils, and proteins, with specialized forms such as amyloplasts, proteinoplasts, and elaioplasts.

• Plant cell organelles
  • central vacuole — The central vacuole is the large fluid-filled organelle in plant cells bounded by the tonoplast. It stores water and nutrients and can occupy much of the volume of a mature plant cell.
• Plant cell structures
  • Chloroplast stroma
    • stroma — In chloroplasts, the stroma is the fluid matrix surrounding the thylakoids, and it is where the Calvin cycle occurs. The same word is also used for the connective tissue framework of some organs, such as the cornea.
  • Vacuolar membrane
    • tonoplast — The tonoplast is the membrane that surrounds the vacuole in a plant cell. It regulates ion movement and contains transport proteins such as tonoplast intrinsic proteins, with proton pumps helping create the gradient across it.
• Actin cytoskeleton
  • treadmilling — Treadmilling is the steady-state behavior of actin filaments in which subunits are added at one end and lost from the other without changing overall length. It is often discussed with actin polymers, phalloidin, and the leading-edge dynamics of cell migration.
• Cell motility

  • chemotaxis — Chemotaxis is movement in response to a chemical stimulus or gradient. It guides immune cells such as neutrophils toward infection and can also be exploited by pathogens like H. pylori.

  • Cellular motion — Cellular motion, or locomotion, is the process by which cells move using cilia, flagella, pseudopodia, or amoeboid crawling. Related clues include the scratch assay, epithelial-mesenchymal transition, and chemotaxis or taxis as stimulus-directed forms of movement.

• Cytoskeletal proteins
  • actin — Actin is the major cytoskeletal protein that polymerizes into microfilaments and interacts with myosin during muscle contraction. It exists in G and F forms, is stained by phalloidin, and appears in many cell biological contexts from motility to cell cortex structure.

    • beta-actin — Beta-actin is a major cytoskeletal protein that forms thin filaments and binds myosin during muscle contraction. Its globular and filamentous forms are often referred to as G-actin and F-actin, and actin-rich structures power cell movement and shape.

    • globular actin — Globular actin, also called G-actin, is the soluble monomeric form of actin. These monomers polymerize into filamentous F-actin, providing a soluble reservoir for actin assembly and disassembly during treadmilling; this polymerization is inhibited by the sponge toxin latrunculin.

  • spectrin — Spectrin is a cytoskeletal protein that forms a membrane-supporting meshwork in red blood cells. Its alpha and beta subunits bind actin and anchor the plasma membrane, giving erythrocytes their flexible shape.
• Cell motility structures
  • flagella — Whip-like motility structures used by bacteria, sperm cells, and some protists and fungi. Bacterial flagella are built around a basal body, hook, rod, and rotor system, with the filament made from flagellin and rotation powered by MotA and MotB; their arrangement can be monotrichous, lophotrichous, or other patterns. Eukaryotic flagella have a 9+2 microtubule axoneme and share this microtubule-based structure with cilia.
    • flagellum or flagella — A flagellum is a whip-like motility structure used by cells and many microbes for locomotion. In bacteria it includes a basal body, hook, and filament, while eukaryotic flagella have a 9+2 microtubule axoneme driven by dynein.
• Cilia and flagella

  • axoneme — The axoneme is the microtubule core of eukaryotic cilia and flagella. It has the classic 9+2 arrangement and is anchored to the cell by the basal body.

  • Basal bodies
    • basal body — A basal body is a centriole-derived, centriole-like structure that anchors a cilium or flagellum to the cell surface or membrane. It is built from nine microtubule triplets and is often treated as a modified centriole; unlike the axoneme, it lacks the central pair of microtubules.
  • axonemes — Axonemes are the microtubule cores of cilia and flagella. Their characteristic 9+2 arrangement and dynein-driven movement are key clues, and defects in them can cause disorders such as situs inversus.
• Cell membrane models
  • Davson-Danielli model — The Davson-Danielli model was an early hypothesis for cell membrane structure that pictured phospholipids sandwiched between layers of protein. It was later displaced by the fluid mosaic model because it could not explain membrane protein mobility and distribution.
• Cell membrane structures
  • caveolae — Caveolae are tiny flask-shaped invaginations in the cell membrane that can pinch off into vesicles. They are a type of lipid raft rich in cholesterol and sphingolipids and are involved in micropinocytosis.
• Nuclear bodies
  • Cajal bodies — Cajal bodies are dense nuclear suborganelles involved in RNA-processing machinery. They are associated with coilin and with the assembly and maturation of snRNPs and telomerase RNA.
• Nuclear transport
  • Importins — Importins are karyopherins that transport cargo into the nucleus through the nuclear pore complex. They recognize nuclear localization signals, and RanGTP binding releases the cargo at the target side.
• Nucleoli
  • nucleolus — A dense, roughly spherical nuclear sub-organelle and darkly staining nuclear body where ribosome biogenesis begins: rRNA is processed and ribosomes are assembled. Its structure includes a fibrillar center, a dense fibrillar component, and a granular component, and it is associated with nucleolar organizer regions, snoRNAs, and rDNA.

Membrane transport

• Membrane transport proteins

  • ABC transporter — ABC transporters are ATP-binding cassette proteins that use ATP hydrolysis to move molecules across membranes. They include clinically important members such as CFTR, TAP, MDR1, and proteins whose defects can cause adrenoleukodystrophy or contribute to drug resistance.

  • ABC transporters — ABC transporters are a large family of membrane proteins that use ATP hydrolysis to move substances across membranes. Members include CFTR, the cystic fibrosis protein, as well as export and import systems such as P-glycoprotein, TAP, and bacterial transporters like BtuCD and MalK.

  • Proton pumps
    • proton pump — A proton pump uses energy to move hydrogen ions across a membrane and build an electrochemical gradient. That gradient can power ATP synthase in mitochondria and drive motility in structures such as flagella.
  • Transporters
    • antiporter — An antiporter is a membrane transport protein that moves one solute out of a cell while moving another in. It uses the gradient of one molecule to drive transport of another against its gradient.

• Passive transport — membrane transport down a concentration or electrochemical gradient without direct cellular energy expenditure. Thermal motion continues at equilibrium; equal opposing movements yield no net transport. Simple diffusion, facilitated diffusion, and osmosis are passive processes, although cells often expend energy elsewhere to maintain the gradients that drive them.³ It includes diffusion through protein channels or carriers and does not require ATP.

• Facilitated diffusion — passive movement through a membrane protein rather than directly through the lipid bilayer. Channels provide selective pores; carriers bind a substance and change conformation. Carrier-mediated transport can saturate when the available carriers are occupied. Protein involvement alone does not make transport active: the direction relative to the electrochemical gradient and the source of energy distinguish the mechanisms.³ It includes both channel-mediated and carrier-mediated transport; the latter is saturable when all carriers are occupied. A common example is glucose transport through GLUT proteins.

• Active transport — requires ATP; moves against concentration gradient. Example: sodium-potassium pump (Na⁺/K⁺-ATPase) moves 3 Na⁺ out and 2 K⁺ in per ATP. It powers the sodium-potassium pump in animal cells and helps load sap into plant phloem; bulk forms include phagocytosis and endocytosis.

• Endocytosis / exocytosis — bulk movement of materials into or out of the cell via vesicles; phagocytosis (solid particles), pinocytosis (fluids).

• Osmosis — net water movement across a selectively permeable membrane toward lower water potential. With equal hydrostatic pressure, an effectively impermeant solute draws water toward its more concentrated side; pressure can oppose that movement. Aquaporins facilitate water passage. A hypotonic environment can swell an animal cell, while a hypertonic one causes shrinkage; in plant cells, water loss can pull the membrane away from the wall (plasmolysis).³ Its reverse form is used in water purification, and the related osmotic pressure is the colligative property that counteracts this movement.
  • Hypotonic solutions
    • hypotonic — A hypotonic environment has a lower solute concentration outside the cell than inside it. In such a solution, water tends to move into the cell and can cause it to swell or burst.

  • isotonic — Isotonic describes solutions with the same osmotic pressure, so there is no net movement of water across a membrane. Intravenous fluids are often required to be isotonic with blood to avoid crenation or hemolysis.

  • Plasmolysis — Plasmolysis is the shrinkage of a plant cell’s cytoplasm away from the cell wall when water leaves by osmosis. It occurs when a plant cell is placed in a hypertonic solution.

  • Tonicity
    • Hypertonic — A hypertonic solution has a higher solute concentration than the cell or fluid it is compared against. In biology, placing cells in a hypertonic solution causes water to leave them by osmosis.
• Membrane fusion

  • cell fusion — Cell fusion is the merging of cells or membranes, a process that can produce syncytia. It appears in exocytosis-related membrane events and in experimentally induced fusion of cultured cells.

  • SNARE complex — The SNARE complex is a set of proteins that mediates vesicle fusion, especially at synapses. It includes proteins such as syntaxin, synaptobrevin, and SNAP-25, which assemble into a fusion machinery.

• Membrane fusion proteins

  • SNARE — SNARE proteins are membrane-fusion proteins that drive vesicle docking and exocytosis. They include vesicle and target forms and are targets of botulinum and tetanus toxins.

  • SNAREs — SNARE proteins mediate membrane fusion during vesicle exocytosis and neurotransmitter release. Examples include syntaxin and synaptobrevin, and they are the targets of botulinum and tetanus toxins.

• Membrane trafficking
  • SNARE proteins — SNARE proteins mediate vesicle docking and fusion with the plasma membrane at synapses. They include vesicle protein synaptobrevin and membrane proteins such as SNAP-25, and botulinum and tetanus toxins disable neurotransmitter release by targeting them.
    • synaptobrevin — Synaptobrevin, also known as VAMP1 or VAMP2, is a vesicle-associated membrane protein involved in neurotransmitter release. It is part of the SNARE complex with SNAP-25 and syntaxin, and it is a target of tetanus toxin and some botulinum toxins.
  • vesicle fusion — Process in which a vesicle docks with and fuses to a membrane, releasing its contents; in cells this usually means fusion with the plasma membrane for secretion and other vesicle-based export pathways. It is used in endocrine cells. The fusion machinery involves SNARE proteins such as syntaxin and VAMP, along with NSF, Rab effectors, and complexin.
• Vesicle coat proteins
  • clathrin — Clathrin is the vesicle-coating protein that forms triskelion-shaped lattices in endocytosis and transport from the Golgi. It is associated with coated pits, adaptor proteins, and vesicle formation during receptor-mediated uptake.
• Vesicle coats
  • COPI
    • COPI (COP-ONE) — COPI is a coat protein complex that forms vesicles involved in retrograde transport from the cis-Golgi to the endoplasmic reticulum. It is recruited by ARF-GTP and is central to ER retrieval and Golgi trafficking.
  • COPII — COPII is the vesicle-coating protein complex that moves cargo from the endoplasmic reticulum to the Golgi apparatus. It is the opposite of COPI, which mediates retrograde transport back toward the ER.
• Solutions
  • Tonicity
    • hypertonic solution — A hypertonic solution has a higher solute concentration than a cell’s interior. When a cell is placed in it, water leaves the cell and the cell shrinks or crenates.

Cell division

• Cell division
  • Cytokinesis — Cytokinesis is the stage of cell division in which the cytoplasm splits to form two daughter cells after mitosis or telophase. In animals it uses a contractile ring and cleavage furrow, while in plants it proceeds through a cell plate formed by vesicle fusion and a phragmoplast.

    • cleavage furrow — The cleavage furrow is the indentation that forms in animal cells during cytokinesis. It is produced by a contractile ring of microfilaments that pinches the cell into two daughter cells.

    • contractile ring — The contractile ring and cleavage furrow are central clues to cytokinesis, the step of cell division that separates one cell’s cytoplasm into two daughter cells. In animals it depends on actin and myosin II, while plants use a phragmoplast and cell plate formation.

  • Hayflick limit — The Hayflick limit is the finite number of times most somatic cells can divide before entering senescence. It is tied to telomere shortening and is often contrasted with the limitless replication of cancer cells or the relative immortality of stem cells.

  • kinetochore — The kinetochore is the protein complex on the centromere that attaches spindle microtubules to chromosomes during cell division. In humans it forms around alpha-satellite DNA and includes centromere proteins such as CENP-A and CENP-C.

  • mitotic spindle — The mitotic spindle is the microtubule-based structure that separates chromosomes during mitosis. Its poles are organized by centrosomes, and its assembly is regulated by checkpoints and motor proteins that ensure correct attachment to kinetochores.

  • mitotic spindle apparatus — The mitotic spindle apparatus is the microtubule structure that attaches to chromosomes at metaphase and pulls sister chromatids apart during anaphase. It is organized by the centrosomes during prophase and is essential for aligning centromeres before chromosome separation.
• Mitosis — division producing two genetically identical daughter cells; phases: PMAT (Prophase, Metaphase, Anaphase, Telophase) plus Cytokinesis. Somatic cell division. Mitosis is the eukaryotic cell-division process that follows G2 and separates sister chromatids through PMAT during the M phase. It produces two genetically identical daughter cells after spindle fibers attach to kinetochores and checkpoints regulate entry and progression.

  • Anaphase — Anaphase is the stage of cell division, including mitosis, in which sister chromatids separate and move toward opposite poles of the cell after metaphase and before telophase. It is driven by APC/C-triggered securin degradation, separase activation, and spindle microtubule shortening; in meiosis, anaphase I and anaphase II are distinct variants.

  • metaphase — Metaphase is the stage of cell division when chromosomes line up at the cell’s equatorial, or metaphase, plate before sister chromatids separate. It is the point at which the spindle checkpoint operates, and it is used for karyotyping because the chromosomes are maximally condensed and visible. In mitosis it follows prophase and precedes anaphase.

  • Mitosis phases — Mitosis is commonly ordered as interphase, prophase, metaphase, anaphase, and telophase. The sequence describes the standard progression of cell division in eukaryotic cells.

  • Phases
    • phases of mitosis — These are the five phases of mitosis in order: prophase, prometaphase, metaphase, anaphase, and telophase. They exclude interphase and cytokinesis.

  • prometaphase — Prometaphase is an intermediate stage of mitosis between prophase and metaphase. During it, the nuclear envelope breaks down and spindle microtubules attach to chromosomes at their kinetochores.

  • prophase — Prophase is the first stage of mitosis, when chromatin condenses into chromosomes and the nuclear envelope and nucleolus disappear. In meiosis, related clues often mention synapsis, crossing over, and the prophase I subdivisions such as diplotene and diakinesis.

  • prophase, metaphase, anaphase, telophase — The stages of mitosis occur in the order prophase, metaphase, anaphase, then telophase. This sequence carries a cell from chromosome condensation through separation of chromatids to the formation of two daughter cells.
• Meiosis — two successive divisions producing four genetically unique haploid cells (gametes). Meiosis I separates homologs (reductive division); Meiosis II separates sister chromatids. Crossing over during Prophase I increases genetic diversity. It can also produce one ovum and polar bodies in females. Homologous chromosomes pair in prophase I, where crossing over and homologous recombination create genetic variation.

  • Anaphase I — Anaphase I is the meiotic stage in which homologous chromosomes separate and move toward opposite poles while sister chromatids remain joined. It corresponds to the segregation of alleles in Mendelian genetics.

  • chiasma — The visible point where homologous chromosomes remain attached during meiosis, marking the site of crossing over between chromatids during prophase I. Chiasmata are the points of contact where paired chromatids exchange genetic material and help hold homologs or tetrads together as chromosomes separate.

  • leptotene — Leptotene is the first stage of prophase I, when chromosomes begin condensing into slender threads. It precedes the later meiotic substages in which homologous chromosomes pair and exchange material.

  • Meiosis II
    • anaphase II — Anaphase II is the meiotic stage in which sister chromatids separate. It follows meiosis I and occurs after fertilization releases human oocytes from arrest before the stage begins.

  • metaphase I — Metaphase I is the meiotic stage in which homologous chromosome pairs line up at the cell’s equator. It is the phase associated with independent assortment as the paired chromosomes orient randomly before being separated in meiosis I.

  • nondisjunction — Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, especially meiosis. It produces aneuploid gametes and can cause disorders such as monosomy in Turner syndrome and trisomy in Down, Edwards, Patau, or Klinefelter syndrome.

  • pachytene — Pachytene is the third substage of prophase I in meiosis, following zygotene and preceding diplotene. It is the stage when homologous chromosomes undergo genetic recombination in the four-strand stage, with synaptonemal complexes fully formed so crossing over can occur and chiasmata can develop, producing recombinant and parental chromatids.

  • prophase I — Prophase I is the first stage of meiosis I, when chromosomes condense, synapsis occurs, and crossing over takes place. It is divided into leptotene, zygotene, pachytene, diplotene, and diakinesis, and in many animal cells oocytes remain arrested in diplotene until ovulation. The phase ends before metaphase I.

    • diakinesis — Diakinesis is the final stage of prophase I in meiosis, when chiasmata terminalize and tetrads become especially visible. It follows leptotene, zygotene, pachytene, and diplotene.

    • pachytene stage — Pachytene is the third stage of meiotic prophase I, after zygotene and before diplotene. During this stage the synaptonemal complexes are fully assembled and chromosomal crossing over occurs. Its name comes from Greek for “thick threads,” and in females the arrest before puberty happens much earlier in meiosis.

  • Prophase II — Prophase II is the first stage of the second meiotic division, when the nuclear envelope breaks down and the spindle apparatus forms. Unlike prophase I, it does not involve synapsis or crossing over.

  • Recombination
    • Crossing over — The meiotic process in which homologous chromosomes exchange corresponding DNA segments during prophase I, typically at chiasmata within synaptonemal complexes during pachytene. This exchange creates new allele combinations, increases genetic variation, and underlies linkage maps; geneticists use crossover frequency to estimate map distance, measured in centimorgans.

  • second metaphase — Metaphase II is the meiotic stage in which individual chromosomes, each still consisting of two sister chromatids, align at the cell’s equator after homologous chromosomes have already separated in meiosis I. In most mammals, oogenesis is arrested at this stage until fertilization completes meiosis.

  • synapsis — This is the pairing of homologous chromosomes in prophase I of meiosis that enables crossing over. It is the alignment or joining of chromosome pairs, especially in discussions of genetic recombination and meiotic division.

  • synaptonemal complex — This zipper-like protein structure holds homologous chromosomes together during prophase I of meiosis. It forms in the zygotene stage, disappears after pachytene, and supports crossing over by keeping the chromosomes in close proximity.

  • tetrad — A tetrad is a group of four, especially the four haploid cells produced at the end of meiosis. In genetics it can also refer to the four chromatids of a paired set during prophase I.

  • tetrads — Tetrads are the paired homologous chromosomes formed in meiosis, also called bivalents. In fungi such as yeasts and molds, tetrads can be recovered from an ascus for genetic analysis.
• Cell cycle — G1 (growth), S (DNA synthesis), G2 (preparation for division), M (mitosis/meiosis); cells may exit to G0 (quiescent state). Duration ~24 hours in rapidly dividing human cells.

  • Cell synchronization — Cell synchronization is the process of bringing a population of cells into the same point in the cell cycle. It can be done by serum deprivation, drug arrest, or physical separation methods such as centrifugal elutriation, then checked by DNA-content sorting or arrest-inducing agents like nocodazole.

  • cellular senescence — Cellular senescence is the state in which cells permanently stop dividing, often after telomere shortening or irreparable DNA damage. It is associated with G0 arrest and can be marked by beta-galactosidase staining at pH 6.

  • checkpoints — Checkpoints are control points in the cell cycle, such as the G1, G2, and M checkpoints. In immunology, the term also appears in immune checkpoint signaling, which can be targeted by checkpoint inhibitors like pembrolizumab.

  • endoreplication — Endoreplication is DNA replication without cell division, producing polyploid cells. It is seen in megakaryocytes and related to repeated genome duplication without cytokinesis.

  • G0 — G0, or the resting phase, is the quiescent stage of the cell cycle after G1 when a cell exits active division. Cells in G0 do not proceed to DNA replication or mitosis, and many specialized cells such as neurons remain there permanently.

  • G2 phase — The G2 phase is the second growth stage of interphase, occurring after DNA replication and before mitosis. Cells use it to continue growth, duplicate organelles, and pass a checkpoint that can block entry into division if damage is detected.

  • interphase — Interphase is the long portion of the cell cycle when the cell grows, replicates its DNA, and prepares for mitosis through the G1, S, and G2 subphases. It is controlled by cyclins and CDKs, and cells may enter G0 from this stage.
    • G1 phase — The G1 phase is the first growth stage of interphase, when cells prepare for DNA synthesis and can exit into quiescence at the restriction point. It is the checkpointed period immediately before S phase and is regulated by proteins such as p53, Rb, and E2F.
      • G1 — G1 is the first gap phase of the cell cycle, occurring after mitosis and before S phase. Cells that do not pass the restriction point in G1 may exit into G0, and in yeast this stage can be triggered into arrest by pheromones.
    • S phase — The S phase is the portion of interphase in which DNA replication occurs. During this synthesis phase, a cell duplicates its DNA and forms sister chromatids before moving on to G2.
      • Synthesis phase — The synthesis phase, or S phase, is the part of interphase when DNA replication occurs. It lies between G1 and G2 and prepares the cell for mitosis.
  • M phase — The M phase, or mitotic phase, is the part of the cell cycle divided into prophase, metaphase, anaphase, and telophase. Chromosome division and cytokinesis occur during this phase.
    • Telophase — Telophase is the final stage of mitosis and meiosis, following anaphase and preceding cytokinesis. During it, chromosomes decondense, nuclear envelopes and nucleoli re-form, and the spindle breaks down as daughter nuclei are reassembled.

  • maturation promoting factor — Maturation promoting factor, or MPF, is the cyclin B–CDK1 complex that drives cells from G2 into mitosis. It phosphorylates proteins such as lamins and condensins, and was first characterized in frog oocytes by Yoshio Masui.

  • Quiescence — Quiescence is a reversible resting state in which cells leave the active cell cycle and stop dividing. It can be induced by starvation or signals such as cytokines and growth factors, and it is important in stem-cell biology and cancer treatment.

  • restriction point — The restriction point is the late G1 checkpoint at which a cell becomes committed to DNA replication and division. After passing it, the cell proceeds through the rest of the cycle even if external growth conditions change.

  • APC — APC can refer to adenomatous polyposis coli, a tumor suppressor mutated in familial adenomatous polyposis. The abbreviation also names the anaphase-promoting complex and antigen-presenting cells.

  • Cell cycle proteins
    • cyclins — Cyclins are cell-cycle regulatory proteins whose concentrations rise and fall as they activate cyclin-dependent kinases, helping drive progression through mitosis and other stages of the cell cycle. Distinct classes include the mitotic B cyclins, the G1 D cyclins, and related viral mimics such as v-cyclins.
      • Cyclin — Cyclins are proteins whose levels rise and fall during the cell cycle and that regulate progression by binding cyclin-dependent kinases. They help form complexes such as MPF, and their destruction is controlled by APC/C and related inhibitors like Wee1 and Sic1.

• Cell cycle checkpoints — G1/S checkpoint (checks DNA integrity before replication), G2/M checkpoint (checks replication completeness), spindle assembly checkpoint (ensures correct attachment). Regulated by cyclins and cyclin-dependent kinases (CDKs). They are transition points where the cell verifies conditions before moving on: the G1/S checkpoint checks DNA integrity before replication, the G2/M checkpoint checks replication completeness, and the spindle assembly checkpoint ensures correct attachment before anaphase. They are regulated by cyclins and cyclin-dependent kinases (CDKs).

• Apoptosis — programmed cell death; two pathways: intrinsic (mitochondria-mediated, activated by DNA damage; Bcl-2 family regulates; cytochrome c released → apoptosome → caspase cascade) and extrinsic (death receptor pathway; Fas/TNF-R); cell shrinks and fragments into apoptotic bodies; distinct from necrosis. Essential for development and immune sculpting. It proceeds through intrinsic mitochondrial signaling or extrinsic death-receptor pathways such as Fas and TNF-R, with Bcl-2 family proteins regulating cytochrome c release, apoptosome formation, and the caspase cascade. The cell shrinks and breaks into apoptotic bodies, with membrane blebbing, chromatin condensation, nuclear fragmentation, DNA laddering, and phosphatidylserine exposure; p53 can trigger it after irreparable DNA damage. It is important in development, tissue maintenance, and immune system sculpting.

  • Bcl-2 — Bcl-2 is an anti-apoptotic protein that helps prevent mitochondrial membrane permeabilization by Bax and Bak. It is overexpressed in some lymphomas and leukemias, and it is a classic member of the apoptosis-regulating Bcl-2 family defined by BH domains.

  • Apoptosis proteins
    • Bcl2 — Bcl2 is a family of proteins that regulate apoptosis, including both pro-apoptotic and anti-apoptotic members. Its anti-apoptotic members help control the intrinsic pathway, while proteins such as Bax, Bak, and Bid belong to the same family.
  • cell death — Cell death is the general process in which cells cease to function, including apoptosis, necrosis, and related programmed forms. The clues mention p53, fas signaling, TNF-alpha, and membrane blebbing as triggers or features of this process.

    • dead cells — Dead cells are cells that have undergone death, especially apoptosis, and are often detected by annexin-V and propidium iodide staining. Dead Cells is also the title of a well-known roguelike game about a revived corpse.

    • ferroptosis — Ferroptosis is an iron-dependent form of regulated cell death driven by lipid peroxidation. It is commonly triggered by blocking glutathione peroxidase 4 or the cystine-glutamate antiporter system Xc−, allowing toxic lipid peroxides to accumulate.

    • necrosis — Necrosis is a form of uncontrolled cell death associated with tissue injury, infection, gangrene, and inflammation. It contrasts with apoptosis and can occur in diseases such as flesh-eating infections or ischemic injury.

• Cancer — results from mutations in proto-oncogenes (become oncogenes) and/or tumor suppressor genes (e.g., TP53, Rb); cells divide uncontrollably. It lacks normal anchorage and density dependence and can spread by metastasis. Commonly associated examples of altered genes include TP53 and Rb, and treatments include chemotherapy, radiation, and targeted drugs such as rituximab or Herceptin.

  • cancer cells — Cancer cells are malignant cells that divide uncontrollably and can form tumors and metastasize. They often rely on aerobic glycolysis, the Warburg effect, and many express telomerase, which helps them bypass the Hayflick limit.

  • metastasis — Metastasis is the spread of cancer cells from a primary tumor to distant sites, creating secondary tumors. It involves invasion, intravasation, and epithelial-mesenchymal transition, and it is responsible for most cancer deaths.

    • primary tumor — Metastasis is the spread of malignant cells from a primary tumor to distant sites, usually through the bloodstream or lymphatic system. It is a major cause of cancer deaths and is linked to epithelial-mesenchymal transition, invasion, and formation of circulating tumor cells.
• Signal transduction — the process by which extracellular signals are converted to intracellular responses; general steps: signal (ligand) → receptor activation → second messengers → kinase cascade → cellular response (gene expression, metabolism, movement). It includes pathways such as Notch, Wnt, and JAK-STAT, and often relies on second messengers and kinase cascades.

  • adenylate cyclase — Adenylate cyclase is the enzyme that converts ATP into cyclic AMP. It is activated by G proteins and receptors such as the beta-adrenergic receptor, making it central to cAMP signaling.

  • Hedgehog signaling pathway
    • hedgehog — The hedgehog signaling pathway is a developmental signaling system, with Hedgehog in flies and Sonic Hedgehog in mammals. It helps establish embryonic patterning and morphogen gradients. A hedgehog is also the spiny omnivorous mammal that inspired Sonic’s animal form and appears in philosophical titles like The Hedgehog and the Fox.
  • JAK-STAT pathway — The JAK-STAT pathway is a cytokine-signaling cascade in which activated receptors recruit Janus kinases and STAT transcription factors. It is inhibited by proteins such as SOCS and PIAS and is implicated in disorders like polycythemia vera.
• Second messengers — intracellular relay molecules: cAMP (adenylyl cyclase activated by GPCRs; activates PKA), cGMP, IP₃ (releases Ca²⁺ from ER), DAG, and Ca²⁺ itself; amplify and transduce extracellular signals.

  • cAMP (“C-A-M-P”) — cAMP, or cyclic adenosine monophosphate, is a second messenger made from ATP by adenylyl cyclase. It activates protein kinase A and helps relay hormonal signals such as epinephrine’s effects on glycogen breakdown.

  • cAMP (“camp”) — cAMP is a second messenger that activates protein kinase A and helps regulate the lac operon through CAP binding. It is produced from ATP by adenylyl cyclase.

  • Cyclic adenosine monophosphate — Cyclic adenosine monophosphate, or cAMP, is a ubiquitous intracellular second messenger formed from ATP by adenylyl cyclase (adenylate cyclase) and broken down by phosphodiesterases. It activates protein kinase A by binding its regulatory subunits, helps mediate hormone signaling and epinephrine responses, and is involved in bacterial catabolite repression, including regulation of the lac operon through CRP/CAP. It also functions in signaling systems such as Dictyostelium aggregation.

  • IP3 — IP3 is a second messenger produced when phospholipase C cleaves PIP2, alongside diacylglycerol. By binding its receptor on the endoplasmic reticulum, it triggers release of calcium ions into the cytoplasm.

  • Second messenger — A second messenger is an intracellular signaling molecule that relays and amplifies signals from cell-surface receptors. Common examples include cAMP and calcium ions.

• G-protein coupled receptors (GPCRs) — largest family of cell-surface receptors; seven transmembrane domains; binding activates a trimeric G protein (Gα-GDP → Gα-GTP), which activates effectors (adenylyl cyclase, phospholipase C); examples: β-adrenergic receptor, rhodopsin.

• Cancer biology

  • Carcinogenesis — Carcinogenesis is the process by which normal cells become cancerous and form tumors.

  • proto-oncogenes — Proto-oncogenes are normal genes that can promote cell growth but cause cancer when mutated or overactive. Examples in the clues include Ras and BRAF, which can become cancer-promoting through activating mutations.

    • c-Myc — c-Myc is a proto-oncogenic transcription factor first discovered in Burkitt lymphoma and known for binding Max. It activates genes involved in proliferation and is commonly described as a basic helix-loop-helix protein. It also appears in stem-cell reprogramming experiments by Shinya Yamanaka.

    • oncogene — An oncogene is a gene that promotes tumor formation when overexpressed or otherwise activated. The term originated in studies of tumor-causing viruses and now applies broadly to cancer genetics.

• Cell signaling
  • Autocrine signaling
    • autocrine — Autocrine signaling is a form of cell communication in which a cell releases a hormone or signal that acts back on itself. It is a standard biology term for self-targeted signaling, including prostaglandin examples.
  • G proteins — G proteins are guanine nucleotide-binding signal transducers and molecular switches in cell signaling. They are activated by GDP-to-GTP exchange in pathways downstream of cell-surface receptors, especially G protein-coupled receptors. Heterotrimeric G proteins consist of alpha and beta-gamma subunits, and on activation the subunits separate to relay signals to effectors such as adenylyl cyclase. Related examples include Ras and transducin. Cholera toxin can ADP-ribosylate the alpha subunit and lock it in an active state, and G protein malfunction is implicated in conditions such as cholera and diabetes.
    • transducin — Transducin is the heterotrimeric G protein activated by light-activated rhodopsin in retinal phototransduction. It helps trigger cGMP breakdown through phosphodiesterase, leading to hyperpolarization of the photoreceptor membrane.
  • hormones — Hormones are signaling chemicals secreted by glands or tissues that travel through the body to regulate target cells and functions. Examples include insulin, adrenaline, estrogen, testosterone, and plant hormones such as auxins and cytokinins.
    • Appetite hormones
      • ghrelin — Ghrelin is the appetite-stimulating hormone that rises before meals and opposes leptin. It is produced mainly in the stomach and activates pathways that increase feeding behavior.
    • Digestive hormones
      • cholecystokinin — A digestive hormone released by I cells in the small intestine and duodenum in response to food entering the small intestine. It stimulates gallbladder contraction, bile release, and pancreatic enzyme secretion, helping coordinate digestion, and is also associated with hunger suppression. It is often paired with secretin and the acinar cells of the pancreas.

    • erythropoietin — Erythropoietin is a kidney-produced glycoprotein hormone released in response to hypoxia that stimulates red blood cell production in the bone marrow. It is made mainly by peritubular fibroblasts in the renal cortex, signals through a JAK2-linked receptor, and synthetic forms such as darbepoetin are used medically to treat some anemias and are notorious for abuse as blood dopants in athletics.

    • Gastrointestinal hormones
      • gastrins — Gastrins are hormones produced in the stomach that stimulate gastric acid secretion. They also promote pancreatic secretion and gallbladder contraction.
    • Gonadotropins
      • Follicle-stimulating hormone — Follicle-stimulating hormone is an anterior pituitary gonadotropin released in response to GnRH and regulated by inhibin and activin. In females, it promotes ovarian follicle maturation and supports the dominant ovum, helping trigger ovulation-related ovarian growth. In males, it acts on Sertoli cells with testosterone to support spermatogenesis and androgen-binding protein production.
    • Growth hormones
      • Recombinant bovine growth hormone — Recombinant bovine growth hormone, or rBGH, is a genetically engineered hormone used to increase milk production in dairy cows. It was approved by the FDA in the 1990s and became controversial over its links to mastitis and dairy labeling disputes.
    • Insect molting hormones
      • ecdysone — Ecdysone is the insect and crustacean molting hormone. Its active form is 20-hydroxyecdysone, and it is produced by the prothoracic glands under stimulation from brain hormone.

    • leptin — Leptin is an adipose-derived hormone that signals satiety and suppresses appetite. It acts in the hypothalamus, opposes ghrelin, and was identified through experiments on ob and db mice.

    • Peptide hormones

      • Calcitonin — Calcitonin is a thyroid hormone produced by parafollicular C cells (thyroid C cells). It lowers blood calcium levels by inhibiting osteoclasts, and also lowers phosphate levels. It counteracts parathyroid hormone and is used in some treatments for osteoporosis and Paget’s disease.

      • parathyroid hormone — Parathyroid hormone is produced by the parathyroid glands and raises blood calcium levels. It acts antagonistically to calcitonin by stimulating bone resorption through osteoblast signaling, increasing renal phosphate excretion, promoting calcium absorption, and helping activate vitamin D.

    • Renin-angiotensin system
      • angiotensin II — Angiotensin II is a peptide hormone in the renin-angiotensin system, produced from angiotensin I by ACE. It raises blood pressure by causing vasoconstriction and stimulating aldosterone and vasopressin release.

    • secretin — Secretin is a duodenal hormone released by S cells when acidic chyme enters the small intestine. It stimulates the pancreas to secrete bicarbonate-rich fluid and helps regulate duodenal pH.

    • somatostatin — Somatostatin is an inhibitory hormone produced by pancreatic delta cells and by the hypothalamus. It suppresses the release of growth hormone, insulin, glucagon, and thyroid-stimulating hormone. It has medical analogues including lanreotide and octreotide, which are used in imaging and treatment.

  • Notch — Notch is a cell-signaling receptor family named for mutant wing indentations in Drosophila. Quizbowl clues often connect it to Jagged and Delta ligands, gamma-secretase cleavage, Alagille syndrome, CADASIL, and angiogenesis control.

  • Notch signaling pathway — A juxtacrine signaling cascade in which neighboring cells activate a transmembrane Notch receptor through Delta, Jagged, or Serrate ligands. The receptor is activated by proteolytic cleavage, then forms a complex with CBF1 and Mastermind to regulate transcription. It is central to developmental cell-fate decisions, including differentiation, proliferation, and organ formation, and is associated with angiogenesis. The pathway is classically linked to Drosophila wing notches and to diseases such as CADASIL and Alagille syndrome.

  • Wnt signaling
    • Wnt proteins — Wnt proteins are secreted signaling glycoproteins named for wingless and integrated. They bind Frizzled and the LRP5/6 coreceptor to activate the canonical pathway that stabilizes beta-catenin, with inhibitors including Dkk, SFRPs, and Cerberus.
• GTPases
  • Ras — Ras is a small GTPase and oncogene that switches between active and inactive states through GEFs and GAPs. Mutations in Ras family proteins drive many cancers, including pancreatic cancer, and are also implicated in disorders such as neurofibromatosis and related signaling defects.
• Signal transduction pathways
  • Wnt signalling pathway — The Wnt signalling pathway is a major developmental pathway named from the Drosophila wingless gene. It acts through Frizzled receptors, influences embryonic development and cancer, and canonically stabilizes beta-catenin.
• Signaling molecules
  • growth factors — Growth factors are signaling proteins that bind cell-surface receptors to promote proliferation, differentiation, and development. Examples include epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, VEGF, and insulin-like somatomedin C.
    • TGF-beta superfamily
      • Bone morphogenetic proteins — Bone morphogenetic proteins are TGF-beta-related growth factors and signaling molecules named for their ability to induce bone formation. They help induce bone and cartilage formation, play major roles in embryonic patterning and development, including neural crest induction, and are involved in cell differentiation. Their signaling works through type I and type II receptors and Smads 1, 5, and 8; inhibition of BMP signaling is important for neural plate development.
    • VEGF-A — VEGF-A is a growth factor that promotes angiogenesis, vasculogenesis, and vascular permeability. Its expression rises under hypoxic conditions and is relevant to treatments that block abnormal blood-vessel growth, such as therapy for macular degeneration.
• Signaling pathways

  • Hippo signaling pathway — The Hippo signaling pathway regulates organ size through a kinase cascade involving Salvador, Warts, YAP, and TAZ. It is named for Drosophila mutants with unusually large heads and is linked to growth control and cancer biology.

  • Wnt — Wnt is a developmental signaling pathway named in part for wingless in Drosophila. Wnt ligands bind Frizzled to stabilize beta-catenin in the canonical pathway. It helps control stem-cell maintenance and tissue polarity, is frequently implicated in cancer, and has been linked to endometriosis in some studies.

  • Wnt signaling pathway — The Wnt signaling pathway is a developmental pathway named for wingless and Wnt-related signals that act through Frizzled and LRP receptors. It regulates beta-catenin and LEF/TCF transcription, helps pattern embryos, and has noncanonical branches involved in planar cell polarity and convergent extension.

• TGF-beta signaling
  • Smads — Smads are intracellular transcription factors used in TGF-beta signaling. They are activated by TGF-beta family receptors and then move to the nucleus, often as complexes containing receptor-regulated, common-mediator, or inhibitory subunits.
• TGF-beta signaling proteins
  • Nodal — Nodal is a TGF-beta-family signaling protein involved in maintaining the primitive streak and establishing left-right asymmetry. It appears in developmental pathways alongside factors such as Lefty, Cerberus, and BMP proteins.
• Transforming growth factor beta superfamily
  • TGF-beta — TGF-beta is a signaling protein family that regulates cell growth and apoptosis through serine/threonine kinase receptors. It includes related groups such as BMPs, activins, inhibins, and GDFs.
• Cellular stress responses
  • unfolded protein response — The unfolded protein response is a cellular stress pathway activated by misfolded proteins in the endoplasmic reticulum. Its major sensors include IRE1, PERK, and ATF6, which trigger effects such as XBP1 splicing and translational shutdown.

• Sonic Hedgehog — Sonic Hedgehog is a developmental gene named for the video game character Sonic the Hedgehog. Mutations in it can cause holoprosencephaly, reflecting its role in establishing the body’s midline during embryonic development.

• Sonic hedgehog — Sonic hedgehog is a secreted signaling protein/morphogen that binds Patched to activate GLI (Gli) transcription factors. It patterns the vertebrate ventral neural tube, including floor plate specification, and contributes to limb bud and tooth development. The protein is named for the video game character Sonic, and its signaling is associated with holoprosencephaly, preaxial polydactyly, and medulloblastoma. In Drosophila, the homolog helps protect cubitus interruptus from cleavage.

• Wnt pathway proteins
  • beta-catenin — Beta-catenin is a dual-function protein central to cell adhesion and the canonical Wnt pathway. It is degraded by the APC-Axin-GSK3 destruction complex, binds TCF in the nucleus, and is also known from Drosophila as armadillo.

Related Concepts and Applications

• Biological organization
  • multicellularity — Multicellularity is the condition of being made of many specialized cells rather than a single cell. It is studied through examples such as Volvox, Dictyostelium, yeast experiments, and the evolution of differentiated tissues in animals and plants.

Biomolecules

• Biomolecules
  • proteoglycans — Proteoglycans are heavily glycosylated extracellular-matrix molecules made of a core protein with one or more attached glycosaminoglycans. They are important in cartilage, corneal transparency, and perineuronal nets, and include examples such as decorin, agrin, and aggrecan.
• Carbohydrates — composed of C, H, O in roughly 1:2:1 ratio; monosaccharides (glucose, fructose, galactose) → disaccharides (maltose, sucrose, lactose) → polysaccharides (starch, glycogen, cellulose, chitin). Glucose and mannose are examples, and these compounds often appear in glycosylation and in reducing-sugar tests such as Benedict’s reagent.

  • Carbohydrate — Carbohydrates are organic compounds made only of carbon, hydrogen, and oxygen, including sugars, starches, and cellulose. They are often built from monosaccharides and serve as a major cellular energy source.

  • Cellulose — Cellulose is a straight-chain β-1,4-linked polymer of glucose (specifically beta-glucose units) and the chief structural material of plant cell walls, where it forms microfibrils strengthened by pectin and lignin. It is used as the raw material for viscose rayon and is associated with rosette synthase complexes, filtration and blotting derivatives. Cellulose is undigested by humans, though many herbivores and insects digest it with microbial help, including ruminants and termites.

  • disaccharide — A disaccharide is a sugar made of two monosaccharides joined by a condensation reaction. Lactose, maltose, and sucrose are common examples.

  • Disaccharides

    • lactose — Lactose is the milk sugar that many adults digest poorly after weaning. It is the substrate of the lac operon in E. coli, and its breakdown is used in blue-white screening and related enzyme assays.

    • trehalose — Trehalose is a disaccharide of two glucose units with a 1,1 linkage. It is important in anhydrobiotic organisms such as tardigrades, where it helps form a protective glassy matrix during cryptobiosis.

  • glucose — Glucose is the six-carbon sugar that serves as a main fuel of the body and the product of photosynthesis. It is broken down in glycolysis, stored as glycogen, and linked with insulin, diabetes, and the Cori cycle.

  • Glycans — Glycans are carbohydrate chains found free or attached to proteins and lipids, including glycoproteins, glycolipids, and proteoglycans. They are central to glycomics and are often analyzed by mass spectrometry and lectin-based purification methods.

  • glycosaminoglycans — Glycosaminoglycans are long, negatively charged polysaccharides made of repeating disaccharides and found in proteoglycans. Examples include hyaluronan and heparin, and defects in their breakdown cause disorders such as Hurler syndrome and Sanfilippo syndrome.

    • Glycosaminoglycan — Glycosaminoglycans are long, often sulfated polysaccharides built from repeating disaccharide units. They are abundant in the extracellular matrix and include heparan sulfate and hyaluronan.

    • hyaluronic acid — Hyaluronic acid, also called hyaluronan or HA, is a nonsulfated glycosaminoglycan built from repeating glucuronic acid and N-acetylglucosamine units. It is a major component of synovial fluid and the extracellular matrix, and it is the substance broken down by sperm acrosomal enzymes during fertilization.

  • Glycosidic linkages — Glycosidic linkages are the covalent bonds that connect monosaccharides into polysaccharides. Alpha-amylase attacks the alpha-1,4 form of this linkage in starch digestion.

  • Monosaccharides

    • D-glucose — D-glucose, also called dextrose, is the six-carbon sugar that enters glycolysis and is the main blood sugar regulated by insulin and glucagon. It is the monomer of starch and glycogen, and high levels of it are reflected in hemoglobin A1c. Its ring form contains a hemiacetal at the anomeric carbon, and a reduced form was used in the Reichstein process to make ascorbic acid. Sodium-glucose cotransport in the kidney and intestine depends on it, which is why it matters in oral rehydration therapy.

    • Galactose — Galactose is a monosaccharide found in lactose, where it pairs with glucose, and it gives the blue-white screening reaction when cleaved from an indole chromophore. It also appears in blood-group antigens and in disorders such as galactosemia and Fabry disease.

    • mannose — Mannose is a monosaccharide closely related to glucose, and mannose-6-phosphate tags proteins for lysosomal transport. It is also important in glycoprotein processing, high-mannose glycans, and endoplasmic reticulum glycosylation pathways.

    • pentoses — Pentoses are five-carbon monosaccharides, including ribose. They are central to the pentose phosphate pathway, which produces NADPH and uses transketolase.

      • ribose and deoxyribose — Ribose and deoxyribose are the five-carbon sugars that form the sugar-phosphate backbone of nucleic acids. Ribose is the sugar in RNA, while deoxyribose is its slightly altered counterpart in DNA.
  • Oligosaccharides
    • verbascose, stachyose, and raffinose — These are three galacto-oligosaccharides commonly found in beans and other legumes. Humans and other monogastric animals cannot digest them well because they lack the enzymes needed to break them down.

  • pentose — A pentose is a five-carbon monosaccharide that may have either an aldehyde or ketone group. Pentoses are involved in the phosphate or pentose phosphate shunt that produces NADPH and related sugars.

  • Polysaccharides

    • glycogen — Glycogen is the highly branched glucose storage polysaccharide in animals, especially liver and muscle. The clues emphasize its synthesis from UDP-glucose, its alpha-1,4 and alpha-1,6 linkages, and its breakdown under the influence of glucagon and epinephrine.

    • Starch — This plant storage polysaccharide is detected by iodine and consists of amylose and amylopectin. It is broken down by amylase into maltose, forms oobleck when suspended in water, and serves as the main energy reserve in plants. It is the main storage carbohydrate in plants, built from long chains of glucose; many natural starches are mixtures of amylose and amylopectin, the two polymers commonly mentioned in its composition.

      • amylose — Amylose is the unbranched component of starch made of long chains of glucose linked by alpha-1,4 bonds. It gives the starch test its characteristic blue-black color with iodine. It contrasts with the branched starch polymer amylopectin and is a basic carbohydrate polymer in plants.

      • amylose and amylopectin — Amylose and amylopectin are the two main polysaccharides that make up starch. Amylose is mostly linear, while amylopectin is highly branched, and their structure determines starch behavior such as retrogradation.

    • starches — Starches are plant storage polysaccharides made of amylose and amylopectin. They are identified by iodine tests that produce a dark blue or black color.
• Lipids — hydrophobic; includes triglycerides (energy storage), phospholipids (membranes), steroids (cholesterol, hormones). Saturated fats have no double bonds; unsaturated have one or more. They also include fats, oils, and waxes; triglycerides are stored in adipose tissue. Phospholipids form the bilayer of cell membranes, and lipid rafts are specialized membrane microdomains made of these compounds. Related examples include sphingolipids, gangliosides, and signaling molecules such as prostaglandins, thromboxanes, and leukotrienes.
  • cannabinoids — Cannabinoids are a class of lipid-based compounds that includes the body’s own endocannabinoids, as well as plant-derived molecules such as THC and CBD. They act at CB1 and CB2 receptors, help regulate appetite, and include the 2018 FDA-approved epilepsy drug Epidiolex.
    • THC — THC is the main psychoactive compound in cannabis and marijuana’s main psychoactive ingredient. It was first isolated in 1964. THC acts on CB1 and CB2 receptors in the brain and inspired the medical synthetic drug Marinol, which it mimics; it is also associated with indica, sativa, and hybrid cannabis strains.

  • fat s — Fats are the common lipids made of glycerol esterified with long-chain carboxylic acids. They include saturated, unsaturated, and polyunsaturated varieties and provide about nine calories per gram.

  • fats — Fats are high-energy macronutrients stored in adipose tissue as triglycerides and commonly found in foods such as oils, butter, and lard. They can be saturated or unsaturated, dissolve fat-soluble vitamins, and are often discussed under the broader category of lipids.

  • fatty acids — Fatty acids are long-chain carboxylic acids that make up triglycerides and are broken down by beta-oxidation. Examples include palmitic and arachidonic acids, and they are also important in lipid modification reactions such as palmitoylation, as well as carnitine transport and other lipid metabolism processes.
    • Arachidonic acid — Arachidonic acid is a polyunsaturated fatty acid that serves as a key precursor to eicosanoids. It is released by phospholipase A2 and then converted into prostaglandins and thromboxanes, making it central to inflammatory signaling.
  • Glycosphingolipids
    • gangliosides — Gangliosides are sialic-acid-containing glycosphingolipids found in cell membranes. They accumulate in Tay-Sachs disease when hexosaminidase is defective, and GM1 gangliosides also serve as binding targets for cholera toxin.
  • Isoprenoids
    • dolichol — Dolichol is a long-chain polyisoprenoid lipid in the endoplasmic reticulum membrane that serves as the carrier for precursor oligosaccharides in N-linked glycosylation. Its phosphorylated form, dolichol phosphate, donates the sugar chain to asparagine residues, and tunicamycin blocks this process to trigger the unfolded protein response.
  • Phospholipids
    • phospholipid — Phospholipids are amphipathic membrane lipids with hydrophilic heads and hydrophobic tails. They form bilayers and micelles, and common examples such as lecithin help make up cell membranes and act as emulsifiers.
  • sphingolipids — Sphingolipids are membrane lipids built on a sphingosine backbone rather than glycerol. Their many forms include cerebrosides and gangliosides, and their carbohydrate groups can determine blood types.

    • sphingomyelin — Sphingomyelin is a membrane sphingolipid made of ceramide and phosphocholine. It is abundant in myelin and is measured in fetal lung maturity tests through its lecithin ratio in amniotic fluid.

    • sphingosine — Sphingosine is an amino alcohol used as the backbone for several sphingolipids. It has a long hydrocarbon chain and is important in neural membrane lipids such as ceramides.

  • Steroids
    • sterol — A sterol is a steroid alcohol with a hydroxyl group on the A-ring and a largely nonpolar side chain. Cholesterol is the best-known sterol and a major component of animal cell membranes.

  • thromboxanes — Thromboxanes are fatty-acid derivatives that promote platelet aggregation and vasoconstriction. Their A2 form is released by activated platelets, and aspirin suppresses their synthesis by inhibiting cyclooxygenase.

  • Triglycerides — Triglycerides are lipids made of glycerol ester-linked to three fatty acids. They are the main constituents of dietary fats and oils, make up most stored body fat, and serve as the main form of long-term energy storage in animals. They are often referred to as oils or fats depending on how saturated the fatty acids are. Elevated levels can contribute to metabolic syndrome and cardiovascular risk.

  • waxes — Waxes are high-melting lipids made from long-chain fatty acids ester-linked to long-chain alcohols. In plants, they form the protective cuticle that helps prevent water loss, and in animals they help protect hair and skin.
• Proteins — polymers of amino acids linked by peptide bonds; four structural levels: primary (sequence), secondary (alpha-helix, beta-sheet via H-bonds), tertiary (3D folding), quaternary (multiple subunits). Functions: enzymes, structural support, signaling, transport, immunity. Misfolded proteins can form prions, including the agents associated with Creutzfeldt-Jakob disease and mad cow disease. Common laboratory and example associations include Western blots, Edman degradation, and sickle-cell hemoglobin misfolding.

  • albumin — Albumin is the main protein in blood plasma and egg white, where it helps transport hydrophobic molecules. Bovine serum albumin is a common lab blocking agent, and albumin levels are used in tests involving calcium, phenytoin, ascites, and diabetic kidney damage.

  • alpha-synuclein — Alpha-synuclein is an intrinsically disordered protein encoded by the SNCA gene and a major component of Lewy bodies. It is associated with Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy.

  • Calcium-binding proteins
    • calmodulin — Calmodulin is a calcium-binding signaling protein with four EF-hand motifs that regulates many enzymes and muscle-related processes. It appears in clues about calcineurin, myosin light-chain kinase, CaM-dependent kinases, and calcium-dependent activation of TRPV1 and other targets.
  • Carbohydrate-binding proteins
    • Lectins — Lectins are carbohydrate-binding proteins found on cell surfaces and in immune pathways that recognize specific sugars. They are important in cell adhesion, innate immunity, and complement activation. In the complement system, mannose-binding lectin helps initiate one of the activating pathways by binding sugars such as mannose and glucose. Examples include concanavalin A and mannose-binding lectin, and some viral and plant proteins belong to this class.

  • catabolite activator protein — Catabolite activator protein, or CAP, is a cAMP-binding transcription factor that helps activate the lac operon when glucose is scarce. It promotes lacZ expression and is also known as the cyclic AMP receptor protein.

  • Conjugated proteins — Conjugated proteins are proteins bonded to a nonprotein prosthetic group such as a sugar, lipid, or nucleic acid. They are distinguished from simple proteins by this attached component.

  • crystallins — Crystallins are structural proteins found almost exclusively in the lens of the eye. Their stability helps maintain the lens’s refractive properties, and damage or misfolding of them is associated with cataracts.

  • Disulfide bonds
    • disulfide bond — A disulfide bond is a covalent linkage formed between the sulfur atoms of two cysteine residues. These bonds help stabilize proteins such as ribonuclease A and antibodies, and they can be broken by reducing agents like beta-mercaptoethanol.

  • dystrophin — Dystrophin is a muscle protein that links actin to the membrane-associated cytoskeleton through the dystrophin-glycoprotein complex. Mutations in its gene cause Duchenne and Becker muscular dystrophy.

  • ferritin — Ferritin is the iron-storage protein found inside cells, where it forms a cage-like shell around its mineral core. It is closely tied to iron regulation, and related proteins such as bacterioferritin and apoferritin often appear in clues about iron homeostasis.

  • flagellin — Flagellin is the protein that makes up the bacterial flagellum. It is recognized by immune receptors such as TLR5 and is often used as an adjuvant or immunological stimulus in research.

  • fluorescent proteins — Fluorescent proteins are beta-barrel proteins used as cellular reporters and localization markers. Green fluorescent protein is the classic example, and many natural versions come from jellyfish or other cnidarians.

    • GFP or Green fluorescent protein — Green fluorescent protein is a jellyfish-derived reporter protein that fluoresces green and is widely used as a molecular tag. Its chromophore forms from a serine-tyrosine-glycine sequence inside a beta-barrel, and engineered variants helped expand its use in imaging.

    • Green fluorescent protein — A jellyfish-derived fluorescent reporter protein from Aequorea victoria that glows green under blue or ultraviolet light and is widely used in molecular biology and cell biology. It is commonly fused to other proteins to tag and track them in live cells, including in transformed E. coli, microscopy, gene-expression studies, vesicular trafficking, FRAP, FRET, and Brainbow. It has a beta-barrel structure; mutant forms such as pHluorin and other color variants are used for imaging. Discovery and development of GFP were central to the 2008 Nobel Prize shared by Shimomura, Chalfie, and Tsien.

  • gelatin — Gelatin is the collagen-derived protein used to make Jell-O, marshmallows, and many aspic dishes. In biology it can also serve as a medium in classic plant-growth experiments, where a barrier is placed between a tip and a responding tissue.

  • glycoproteins — Glycoproteins are proteins with carbohydrate chains attached after translation, commonly through N-linked or O-linked modification. Examples include mucins, SHBG, and other lectin-associated proteins.
    • mucins — Mucins are heavily glycosylated proteins found in mucus and in signet-ring cell vacuoles. Several mucins, including MUC5AC and related transmembrane forms, are important cancer markers and drug targets.
  • Hemoglobin — Iron-containing oxygen-transport protein in red blood cells, built from four subunits and known for cooperative oxygen binding, its characteristic dissociation curve, and the Bohr effect. It contains porphyrin heme groups, binds carbon monoxide with high affinity, and is associated with sickle-cell anemia when a glutamate-to-valine mutation occurs; fetal variants and BOLD fMRI oxygenation changes are also commonly noted.

    • 2,3-BPG — 2,3-BPG is an erythrocyte metabolite made in the Luebering–Rapoport pathway that allosterically lowers hemoglobin’s oxygen affinity. By stabilizing the tense state of hemoglobin, it helps unload oxygen to tissues, especially in hypoxic conditions.

    • Bohr effect — The Bohr effect is the drop in hemoglobin’s oxygen affinity in acidic, carbon-dioxide-rich conditions. It helps oxygen unload in metabolically active tissues and is sometimes called the Bohr shift.

    • fetal hemoglobin — Fetal hemoglobin, or hemoglobin F, is the oxygen-binding hemoglobin isoform with gamma chains instead of beta chains. It is central to clues about sickle-cell treatment, hereditary persistence of fetal hemoglobin, and the Kleihauer-Betke test.

  • Intermediate filament proteins
    • cytokeratins — Cytokeratins are intermediate filament proteins found in epithelial structures such as hair and nails. They are type I and type II heterodimers and are strengthened by cysteine-rich disulfide bridges.
  • Intrinsically disordered proteins
    • intrinsically disordered — Intrinsically disordered proteins or regions lack a fixed three-dimensional structure and therefore resist ordinary fold prediction. They are often discussed in connection with NMR studies and proteins such as alpha-synuclein and p53.
  • molecular chaperones — Molecular chaperones are proteins that assist other proteins in folding correctly and avoiding misfolding by shielding exposed hydrophobic regions; they can also prevent denaturation. Heat shock proteins are a major class of chaperones. Examples include Hsp70, BiP, GRP94, GroEL, GroES, and the ATP-dependent TCP1 complex. They are sometimes divided into holdases, foldases, and disaggregases.
    • chaperonins — Chaperonins are molecular chaperones that help other proteins fold into their correct three-dimensional shapes. Examples include GroEL and GroES, which are heat-shock proteins involved in protein folding.
      • GroEL and GroES — GroEL and GroES are paired chaperone proteins that form a barrel-shaped folding chamber for other proteins. Their eukaryotic homologs are Hsp60 and Hsp10, and the complex is sometimes described as an Anfinsen cage.
    • heat shock proteins — Heat shock proteins are chaperones that help other proteins fold correctly under stress, especially heat. They are often named by molecular weight, such as Hsp70 and Hsp60, and include chaperonin complexes like GroEL/GroES. Their expression rises under stressful conditions and can be controlled by RNA thermometers and sigma-factor pathways in bacteria.
      • HSP70 proteins
        • BiP — BiP, also called binding immunoglobulin protein or GRP78, is an ER chaperone in the HSP70 family. It helps proteins fold properly in the endoplasmic reticulum and is highly expressed in plasma cells.
      • HSP90 proteins
        • Hsp90 — Hsp90 is an abundant molecular chaperone that helps stabilize partly folded client proteins. It has been targeted by geldanamycin, and cancer-related studies have linked its inhibition to degradation of proteins such as p53.

  • myosins — Myosins are ATP-driven motor proteins that move along actin and make up the thick filaments of sarcomeres. Different classes perform specialized roles, including muscle contraction, vesicle transport, and sensory-hair-cell function.

  • Peptides — Peptides are short chains of amino acids joined by peptide bonds and are often contrasted with longer proteins. The term can cover oligopeptides and polypeptides, and peptide families include tachykinins, vasoactive intestinal peptides, and pancreatic peptides.

    • bradykinin — Bradykinin is a short peptide vasodilator produced from kininogen by kallikrein and degraded by ACE. It increases capillary permeability, is implicated in the dry cough caused by ACE inhibitors, and is associated with several inflammatory signaling pathways.

    • polypeptides — Polypeptides are chains of amino acids joined by peptide bonds, and proteins are a broader related term often used for the same class in this context. Their synthesis can be done by solid-phase methods, and natural cyclic examples include nonribosomal peptides such as ciclosporin.

  • Protease inhibitors
    • alpha-1-antitrypsin — Alpha-1 antitrypsin is a liver-produced protease inhibitor that protects the lungs by blocking neutrophil elastase. Mutations in SERPINA1 can cause panacinar emphysema, cirrhosis, and PAS-positive granules in the liver.

  • protein — Proteins are polymers of amino acids that fold into specific three-dimensional structures and can be detected by techniques such as Western blotting. They include the macromolecules purified in affinity chromatography and the molecules analyzed by Ramachandran plots and Anfinsen’s folding experiments.

  • Protein structure
    • globular proteins — Globular proteins are compact proteins whose hydrophobic residues tend to face inward while hydrophilic residues face outward. They are contrasted with fibrous proteins and can pass through a molten globule folding intermediate.
      • Albumins — Albumins are water-soluble proteins that make up about half of the protein in human serum. They help maintain osmotic balance between blood and tissues and act as transport proteins for less soluble substances.
    • tertiary structure — Tertiary structure is the overall three-dimensional folding of a single protein chain, higher than secondary structure such as alpha-helices and beta sheets. Disulfide bridges and other interactions help stabilize this level of protein structure.
  • Sequencing
    • protein sequencing — Protein sequencing is the process of determining the amino-acid order of a protein or polypeptide. Classical methods include Edman degradation, while modern approaches often use tandem mass spectrometry.
      • Edman degradation — Edman degradation is a protein-sequencing method that removes amino acids one at a time from the N terminus. The procedure yields phenylthiohydantoin derivatives and is named for the Swedish chemist Pehr Edman.
  • Simple proteins
    • Simple protein — Simple proteins are proteins that hydrolyze to give only amino acids. Examples include insulin, ribonuclease, oxytocin, and bradykinin.

  • thermogenin — Thermogenin, or uncoupling protein 1, is a brown-fat mitochondrial protein that dissipates the proton gradient to generate heat. It uncouples oxidation from ATP synthesis, is activated by norepinephrine, and is central to nonshivering thermogenesis.

  • Transport proteins
    • Aquaporin — A transmembrane water channel, a membrane protein that allows water to cross cell membranes efficiently and in and out of cells. Aquaporins are important in the kidney collecting duct under ADH control, and some family members also transport glycerol. Peter Agre was recognized for discovering this long-hypothesized mechanism for regulating osmotic pressure.
• Nucleic acids — DNA (deoxyribose, double-stranded, A-T and G-C base pairs; stores heritable information) and RNA (ribose, single-stranded, A-U and G-C). Nucleic acids are polymers built from nucleotides linked by phosphodiester bonds, including DNA and RNA. They form the genetic material of cells and can be synthesized or sequenced using methods such as phosphoramidite chemistry and Sanger sequencing.

  • Adenine and Thymine — Adenine and thymine are the DNA bases that pair with two hydrogen bonds. In RNA, adenine pairs with uracil, while thymine is one of the four standard DNA nucleobases.

  • adenine, cytosine, guanine, and thymine — The four nitrogenous bases of DNA are adenine, cytosine, guanine, and thymine.

    • adenine, cytosine, guanine, thymine — The four standard DNA bases are adenine, cytosine, guanine, and thymine. These nitrogenous bases pair along the strands of the DNA double helix to encode genetic information.

    • Adenine, Guanine, Cytosine, and Thymine — The four bases found in DNA are adenine, guanine, cytosine, and thymine. These nitrogenous bases make up the standard DNA alphabet.

    • Adenine, Guanine, Cytosine, Thymine — Adenine, guanine, cytosine, and thymine are the four nitrogenous bases found in DNA. They are the standard nucleobases of the DNA double helix.

  • Antisense oligonucleotides
    • morpholinos — Morpholinos are synthetic antisense oligomers used to block RNA binding sites and knock down gene function. They are useful for altering pre-mRNA splicing or inhibiting ribozyme activity, and their backbone uses phosphorodiamidate linkages rather than natural nucleic acid chemistry.

  • aptamers — Aptamers are synthetic oligonucleotides or peptides selected to bind specific target molecules. They are commonly generated by SELEX, and their structures can use motifs such as G-quadruplexes or Spiegel variants made from L-ribose.

  • DNA — DNA is the double-helical nucleic acid that stores hereditary information in living organisms and genes. It is made of two nucleotide strands with adenine, thymine, guanine, and cytosine paired by hydrogen bonds; replication is semiconservative. DNA can be amplified by PCR, detected with stains such as DAPI or Hoechst, and packaged around histones as chromatin, though bacteria may lack histones. Its chemistry includes a phosphodiester backbone, supercoiling, and conformations such as A, B, and Z forms, and it can form i-motifs under acidic conditions. Classic associations include bacterial transformation, helicase, Watson and Crick, and Rosalind Franklin’s X-ray data; in bacteria replication can be organized around oriC.

    • DNA binding — DNA binding is the activity by which proteins recognize and attach to DNA, often through the major or minor groove. Common DNA-binding motifs include zinc fingers, helix-turn-helix domains, leucine zippers, and TALE repeats.

    • DNA components
      • deoxyribose — Deoxyribose is the five-carbon sugar in DNA’s sugar-phosphate backbone. It pairs with phosphate to form the structural framework that links DNA bases together.

    • DNA intercalation — DNA intercalation is the insertion of a molecule between adjacent base pairs in double-stranded DNA. Intercalating agents such as ethidium bromide, doxorubicin, and daunomycin can disrupt DNA unwinding and are strongly mutagenic.

    • DNA sequences — DNA sequences are strings of nucleotide letters such as A, C, G, and T that record the order of bases in genetic material. They are stored in formats like FASTA and analyzed with tools such as BLAST and WebLogo.
      • contigs — Contigs are contiguous DNA sequences assembled from overlapping reads in shotgun sequencing. They can be combined into scaffolds, but unlike scaffolds they contain no gaps.
        • contig — A contig is a contiguous sequence assembled from overlapping DNA fragments. The term comes up in shotgun sequencing and in building larger scaffolds from shorter fragments.

    • heteroduplex DNA — Heteroduplex DNA forms when strands from different sources anneal into a mismatched double helix. It can arise in recombination or TILLING and may create local mosaicism when the mismatch is present.

    • junk DNA — Junk DNA is a term for nonfunctional or apparently noncoding DNA that may make up much of a genome. It is often discussed in connection with the C-value paradox and with debates over how much of the genome has a biological role.

    • sticky ends — Sticky ends, also called cohesive ends, are the single-stranded DNA overhangs left when restriction enzymes cut DNA unevenly. Because complementary overhangs can base-pair, ligase can join them after enzymes such as EcoRI or HindIII make the cut.

    • Strands
      • DNA strands — DNA strands are the single polymer chains that base-pair to form the DNA double helix. They can be separated by helicases, measured in microarrays, and used in strand-displacement systems and DNA origami.
  • nitrogenous bases — Nitrogenous bases are the ringed components of nucleotides found in DNA and RNA. Examples include adenine, thymine, cytosine, guanine, and uracil.
    • Purines or pyrimidine — Purines and pyrimidines are the two broad classes used to group nitrogenous bases. The distinction is based on whether the base has a single ring or a double ring structure.

  • nucleic acid — Nucleic acids are polymeric molecules made of nucleotide chains, including DNA and RNA. Unlike viruses, prions lack nucleic acid entirely.

  • nucleotides — Nucleotides are the monomers or building blocks of nucleic acids, made from a sugar, a phosphate group, and a nitrogenous base. DNA and RNA are composed of nucleotides linked into long chains. Their bases include adenine, guanine, cytosine, thymine, and uracil, and they are grouped into purines and pyrimidines.

    • adenosine monophosphate — Adenosine monophosphate, or AMP, is a nucleotide formed from ATP hydrolysis and a key indicator of low cellular energy. It allosterically activates phosphofructokinase-1, and its cyclic form, cAMP, functions as an important second messenger.

    • ATP or adenosine triphosphate — ATP, or adenosine triphosphate, is the cell’s main energy currency and a major product of cellular respiration. Its hydrolysis powers many biological processes, and its synthesis is central to oxidative phosphorylation and ATP synthase.

    • Cyclic nucleotides
      • cyclic guanosine monophosphate — Cyclic guanosine monophosphate is a cyclic nucleotide second messenger made by guanylyl cyclase in signaling pathways such as nitric oxide and natriuretic peptide signaling. It activates protein kinase G, mediates smooth muscle relaxation and erection, and is broken down by phosphodiesterase-5, making it relevant to Viagra and to phototransduction in the retina.

    • GTP — Guanosine triphosphate is a guanine nucleotide triphosphate used in RNA synthesis, translation, signal transduction, microtubule dynamics, nuclear transport, and vesicle trafficking. It powers G proteins and small GTPases such as Ras, Rho, and Ran, and its hydrolysis drives many exchange and hydrolysis systems in bacteria and eukaryotes. It can be converted to cyclic GMP by guanylate cyclase and is also produced in substrate-level phosphorylation when succinyl CoA forms GTP in the Krebs cycle.

    • Nucleosides
      • Nucleoside — A nucleoside is the base-sugar portion of a nucleotide after the phosphate group has been removed. It consists of a nitrogenous base attached to a sugar.
  • RNA — RNA, or ribonucleic acid, is a single-stranded nucleic acid transcribed from DNA that uses uracil. It appears in messenger, transfer, ribosomal, and many regulatory forms, and has central roles in translation, splicing, gene regulation, and RNA interference. Messenger RNA is processed with a 5’ cap and poly-A tail before protein synthesis; transfer RNA carries amino acids during translation; ribosomal RNA is a core component of ribosomes. Small RNAs can be bound by Argonaute proteins in RISC, and RNA also figures in ribozymes and spliceosomes.
    • microRNAs — MicroRNAs are short noncoding RNAs that regulate gene expression by binding target mRNAs. They are produced from hairpin precursors processed by Drosha and Dicer and include examples such as let-7 and lin-4.
      • microRNA or miRNA — MicroRNA, or miRNA, is a short noncoding RNA that can silence gene expression after transcription. It is used by RNA interference and has been studied for roles in development and cancer.

    • mRNA — mRNA is the single-stranded RNA transcribed from DNA that carries protein-coding information to ribosomes for translation. It is read in codons as the template for amino acid sequencing during protein synthesis. In eukaryotes, precursor mRNA is processed by splicing, capped at the 5’ end, polyadenylated with a poly(A) tail, and exported from the nucleus to the cytoplasm. mRNA can be targeted by RNA interference, and in bacteria and archaea it may be polycistronic; it is also studied in transcriptomics.

    • ribosomal RNA — Ribosomal RNA is the RNA that, with proteins, forms the ribosome and carries out translation. It is a single-stranded RNA that folds into helical regions by internal base pairing. Bacterial rRNA includes the 16S and 23S molecules; its conserved sequences, especially 16S rRNA, are widely used in microbial classification, taxonomy, phylogeny, and metabarcoding, including Carl Woese’s work on the three domains of life and the Archaea.
      • 16S rRNA — 16S rRNA is the conserved ribosomal RNA gene used to study bacterial and archaeal phylogeny. It is part of the small ribosomal subunit and contains the anti-Shine-Dalgarno sequence near its 3’ end.

    • RNAs — RNAs are single-stranded ribonucleic acids that use uracil and help carry genetic information and perform translation-related roles. They include mRNA, tRNA, rRNA, ribozymes, and regulatory forms such as Xist and siRNA, with catalytic examples supporting the RNA world hypothesis.

    • snRNA — snRNA stands for small nuclear RNA, a class of RNA found in spliceosomes. These molecules help recognize introns during pre-mRNA splicing.

    • transfer RNA — Transfer RNA is the adaptor RNA used in translation to deliver amino acids to the ribosome. Its anticodon loop base-pairs with mRNA codons, and aminoacyl-tRNA synthetases charge it with an amino acid before protein synthesis. It typically has a cloverleaf secondary structure that folds into an L-shaped tertiary structure, with an acceptor stem and 3′ CCA end plus the D (DHU) loop, TψC loop, variable loop/arm, and anticodon loop. Modified bases such as inosine and pseudouridine are common.
      • anticodon — An anticodon is the three-base sequence on tRNA that pairs with a complementary mRNA codon. It lies in the middle arm of the tRNA cloverleaf structure and is read in reverse complement to the codon.

• Guanine — a purine nitrogenous base found in both DNA and RNA; pairs with cytosine via three hydrogen bonds (G≡C); one of the four DNA bases along with adenine, thymine, and cytosine; its nucleoside is guanosine; part of GTP (guanosine triphosphate), a key energy carrier in the citric acid cycle. It also appears in the 5-prime cap structure of messenger RNA and is central to many signal-transduction proteins.

• Thymine — a pyrimidine nitrogenous base exclusive to DNA (replaced by uracil in RNA); pairs with adenine via two hydrogen bonds (A=T); contains a methyl group at the 5-position that distinguishes it from uracil. UV light commonly causes thymine dimers in DNA, and thymine is part of the activated form of the anti-HSV drug acyclovir.

• Uracil — a pyrimidine nitrogenous base found in RNA but not DNA; pairs with adenine; lacks the methyl group present on thymine; incorporated in place of thymine during transcription. It also appears in classic genetics experiments with poly-U RNA and in biochemical derivatives such as 5-fluorouracil and uridine nucleotides.

• Pyrophosphate — the inorganic diphosphate ion (PPi); released when nucleoside triphosphates (ATP, GTP, etc.) are used in polymerization reactions (e.g., DNA/RNA synthesis); its subsequent hydrolysis by pyrophosphatase to two phosphate ions drives many biosynthetic reactions forward by making them thermodynamically irreversible. It is also a product of ATP hydrolysis to AMP.

• ATP (adenosine triphosphate) — the universal cellular energy currency; hydrolysis of the terminal phosphate bond releases ~7.3 kcal/mol (standard conditions). It consists of adenine, ribose, and three phosphate groups. ATP synthase makes it using a proton gradient via chemiosmosis.

  • adenosine triphosphate or ATP — Adenosine triphosphate, or ATP, is the cell’s main energy currency and is produced in respiration, especially by ATP synthase in the mitochondrion. It is hydrolyzed to power cellular work and also serves in signaling through cAMP-related pathways.

  • ATP hydrolysis — ATP hydrolysis is the cleavage of ATP to ADP and phosphate, releasing energy for cellular work. It powers processes such as myosin movement, the sodium-potassium pump, and protein-folding machines like GroEL and GroES.

• Enzymes — biological catalysts (mostly proteins, some RNA = ribozymes); lower activation energy; substrate binds at the active site; inhibited competitively (at active site) or non-competitively (allosteric site). They are not consumed in the reactions they catalyze and are often described by Michaelis-Menten kinetics. Their specificity is determined by the active site, and they may be classified by reaction type as oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Activity can also be regulated by allosteric effects and covalent modification, and examples include restriction enzymes, ligases, and polymerases.

  • active site — The active site is the region of an enzyme where a substrate binds and the reaction occurs. It is the part described by the lock-and-key model and is often discussed in terms of Km, Vmax, and competitive inhibition.

  • adenylyl cyclase — Adenylyl cyclase is the enzyme that converts ATP into cyclic AMP in signal transduction. It is activated by forskolin and Gs alpha subunits and is a key target in pathways involving cAMP production.

  • aldolase — Aldolase is a glycolytic enzyme that cleaves fructose-1,6-bisphosphate into two three-carbon products, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Its two classes differ in mechanism, with class I forming a Schiff base and class II lacking that feature.

  • allosteric enzymes — Allosteric enzymes are enzymes whose activity is altered when effectors bind to regulatory sites separate from the active site. This binding changes the enzyme’s shape, allowing allosteric control of catalysis.
    • allosteric enzyme — An allosteric enzyme is regulated by effectors that bind at a site other than the active site. These enzymes often produce sigmoidal Michaelis-Menten curves.
  • amylases — Amylases are enzymes that break starch into smaller sugars by hydrolyzing alpha-1,4-glycosidic bonds. They include salivary ptyalin and related alpha and beta forms. They are produced in the digestive system, including by the pancreas, and are important in the breakdown of carbohydrates.
    • alpha-amylase — Alpha-amylase is the enzyme that hydrolyzes starch, and its salivary form begins carbohydrate digestion in the mouth. It is also released in plants after gibberellin signaling and was the first enzyme to be characterized.
  • Antibiotic resistance enzymes
    • beta-lactamase — Beta-lactamases are bacterial enzymes that destroy beta-lactam antibiotics by hydrolyzing the four-membered ring. They include enzymes such as NDM-1 and TEM and can be inhibited by drugs like clavulanic acid or sulbactam.
  • ATPases — ATPases are enzymes that hydrolyze ATP, often using that energy to drive transport or mechanical work. Examples include rotary ATPases and P-type pumps such as the sodium-potassium pump.
    • ATPase — ATPase is the enzyme class that hydrolyzes ATP to power many cellular processes. Examples include the sodium-potassium pump and enzymes in the endoplasmic reticulum that consume ATP during transport or folding work.
  • Carbohydrases
    • β-galactosidase — β-galactosidase is the lacZ enzyme produced from the lac operon that cleaves lactose into glucose and galactose. It was the classic enzyme used in studies of operon regulation and is commonly used as a reporter in blue-white screening and as a standard senescence marker in cell biology.
  • Coenzymes — Coenzymes are small organic molecules that assist enzymes, often by carrying atoms or functional groups during catalysis. Examples include NAD and FAD, as well as biotin, thiamine pyrophosphate, lipoamide, and coenzyme A, and some forms such as ubiquinone function in electron transport.

    • NAD+ — NAD+ is the oxidized form of nicotinamide adenine dinucleotide, a dinucleotide coenzyme and central electron carrier in metabolism. Its reduced form, NADH, carries electrons in redox reactions; NAD+ is reduced in glycolysis, the citric acid (Krebs) cycle, and beta oxidation, and it is regenerated by oxidative phosphorylation. It also serves as a cofactor for sirtuins, and defects in its salvage pathway can contribute to pellagra.

    • NADP+ — NADP+ is the oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier in photosynthesis. It accepts high-energy electrons in the light reactions to become NADPH, then is reduced again in the Calvin cycle as it helps carry reducing power needed to build sugars.

  • Cofactors — Cofactors are the nonprotein helpers some enzymes need in order to become active. They can be metal ions or vitamin-derived molecules such as FAD or coenzyme A.
    • Flavin cofactors
      • FAD — FAD, or flavin adenine dinucleotide, is a riboflavin-derived redox cofactor that carries electrons in metabolism. It appears in succinate dehydrogenase and other reactions of fatty acid and amino acid breakdown, where its reduced form FADH2 feeds electrons directly into the electron transport chain.
    • prosthetic group — This is the non-protein component of a conjugated protein or enzyme. Examples of such groups include cofactors that are tightly bound to the protein.
  • Digestive enzymes
    • salivary amylase — Salivary amylase, also called ptyalin, is an enzyme in saliva that begins digestion by breaking starch and glycogen into smaller sugars. It works best near neutral pH and is inactivated by the stomach’s acidic environment.

  • DNA gyrase — DNA gyrase is a bacterial type II topoisomerase that introduces negative supercoils using ATP. It relieves torsional stress during unwinding and is targeted by quinolones and coumarins.

  • DNA ligase — DNA ligase is the enzyme that seals breaks in DNA by forming phosphodiester bonds between a 5-prime phosphate and a 3-prime hydroxyl. It joins Okazaki fragments during replication and is used in cloning, repair, and lab ligation reactions, often with ATP or NAD as a cofactor depending on the source. It is essential in sealing nicks during DNA replication and is widely used in laboratory cloning and sequencing workflows.

  • DNA polymerases — DNA polymerases are the enzymes that synthesize DNA by adding nucleotides to a growing strand. They are central to replication and PCR, and many forms have proofreading exonuclease activity.

    • DNA polymerase I — DNA polymerase is the enzyme that extends a DNA strand by adding nucleotides during replication and during PCR. Thermus aquaticus provides the heat-stable Taq form, and Arthur Kornberg is associated with its discovery.

    • Klenow fragment — The Klenow fragment is the large proteolytic fragment of E. coli DNA polymerase I produced by subtilisin cleavage. It retains the polymerase activity and the 3’ to 5’ exonuclease activity, but lacks the 5’ to 3’ exonuclease domain. The fragment corresponds roughly to residues 324 to 928 of the enzyme.

    • Taq polymerase — Taq polymerase is the heat-stable DNA polymerase from Thermus aquaticus that made PCR practical. Because it survives repeated heating, it can be reused through the amplification cycle without being replaced each round.

      • Taq — Taq polymerase comes from the heat-loving bacterium Thermus aquaticus. The enzyme’s heat resistance made automated PCR possible.

  • enzyme — Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering activation energy. Standard clues mention the active site, induced fit, and Michaelis-Menten kinetics, along with inhibition and cofactors.

  • enzyme inhibition — Enzyme inhibition is the process in which molecules bind to enzymes and reduce their activity. It includes competitive, noncompetitive, allosteric, and suicide forms of inhibition.

  • enzyme inhibitors — Enzyme inhibitors are molecules that reduce enzymatic activity by blocking substrate binding or altering enzyme conformation. They may act competitively, noncompetitively, allosterically, mixedly, or covalently.
    • competitive inhibitors — Competitive inhibitors are enzyme inhibitors that bind the active site and block the substrate from binding. They are a standard drug-screen target, and their binding is often described with the lock-and-key model.

  • glucose-6-phosphate dehydrogenase — Glucose-6-phosphate dehydrogenase catalyzes the first, rate-limiting step of the pentose phosphate pathway and helps generate NADPH. Deficiency of this enzyme causes G6PD deficiency, a common hereditary disorder that can produce bite cells and hemolysis after oxidative stress such as fava beans or primaquine.

  • Glycosidases
    • Galactosidase — Galactosidase is an enzyme class involved in breaking down galactosides. Its synthesis is classically discussed through the first operon model used to explain gene regulation.
  • Helicases — Helicases are enzymes that unwind double-stranded DNA or RNA during replication and related nucleic-acid processes. Common examples include DnaB, RecQ-family helicases, DEAD-box proteins, viral helicases such as the hepatitis C enzyme NS3, and the Mcm2-7 family, which is important in DNA replication and repair.

    • DNA helicase — DNA helicase is the enzyme that uses ATP to unwind the double helix and separate DNA strands during replication and transcription. It helps form the replication fork, and defective helicases such as RecQ-family members are linked to Werner syndrome and other premature-aging disorders.

    • DNA helicases — DNA helicases are the protein complexes that unwind DNA during replication, including bacterial DnaB, the RecQ family, and the MCM complex. They use ATP to open the double helix and are countered by topoisomerases, which relieve the torsional strain they create.

    • helicase — A helicase is an enzyme that unwinds nucleic acids by breaking hydrogen bonds during replication or transcription. Clues often mention DNA helicases such as DnaB or RecQ proteins, as well as RNA helicases like DEAD-box proteins.

  • hexokinase — Catalyzes the first step of glycolysis by phosphorylating glucose to glucose-6-phosphate using ATP. In pancreatic cells, the major glucose-phosphorylating enzyme is called glucokinase.

  • Hydrolases

    • protease — A protease is an enzyme that breaks peptide bonds and digests proteins. HIV codes for a protease that cleaves its large polyprotein into functional components, and examples include pepsin and trypsin.

    • urease — Urease is a nickel-dependent enzyme that hydrolyzes urea into ammonia and bicarbonate. It was the first enzyme crystallized, and its role in H. pylori helps the bacterium survive acidic conditions.

  • induced fit — Induced fit is the enzyme-binding model in which a substrate changes the shape of the active site rather than fitting a rigid lock. It replaced the older lock-and-key conception by emphasizing conformational change and stronger binding after association.
    • Induced-fit model — A model of enzyme action in which substrate binding at the active site causes the enzyme to change shape slightly, improving the fit. It is commonly contrasted with the lock-and-key model and used to explain flexible enzyme-substrate binding and related ligand-binding studies.

  • inhibitors — Inhibitors are molecules that reduce enzyme activity, including competitive inhibitors that occupy the active site and allosteric inhibitors that change enzyme conformation. Their effects are often discussed with Lineweaver-Burk plots, and some act as transition-state mimics.

  • Isomerases — Isomerases are enzymes that rearrange atoms within a molecule or interconvert different isomers. Examples include epimerases, mutarotases, mutases, and enzymes such as glucose-6-phosphate isomerase.

  • kinases — Kinases are enzymes that transfer phosphate groups, typically by using ATP to phosphorylate other molecules. They include classes such as protein kinases, tyrosine kinases, and cyclin-dependent kinases, are central to signaling cascades such as MAP kinase pathways, are opposed by phosphatases, and mutations in them can contribute to cancer.

    • kinase — A kinase is an enzyme that transfers a phosphate group from ATP onto another molecule. These enzymes are central to signaling cascades and often phosphorylate serine, threonine, or tyrosine residues, where they are opposed by phosphatases.

    • protein kinase — A protein kinase is an enzyme that transfers phosphate groups from ATP to target proteins, making it central to cellular signaling pathways. Cyclic AMP activates protein kinase A, one prominent member of this class.

    • protein kinases — Protein kinases are enzymes that transfer phosphate groups, usually from ATP, onto target proteins. Many act on serine, threonine, or tyrosine residues and include cAMP-dependent, calcium/calmodulin-regulated, and cyclin-activated forms.

      • AGC kinases
        • Akt — Akt, also called protein kinase B, is a serine-threonine kinase in the PI3K signaling pathway. It is regulated by PTEN and can promote downstream targets such as mTOR.

      • MAPK — MAPK refers to the mitogen-activated protein kinase family, a group of serine/threonine kinases activated in signaling cascades downstream of Ras and other extracellular cues. They are activated by dual phosphorylation on threonine and tyrosine residues. Better-known members include ERK, JNK, and p38; the family includes classical RAF-related pathways as well as JNK and p38 branches involved in stress, inflammation, differentiation, and apoptosis.

      • protein kinase A — Protein kinase A is the cAMP-dependent serine-threonine kinase activated when cyclic AMP binds its regulatory subunits and releases the catalytic subunits. It phosphorylates downstream targets in signal transduction and is associated with Carney complex when mutated.

      • protein kinase C — Protein kinase C is a family of signaling enzymes that phosphorylate serine and threonine residues and are activated by diacylglycerol. Its activity is tied to membrane translocation by RACK proteins and to roles in epithelial polarity, MARCKS phosphorylation, and bryostatin binding.

      • receptor tyrosine kinases — Receptor tyrosine kinases are cell-surface enzymes that dimerize and autophosphorylate when activated. They include receptors such as EGFR and the insulin receptor, and many targeted cancer drugs ending in -tinib inhibit this class.
        • ErbB receptors
          • epidermal growth factor receptor — The epidermal growth factor receptor is a receptor tyrosine kinase in the ErbB family. It is often overexpressed in cancers and is targeted by drugs such as cetuximab and erlotinib.
        • HER2/neu — HER2/neu is a growth-factor receptor often used as a biomarker in breast cancer, where its overexpression marks an aggressive subtype. Quizbowl clues pair it with Herceptin, BRCA1 and BRCA2, ductal and lobular disease, and the triple-negative category.

      • RNA-activated protein kinase — RNA-activated protein kinase, or PKR, is turned on by double-stranded RNA and can shut down protein synthesis by phosphorylating targets involved in translation. It is often discussed in connection with viroid pathogenicity and antiviral responses.

      • tyrosine kinase — Enzymes that phosphorylate tyrosine residues and function in signaling pathways including JAK-STAT and receptor growth-factor signaling. Examples include receptor tyrosine kinases such as HER2 and non-receptor kinases such as Src and JAK. They are major drug targets in cancer therapy, with examples including Bcr-Abl and receptor tyrosine kinases inhibited by imatinib and related drugs.

  • lactase — Lactase, encoded by LCT, is the enzyme that breaks down lactose into glucose and galactose. It is central to lactose tolerance and is often tested with a hydrogen breath test when deficiency is suspected.

  • ligases — Ligases are enzymes that join molecules together by forming phosphodiester bonds, especially in DNA repair and replication. They catalyze bond formation that links two smaller molecules into one larger product.
    • ligase — A ligase is an enzyme that joins molecules together, including DNA ligase, which seals Okazaki fragments during DNA replication.

  • lipase — Lipase is the enzyme that breaks down fats into fatty acids and glycerol. It is produced by the pancreas and acts in the digestive tract, including acidic environments in some contexts.

  • lock-and-key model — The lock-and-key model is the classic explanation of enzyme-substrate fit in which the two structures match directly. It is contrasted with the induced-fit model, which allows the enzyme to change shape around the substrate.

  • luciferase — Luciferase is the bioluminescent enzyme used in firefly-style reporter assays and ATP-dependent light-producing reactions. Related commercial variants include firefly, Renilla, NanoLuc, and other engineered forms used to measure gene expression and protein interactions. Because it requires a substrate but not an external light source, luciferase is a standard tool in biochemistry and cell biology. Assays such as NanoBiT and dual-luciferase systems rely on its light output for sensitive detection.

  • lyases — Lyases are enzymes that break single bonds or add and remove groups to form double bonds. They are one of the six major enzyme classes.
    • thiolase — Thiolase is the enzyme that catalyzes the final cleavage step in beta-oxidation, splitting fatty-acid intermediates into acetyl-CoA units. It works in the pathway that also involves carnitine transport, peroxisomal processing of very long fatty acids, and the production of ketone bodies when oxaloacetate is scarce.
  • Metabolic enzyme complexes
    • pyruvate dehydrogenase complex — The pyruvate dehydrogenase complex links glycolysis to the citric acid cycle by converting pyruvate into acetyl-CoA. Its multi-enzyme structure uses TPP, lipoate, FAD, and NADH-producing steps, and is the target of the Swanson conversion clue and primary biliary cirrhosis antibodies.

  • Monoamine oxidase — Monoamine oxidase is the mitochondrial enzyme that breaks down neurotransmitters such as serotonin, norepinephrine, and dopamine. It is targeted by MAOI antidepressants, and selegiline selectively inhibits the B isoform.

  • nitrogenases — Nitrogenases are enzymes used by diazotrophs to reduce atmospheric nitrogen to ammonia. They contain the FeMoco cofactor in the common molybdenum form, and related vanadium versions can also catalyze unusual reductions.

  • Oxidoreductases

    • Oxidases — Oxidases are enzymes that catalyze oxidation-reduction reactions involving oxygen, often producing water as a product. Examples include catalase and monoamine oxidase.

    • peroxidases — Peroxidases are enzymes that use peroxide substrates, including catalase and thyroid peroxidase, and they participate in oxidative reactions such as leukotriene synthesis. They are also important in host defense and in thyroid hormone metabolism, where selenium-containing peroxidases help convert T4 to T3.
      • horseradish peroxidase — Horseradish peroxidase is an enzyme from horseradish root widely used as a reporter in western blots and immunohistochemistry. It is commonly conjugated to secondary antibodies and can oxidize substrates such as ABTS and DAB for detection.
    • tyrosinase — Tyrosinase is the copper-containing enzyme that catalyzes melanin production from tyrosine and related substrates. Defects in tyrosinase cause many forms of albinism.

  • phosphodiesterases — Phosphodiesterases are enzymes that break phosphodiester bonds and regulate signaling by degrading second messengers such as cAMP and cGMP. Specific isoforms are targeted by drugs including sildenafil, milrinone, and apremilast.

  • Phospholipases
    • phospholipase C — Phospholipase C is an enzyme that cleaves membrane phospholipids, especially PIP2, to produce the second messengers diacylglycerol (DAG) and IP3. It functions downstream of Gq-coupled signaling and can also be stimulated in immune and sensory pathways. In plants, it acts in wound responses that help trigger jasmonate biosynthesis.
  • proteases — Proteases are enzymes that cleave proteins into smaller peptides or amino acids. Examples include serine proteases such as chymotrypsin and trypsin, which use catalytic residues and an oxyanion hole to break peptide bonds.

    • caspases — Caspases are cysteine proteases that cleave after aspartate residues and drive the apoptotic cascade. They are activated through pathways involving Apaf-1, cytochrome c, and CARD-domain interactions, and they also process targets such as PARP and lamins.

    • matrix metalloproteinases — Matrix metalloproteinases are zinc-dependent proteases that break down the extracellular matrix and basement membrane. They include collagenases, gelatinases, and stromelysins, and are implicated in processes such as angiogenesis and tumor invasion.

    • Proprotein convertases
      • furin — Furin is a subtilisin-like protease that cleaves proteins at an RRxR motif. It is notable for the proposed cleavage site in the SARS-CoV-2 spike protein and for inhibition by alpha-1 antitrypsin Portland.
    • serine proteases — Serine proteases are enzymes that cleave peptide bonds using a catalytic triad that includes serine, histidine, and aspartate. Examples include trypsin, elastase, thrombin, and the digestive enzyme subtilisin.
      • serine protease — Serine proteases are peptide-cleaving enzymes whose active-site amino acid is serine. Trypsin, chymotrypsin, and elastase are classic examples, and their catalytic triad is a standard point of identification.
  • Regulation
    • allosteric — Allosteric regulation occurs when a molecule binds somewhere other than an enzyme’s active site and changes the protein’s activity. The concept also explains how the lac repressor responds to its effector molecule by changing conformation.

  • ribonuclease — Ribonuclease is an enzyme that breaks down RNA and is secreted by the pancreas. One classic example is a small, beta-sheet-rich protein stabilized by four disulfide bonds.

  • Sirtuins — Sirtuins are a family of NAD-dependent enzymes that include deacetylases and ADP-ribosyl transferases. They are named from the silent information regulator 2 protein and are often discussed in connection with chromatin regulation and aging in yeast.

  • Steroidogenic enzymes
    • aromatase — Aromatase is the enzyme that converts testosterone into estradiol. It is central to estrogen production in both male development and some breast cancers, which is why aromatase inhibitors are used therapeutically.

  • substrate — A substrate is the reactant or molecule that an enzyme acts on in a biochemical reaction. Enzymes bind their substrates to form a short-lived intermediate during catalysis.

  • topoisomerases — Topoisomerases are enzymes that regulate DNA topology by changing the linking number through cutting and rejoining strands. Type I and type II forms include DNA gyrase, and they are targets of drugs such as camptothecin, irinotecan, and quinolones.

    • DNA gyrase — A bacterial type II topoisomerase that cuts both strands of DNA, passes another duplex through the break, and then re-ligates it to relieve supercoiling and change linking number in steps of two. It helps untangle and unwind DNA during replication and is inhibited by etoposide.

    • DNA topoisomerase — DNA topoisomerases relieve supercoiling in DNA by making transient breaks and then resealing the strands. They act ahead of the replication fork and are classified by whether they cut one strand or both strands.

    • topoisomerase — Topoisomerases are enzymes that alter DNA supercoiling and linking number by cutting and rejoining strands. They include DNA gyrase and other targets of chemotherapy drugs such as etoposide and irinotecan, and they are crucial for untying replicated circular DNA.

    • topoisomerase(s) — Topoisomerases are enzymes that alter DNA topology by cutting and resealing strands to relieve supercoiling and untangle DNA. Type II topoisomerases use ATP and can catenate or decatenate circular DNA, and bacterial DNA gyrase is a special form of the class.

  • Transferases

    • transaminases — Transaminases, or aminotransferases, are enzymes that transfer amino groups between an amino acid and an alpha-keto acid. Important examples include alanine transaminase, which is widely used as a marker of liver damage.

    • transketolase — Transketolase is a thiamine-dependent enzyme of the pentose phosphate pathway that transfers two-carbon units between sugar phosphates. It is also used in the Calvin cycle, and deficiency of its cofactor is associated with disorders such as beriberi and Wernicke-Korsakoff syndrome.

  • ubiquitin ligases — Ubiquitin ligases, also called E3 ligases, attach ubiquitin to target proteins and send them toward proteasomal degradation or other cellular fates. They include HECT and RING-family enzymes and such complexes as SCF and the anaphase-promoting complex.

    • anaphase promoting complex — The anaphase promoting complex, or APC/C, is an E3 ubiquitin ligase that drives the metaphase-to-anaphase transition. Activated by Cdc20 or Cdh1, it targets securin and cyclin B so separase can cleave cohesin. It is tied to the spindle assembly checkpoint and to cell-cycle control of proteins like D-box and KEN-box substrates.

    • E3 ubiquitin ligases — E3 ubiquitin ligases are enzymes that attach ubiquitin to target proteins, marking them for degradation by the proteasome. They include MDM2 and SCF complexes and are commonly classified into HECT and RING types.

  • xanthine oxidoreductase — Xanthine oxidoreductase is a molybdenum-containing enzyme involved in the final steps of purine degradation, converting substrates toward uric acid. It is commonly discussed through its oxidase and dehydrogenase forms and is inhibited by drugs such as allopurinol and febuxostat.

  • zymogens — Zymogens are inactive enzyme precursors that must be cleaved or otherwise modified to become active. Examples include trypsinogen, pepsinogen, procaspases, and chymotrypsinogen.
    • zymogen — A zymogen is an inactive enzyme precursor, also called a proenzyme, that must be activated after secretion. Examples include trypsinogen and pepsinogen, which are kept dormant until proteolytic cleavage turns them into active enzymes.

• Ubiquitin — a small (76-amino-acid), highly conserved protein expressed by all eukaryotic cells; covalently attached to a lysine residue of a target protein, and a poly-ubiquitin chain tags that protein for degradation by the 26S proteasome; conjugation proceeds through a three-enzyme cascade — E1 (activating), E2 (conjugating), and E3 (ligase, which confers substrate specificity; HECT and RING-finger classes); structurally similar to SUMO; ubiquitination of p53 (by the MDM2 ligase) marks it for degradation, and mono-ubiquitination of histone H2B marks transcriptionally active DNA; the N-end rule pathway uses E3 ligases to recognize destabilizing N-terminal residues; Aaron Ciechanover, Avram Hershko, and Irwin Rose won the 2004 Nobel Prize in Chemistry for the ubiquitin-mediated degradation system.

• Amino sugars
  • N-acetylglucosamine — N-acetylglucosamine is a glucose derivative found in peptidoglycan, where it alternates with N-acetylmuramic acid in bacterial cell walls. It is also the monomer of chitin and the sugar residue that begins many human N-linked glycans.
• Polysaccharides
  • Structural polysaccharides
    • Chitin — Chitin is a nitrogen-containing structural polysaccharide built from N-acetylglucosamine units. It is a key component of arthropod exoskeletons, crustacean shells, fungal cell walls, and other tough biological coverings, and it also occurs in mollusk radulae and cephalopod beaks. It can be strengthened by sclerotin, and it was long thought to be present in early sponges and other ancient organisms.
• Eicosanoids
  • prostaglandins — Prostaglandins are arachidonic-acid-derived signaling lipids made by cyclooxygenase and involved in inflammation, vasodilation, and smooth muscle activity. Aspirin and other NSAIDs reduce their synthesis, while some analogs are used clinically for glaucoma or to keep the ductus arteriosus open. Their chemistry also connects to thromboxanes and lipoxins.
• Lipid signaling molecules
  • prostaglandins — Prostaglandins are lipid signaling molecules derived from fatty acids and involved in inflammation, blood pressure, clotting, and muscle contraction. Aspirin works by inhibiting COX enzymes that synthesize prostaglandins.
• lipoproteins — Lipoproteins are blood-borne complexes of lipids and proteins that transport cholesterol and triglycerides. Their major subclasses include LDL, which is often called bad cholesterol, and HDL, which is associated with cholesterol removal.

  • apolipoproteins — Apolipoproteins are lipid-binding proteins that help package cholesterol and triglycerides for transport in lipoprotein particles. Examples include ApoA-I, ApoB, ApoC, and ApoE, with ApoE’s E4 variant closely associated with Alzheimer’s disease.

  • chylomicrons — Chylomicrons are the largest triglyceride-rich lipoprotein particles in the bloodstream. They are assembled in intestinal villi, enter lacteals, and transport dietary fats and fat-soluble vitamins through the lymph and blood.

  • LDL — LDL, or low-density lipoprotein, is a cholesterol-rich blood lipoprotein often called “bad” cholesterol because it carries cholesterol to tissues and contributes to atherosclerosis. It contains ApoB-100, can form oxidized foam-cell–producing particles, and is lowered by statins, ezetimibe, and PCSK9 inhibitors.

  • VLDL — VLDL, or very-low-density lipoprotein, is one of the major classes of lipoproteins. It is produced in the liver and functions in lipid transport through the bloodstream.

• Steroids
  • Androgens
    • androstenedione — Androstenedione, often called andro, is a testosterone precursor that became notorious in baseball performance-enhancing drug controversies. It was linked in clues to Mark McGwire and Dante Bichette.
• Sterols

  • Cholesterol — Cholesterol is a four-ringed lipid that helps regulate cell-membrane fluidity and supplies the starting material for steroid hormones. HDL and LDL transport it in lipoprotein particles. Its biosynthesis involves conversion of HMG-CoA to mevalonate; cholesterol-loaded macrophages form foam cells associated with atherosclerosis.

  • lanosterol — Lanosterol is a 30-carbon sterol formed by epoxidation of squalene and the first recognizable steroid ring system in cholesterol biosynthesis. All animal steroids are ultimately derived from it.

• Amino acids
  • Acidic amino acids

    • Aspartic acid — Aspartic acid is a nonessential amino acid with a carboxylic acid side chain; its anion is aspartate, abbreviated Asp or D in some contexts. It is one carbon shorter than glutamate, is formed from oxaloacetate by transamination, and participates in the malate-aspartate shuttle, the urea cycle, and pyrimidine synthesis. Its D-form can stimulate NMDA receptors, and aspartate is used in several biochemical assays and receptor interactions.

    • Aspartic acid and glutamic acid — Aspartic acid and glutamic acid are the two standard amino acids with acidic side chains. At neutral pH they are negatively charged and are commonly written as D and E, respectively.

    • Aspartic acid or glutamic acid — Aspartic acid and glutamic acid are the two common amino acids whose side chains are acidic.

  • alanine — Alanine is a nonpolar amino acid with a methyl side chain, abbreviated Ala or A. It is formed from pyruvate by transamination and is central to the Cahill cycle and alanine scanning experiments, in which residues are replaced with alanine to test protein function.

  • arginine — A basic proteinogenic amino acid with a protonated guanidinium side chain and one-letter symbol R. It is the precursor to nitric oxide, being converted to citrulline in that pathway, and in the urea cycle it is broken down to ornithine and urea; it can also be metabolized by microbes into putrescine.

  • Asparagine — Asparagine is one of the standard amino acids and, with glutamine, the only one whose side chain is an amide. It is the residue targeted in N-linked glycosylation and appears in a conserved aquaporin motif with proline and alanine.

  • Basic amino acids — The three basic amino acids are lysine, arginine, and histidine. They are the amino acids commonly listed as non-acidic and positively charged at physiological pH.

  • cysteine — Cysteine is the sulfur-containing amino acid with a thiol side chain, symbolized by C and with one-letter code C. It forms disulfide bonds in proteins that help stabilize structure and is associated with keratin, zinc fingers, papain, and PDI. It is also the precursor of acetylcysteine and glutathione-related chemistry, and N-acetylcysteine is used in acetaminophen overdose and as an expectorant.

  • essential amino acids — Essential amino acids are amino acids that humans cannot synthesize on their own and must obtain from the diet. Examples include histidine, methionine, valine, and lysine.
    • leucine — Leucine is a hydrophobic amino acid with an isobutyl side chain and the one-letter symbol L. It is a key clue in leucine zippers, leucine-rich repeat proteins, and the fact that it is one of the purely ketogenic amino acids.

  • glutamine — Glutamine is the amino acid abbreviated Q, with a polar amide side chain and a central role in nitrogen transport. It is the most abundant free amino acid in blood and is central to polyglutamine diseases and several metabolic pathways.

  • histidine — Histidine is an amino acid with an imidazole side chain and a pKa near 6, making it one of the more weakly basic residues in proteins. It is central in the catalytic triads of serine proteases, common zinc-finger motifs, nickel affinity tags, and the Ames test. Its decarboxylated product is histamine, a major mediator of allergic responses.

  • Isoleucine — Isoleucine is an essential amino acid with symbol I and an isobutyl side chain. Its cyclized derivative is associated with maple syrup urine disease, making it a familiar biochemistry answer.

  • lysine — An essential basic amino acid and one of the building blocks of proteins. It is symbolized by K, has a side chain ending in an amine group with a usually protonated epsilon amino group, and is important in histones, collagen crosslinking, and protein tagging. On decarboxylation it forms cadaverine. It also occurs in lysine vasopressin used in pigs and related animals.

  • methionine — Methionine is a sulfur-containing amino acid encoded by AUG and used to start protein synthesis. It also appears in metabolism clues involving S-adenosylmethionine, homocystinuria, and the formylated bacterial initiator.

  • Nonproteinogenic amino acids

    • homocysteine — Homocysteine is a sulfur-containing non-proteinogenic amino acid formed in methionine metabolism. Elevated levels are linked to vascular risk and homocystinuria, which can cause lens subluxation and marfanoid features, and the compound is closely tied to methylation chemistry.

    • ornithine — Ornithine is a nonproteinogenic amino acid central to the urea cycle and to arginine biosynthesis. It is produced from arginine by arginase and helps form citrulline, and its name is tied to the Greek root for bird.

  • Nonstandard amino acids
    • Selenocysteine — Selenocysteine is the 21st amino acid, a selenium-containing analog of cysteine inserted at UGA codons with the help of a SECIS element. It appears in proteins such as glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinase.

  • phenylalanine and glycine — Phenylalanine and glycine are the two amino acids whose FG repeats help make nuclear pore complexes selectively permeable. They also appear in intrinsically disordered regions that promote phase separation, including DDX4-driven germ granules.

  • Proteinogenic amino acids

    • glycine — Glycine is the simplest amino acid and the smallest amino acid, with hydrogen as its side chain. It is the only achiral amino acid and is not optically active. It is an inhibitory neurotransmitter in the spinal cord, appears frequently in collagen and beta turns, and is used in lab procedures such as blot stripping and quenching crosslinking. It is also found in motifs such as RGD and in nuclear pore proteins with phenylalanine-rich repeats.

    • serine — Serine is the hydroxyl-bearing amino acid abbreviated S, and it is a key residue in serine proteases such as chymotrypsin and trypsin. It also appears in O-linked glycosylation and in phosphatidylserine, a membrane signal for apoptosis.

  • Sulfur amino acids
    • Methionine and cysteine — Methionine and cysteine are the two amino acids that contain sulfur. Methionine is associated with the start codon, while cysteine helps form disulfide bridges in proteins.
      • Cysteine or cystine — Cysteine, and its oxidized disulfide-linked form cystine, are sulfur-containing amino acids. Their disulfide bonds help stabilize proteins such as keratin.

  • Tryptophan — Tryptophan is the essential amino acid with an indole side chain and one-letter symbol W. It is the only standard amino acid encoded by UGG, absorbs strongly at 280 nm, and is a precursor to serotonin and niacin; its synthesis is also the classic example of substrate channeling and attenuation in the trp operon.

  • valine — Valine is a branched-chain amino acid found among the proteinogenic amino acids. It is famous for replacing glutamic acid in the hemoglobin mutation that causes sickle-cell anemia.
• Blood proteins
  • Coagulation factors
    • fibrinogen — Fibrinogen is the soluble plasma glycoprotein and blood protein that thrombin cleaves to form fibrin during blood clotting. The resulting fibrin forms threads that entangle platelets and help create a clot. Fibrinogen is also an important surface-adsorbing blood protein and can bridge platelets through GPIIb/IIIa.
  • serum albumin — Serum albumin is the major protein of blood plasma and helps maintain oncotic pressure while transporting hydrophobic molecules. It is also widely used as a blocking agent in assays and as a carrier for drugs such as paclitaxel in Abraxane.
• Milk proteins
  • Casein — Casein is the chief protein in milk and a major component of cheese, making up most of the proteins in cow’s milk. It occurs in micelles and is cleaved by rennin or chymosin; its derivatives have been used in plastics, paper, and synthetic fibers such as Lanital and Galalith.
• Protein complexes
  • Protein cages
    • encapsulin — Encapsulins are capsid-like protein cages found in bacteria, often described as nanocompartments. They are icosahedral structures studied for cargo loading and potential drug-delivery applications.
• Protein domains

  • leucine zipper — A leucine zipper is a dimerization motif in transcription factors made of two alpha helices with leucines repeating every seventh residue. It appears in proteins such as CREB and AP-1, where it helps basic DNA-binding regions form stable dimers.

  • Leucine zipper motifs
    • leucine zippers — Leucine zippers are protein dimerization motifs in which leucines recur every seventh residue along an alpha helix. They occur in DNA-binding transcription factors such as c-Jun and c-Fos, including AP-1 proteins.

  • PDZ domains — PDZ domains are protein-protein interaction modules that bind short peptide sequences, often at the C-terminus of target proteins. They appear in scaffolding proteins such as PSD-95 and help organize receptor complexes at cell membranes and synapses.

  • Zinc finger — Zinc fingers are small DNA-binding protein domains that use conserved cysteines and histidines to coordinate a zinc ion. They appear in many transcription factors and ubiquitin ligases, and engineered versions have been used to build custom nucleases for gene targeting.

  • Zinc finger domains
    • zinc finger domain — A zinc finger domain is a compact DNA-binding motif stabilized by a zinc ion coordinated by conserved cysteine and histidine residues. It appears in many transcription factors, including members of the SRY-SOX family.
• Protein families

  • Lipocalins — Lipocalins are a family of small extracellular beta-barrel proteins that transport hydrophobic molecules such as retinol, fatty acids, and related ligands. One medically relevant example is neutrophil gelatinase-associated lipocalin, a biomarker for kidney disease.

  • Thioredoxins

    • thioredoxin — Thioredoxin is a small redox protein with a CXXC motif that reduces disulfide bonds and helps regulate biosynthetic enzymes. It is also used by bacteriophage T7 as a host processivity factor and works with thioredoxin reductase and NADPH in antioxidant pathways.
• Protein folds
  • Rossmann fold — A common protein structural motif made of alternating beta strands and alpha helices. It is found in many nucleotide-binding enzymes, especially dehydrogenases, and often binds dinucleotides such as NAD and FAD or related cofactors.
• Protein motifs

  • KEN-box — The KEN box is a short protein motif recognized by the APC/C complex during cell-cycle regulation. Together with the D-box, it helps target specific proteins for destruction during mitosis.

  • EF-hand motif
    • EF hand — The EF-hand is a helix-loop-helix calcium-binding motif found in proteins such as calbindin and calmodulin. Its structure coordinates calcium at the center of the loop and is important in calcium transport and signaling.

  • Greek key — A Greek key is a recurring structural motif in proteins made of beta sheets and turns. The name is borrowed from the classical meander or key pattern seen in Greek decoration.

  • leucine zipper motif — A leucine zipper is a DNA-binding motif made of two amphipathic alpha helices that dimerize into a coiled-coil. A nearby basic region binds the DNA, and the motif appears in proteins such as C/EBP, Fos, and Jun.
• Protein secondary structure
  • alpha helices — Alpha helices are a common type of protein secondary structure, stabilized by hydrogen bonding along the peptide backbone. They appear in proteins such as myoglobin and growth hormone and are identified by their characteristic circular dichroism signal.
• Protein structure

  • alpha helix — An alpha helix is a common protein secondary structure stabilized by hydrogen bonds between residues i and i+4. It is right-handed in the usual biological form, shows characteristic circular dichroism minima near 208 and 222 nm, and appears in motifs such as leucine zippers and transmembrane bundles.

  • beta sheets — A common protein secondary structure formed by hydrogen-bonded strands aligned into pleated sheet motifs. Beta sheets occur in many proteins, including globular proteins, immunoglobulin domains, amyloid, and prion structures, and may be parallel or antiparallel; they contrast with alpha helices.

  • Coiled coils
    • Coiled coil — A coiled coil is a protein structural motif in which two or more alpha helices wrap around one another through repeating hydrophobic and charged residues. It appears in proteins such as c-Fos, c-Jun, and soluble methane monooxygenase.

  • Coiling — Coiling is the wrapping of material into spirals, whether in biochemical motifs like coiled coils or in crafts such as baskets and pottery. In proteins, the related supercoiling idea also describes DNA that is overwound or underwound.

  • denaturation — Denaturation is the loss of a protein’s or nucleic acid’s native structure, often caused by heat, pH changes, or chemical agents. It is exploited in techniques like PCR and SDS-PAGE, where biomolecules are forced into more linear forms for analysis.
    • protein denaturation — Protein denaturation is the loss of a protein’s native three-dimensional structure, often driven by heat, chaotropes, or detergents such as SDS. It is commonly discussed with CD ellipticity, thermal shift assays, and the classic Anfinsen experiment using urea and beta-mercaptoethanol.
      • Denaturing — Denaturing is the unfolding of a protein from its native state, usually caused by heat, extreme pH, or chemical solvents. It is the process behind damage to hair proteins and the loss of enzyme function under harsh conditions.
  • helices — Helices are common protein structures, including alpha helices and other related variants, and the double helix is the form of DNA. They are a standard clue in biology and structural chemistry, especially when paired with beta sheets or coiled coils.
    • Helix — A three-dimensional spiral curve or structure that extends along an axis, rather than lying flat like a planar spiral. In biology, DNA’s familiar double form is the best-known example; collagen forms a triple helix. In physics, a charged particle moving in a constant magnetic field traces a helix, and the term also appears in screw motions and particle helicity.
  • hydrophobicity — Hydrophobicity is the tendency of molecules or regions to repel water and is associated with nonpolar side chains and lipid tails. It is measured by log P and related scales, helps drive protein folding and aggregation, and underlies the lotus effect and reversed-phase chromatography.
    • hydrophobic — Hydrophobicity is the tendency of nonpolar substances to repel water, and hydrophobic substances are often lipophilic. It applies to nonpolar materials such as alkanes, oils, greases, and fats, and to structures like the nonpolar core of a micelle or the buried interior of many proteins. Exposed hydrophobic patches can drive protein aggregation. The concept contrasts with hydrophilicity in discussions of surface tension and wetting.

  • Intron(s) — Introns are noncoding sections of pre-mRNA that are removed during splicing before translation. Their discovery helped explain gene structure in eukaryotes and the exon-intron organization of many genes.

  • primary structure — Primary structure is the linear sequence of amino acids in a protein. It is the level encoded by mRNA and comes before secondary and tertiary structure.

  • Protein folding — Protein folding is the process by which a polypeptide adopts its functional three-dimensional structure after synthesis. It is shaped by the amino-acid sequence, hydrophobic collapse, primary structure, and chaperones such as GroEL/GroES and Hsp70; failures in folding can produce prions. Levinthal’s paradox and Anfinsen’s dogma are standard associated concepts.

    • Anfinsen’s dogma — Anfinsen’s dogma is the principle that a protein’s amino-acid sequence determines the three-dimensional structure it will adopt. It is closely tied to protein folding, Levinthal’s paradox, and the role of chaperones in helping proteins reach their native shapes.

    • Anfinsen’s experiment — Anfinsen’s experiment showed that ribonuclease could refold spontaneously after denaturation if the correct disulfide bonds were restored. The result supported the idea that a protein’s amino acid sequence determines its final three-dimensional structure.

    • Christian Boehmer Anfinsen — Christian B. Anfinsen showed that a protein’s amino-acid sequence determines its final structure. His experiments on ribonuclease folding helped establish that the native state can be recovered after denaturation if the proper conditions are restored.

    • folding — Folding is the process by which proteins adopt their native three-dimensional shapes, a phenomenon explained by Anfinsen’s dogma and helped by chaperones such as BiP. The same word also describes bending strata into anticlines and synclines or turning a net into a polyhedron.

    • Foldit — Foldit is the 2008 crowdsourced protein-folding game in which players solve puzzles by manipulating virtual protein structures. A 2010 Nature paper reported that its human players produced solutions better than algorithmic methods in some cases.

    • Levinthal’s paradox — Levinthal’s paradox is the observation that a protein could not find its native fold by blindly sampling all possible conformations in anything like a reasonable time. It is used to motivate the idea that protein folding follows an energy funnel rather than a random walk, and that folding is guided by the information in the amino-acid sequence and by chaperone-assisted pathways.

  • Protein folding intermediates
    • molten globule — A molten globule is a compact protein-folding intermediate with much of its secondary structure intact but without a fully fixed tertiary structure. It is a common way to describe partially folded states that are not tightly packed.
  • Protein folds
    • beta barrel — A beta barrel is a protein fold made of beta strands wrapped into a closed cylindrical structure. It appears in GFP and many membrane proteins, including porins.
  • Protein structure diagrams
    • Richardson diagrams — Richardson diagrams, also called ribbon diagrams, are stylized drawings used to show biomolecular structure. They emphasize repeating features such as alpha helices and beta sheets or barrels with sweeping bands.

  • Ramachandran plot — A plot of protein backbone conformations using the phi and psi dihedral/torsional angles of amino-acid or polypeptide residues. It visualizes allowed and favored regions in protein secondary structure, including alpha helices, beta sheets, left-handed helices, and collagen. Named for the Indian scientist G. N. Ramachandran.

  • secondary structure — The level of protein structure formed by local hydrogen-bonded motifs along the polypeptide backbone, giving the protein its local three-dimensional form rather than its overall shape. Classic examples are alpha helices and beta pleated sheets; common methods used to study it include circular dichroism and prediction schemes such as Chou-Fasman.

    • beta sheet — A beta sheet is a common protein secondary structure made of hydrogen-bonded strands arranged in parallel or antiparallel fashion. It is often shown as flat arrows in ribbon diagrams and is a major structural element in fibrous proteins such as silk fibroin. Repeated beta sheets can fold into beta barrels such as those found in GFP and porins.

    • Beta-pleated sheet and alpha-helix — Ramachandran identified two stable protein secondary structures: the alpha helix and the beta-pleated sheet. The beta sheet can be arranged in parallel or antiparallel strands, while the alpha helix can be left- or right-handed.

    • Corey-Pauling rules — The Corey-Pauling rules are classic proposed rules for protein secondary structure, especially peptide-bond planarity and hydrogen bonding. They were later tested and refined by work on peptide conformations and Ramachandran plots.

• Serum albumins
  • bovine serum albumin — Bovine serum albumin is a common laboratory protein used as a blocking agent, protein standard, and stabilizer in molecular biology. It is often abbreviated BSA and is derived from cows.
• Molecular structures
  • triple helix — A triple helix is a three-stranded molecular structure found in collagen. Pauling incorrectly proposed it for DNA, though related triple-stranded forms can also occur in nucleic acids.
• Nucleobases

  • adenine — Adenine is a purine nucleobase symbolized by A and complementary to thymine in DNA and uracil in RNA. It also appears in ATP, NAD, FAD, and the poly-A tail of mRNA.

  • adenine and guanine — Adenine and guanine are the two purines found in DNA. They are the double-ringed nitrogenous bases that pair with the pyrimidines in nucleic acids.

  • cytosine — Cytosine is a pyrimidine nitrogenous base that pairs with guanine and forms three hydrogen bonds in DNA and RNA. It is also the base most associated with DNA methylation and spontaneous deamination to uracil, a reaction that matters in replication, PCR, and mutation studies.

  • cytosine and guanine — Cytosine and guanine are the two DNA bases that pair with three hydrogen bonds and are associated with CpG sites and islands. Their high GC content increases DNA stability and is important in methylation and genomic imprinting.

  • guanine and cytosine — Guanine and cytosine are the DNA bases that pair with three hydrogen bonds. A high GC content raises DNA melting temperature and is relevant to PCR, primer design, and sequence composition analyses.

  • purines and pyrimidines — The two classes of nitrogenous bases in DNA are purines and pyrimidines. Purines have a fused double-ring structure, while pyrimidines consist of a single six-membered ring.

• Nucleosides

  • adenosine — Adenosine is a purine nucleoside found in ATP and other important cellular molecules. It accumulates during wakefulness, is blocked by caffeine receptors, and also appears in cyclic AMP and in deoxyadenosine-related biology.

  • inosine — In RNA biology, inosine is the hypoxanthine-containing nucleoside commonly written as I. It appears in tRNA wobble positions and can arise from adenosine deamination, while inosine pranobex and related pathways make it a recurring clue in nucleotide metabolism.

• Antioxidant enzymes
  • superoxide dismutase — Superoxide dismutase is an antioxidant enzyme that converts superoxide into oxygen and hydrogen peroxide. Its copper-zinc form is tied to ALS-linked mutations such as A4V and G93A, and it is also invoked in discussions of catalytic perfection.
• Enzyme kinetics

  • Eadie-Hofstee plot — The Eadie-Hofstee plot is a linearized enzyme-kinetics graph that plots velocity against velocity over substrate concentration. Its slope gives the Michaelis constant and its intercept gives the maximum reaction rate.

  • Lineweaver-Burk plot — The Lineweaver-Burk plot is the double-reciprocal linearization of the Michaelis-Menten equation. It graphs reciprocal velocity against reciprocal substrate concentration, with intercepts and slope used to estimate enzyme constants. It is a standard tool in enzyme kinetics and in comparing competitive, uncompetitive, and mixed inhibition.

  • Michaelis-Menten plots
    • Michaelis-Menten plot — The Michaelis-Menten plot shows reaction velocity as a function of substrate concentration for an enzyme-catalyzed reaction. It rises with increasing substrate until it levels off at a maximal velocity and can be used to estimate Vmax and Km.

  • Ping-pong mechanism — The ping-pong mechanism is an enzyme kinetic model in which substrates are processed without forming a ternary complex. It is also called double displacement and gives parallel lines on Lineweaver-Burk plots.

  • turnover number — The turnover number, often written kcat, is the maximum number of substrate molecules an enzyme can convert per active site per unit time. It appears in Michaelis-Menten kinetics and in catalytic models for heterogeneous reactions.

  • Vmax — Vmax is the maximum reaction rate in Michaelis-Menten kinetics. It is the horizontal asymptote of the rate-versus-substrate curve and appears in the Lineweaver-Burk plot as the reciprocal of the y-intercept.
• Enzyme regulation

  • allosteric inhibition — Allosteric inhibition occurs when a molecule binds to an enzyme at a site away from the active site and changes its shape or activity. It is a form of regulation important in Michaelis-Menten discussions and classic examples such as ATCase control by CTP.

  • allosteric regulation — Allosteric regulation is control of an enzyme or protein by a molecule binding at a site other than the active site. It is classically described by cooperative binding, conformational change, and the Monod-Wyman-Changeux model of tense and relaxed states.

    • allosteric effectors — Allosteric effectors are agents that alter hemoglobin’s affinity for oxygen by binding away from the active site. Examples include protons, carbon dioxide, and 2,3-DPG.

    • cooperativity — Cooperativity is the phenomenon in which binding at one site changes the affinity of additional sites, as in hemoglobin’s oxygen binding. It is often described with the Hill coefficient and contrasted with Michaelis-Menten systems that lack allosteric feedback.

      • Hill coefficient — The Hill coefficient measures cooperativity in ligand binding and appears as the exponent in the Hill equation. A Hill coefficient above one indicates positive cooperativity, while a value of one describes noncooperative binding.
• Enzymology
  • Active sites — Active sites are the regions of enzymes where substrates bind and reactions occur. In heterogeneous catalysis, the term can also mean the vacant surface positions where the reaction takes place.
• Metalloproteins
  • iron-sulfur proteins — Iron-sulfur proteins are electron-transfer proteins built around iron-sulfur clusters and include ferredoxins, rubredoxins, aconitase, and the Rieske protein. They are important in metabolism and biosynthesis, and their clusters have been proposed in theories of the origin of life.
    • iron and sulfur — Iron and sulfur are central to ferredoxins, aconitase, and many other iron-sulfur proteins and clusters. They also give their name to the iron-sulfur world hypothesis, which proposes an origin of life near hydrothermal vents.
• Biochemistry
  • Amino Acids

    • amino acid — An amino acid is an organic molecule with both an amino group and a carboxyl group. Amino acids are the building blocks of proteins and are often grouped by the properties of their side chains. Examples include glycine and tryptophan, and they are delivered to the ribosome by tRNA during translation.

    • tyrosine — Tyrosine is an aromatic amino acid with a phenyl ring and a para-hydroxyl (phenol) side chain. Named from the Greek word for cheese, it has single-letter code Y and is a precursor to dopamine and melanin; it is converted to L-DOPA by tyrosine hydroxylase and appears in signaling proteins and kinases such as JAKs and Bcr-Abl.

  • antioxidants — Antioxidants are compounds that neutralize reactive oxygen species and free radicals, helping prevent cellular damage to cells and tissues. Common examples include vitamins C and E, glutathione, catalase, superoxide dismutase, and carotenoids found in foods such as tomatoes, broccoli, and blueberries.

  • Beta-galactosidase substrates
    • ONPG — ONPG is a colorimetric substrate for beta-galactosidase that is hydrolyzed into ortho-nitrophenol and a sugar. It turns yellow on cleavage and is often used in assays alongside X-gal.
  • Biotin-binding proteins
    • avidin — Avidin is the biotin-binding protein from egg whites that is widely used in molecular biology because of its extremely high affinity for biotin. Its bacterial analog streptavidin is also used in affinity chromatography, western blots, and related assays.

  • Calcium — Calcium, Ca, is an alkaline earth metal and the major element in bones and teeth, where it helps form hydroxyapatite. As a Ca2+ ion it is stored largely in the sarcoplasmic reticulum of muscle cells, helps drive muscle contraction through troponin binding, and is central to neurotransmitter release, parathyroid hormone regulation, vitamin D absorption, calcium channels, and ATP-driven ion transport in cells. It is also a major element in milk.

  • Chromogenic substrates
    • X-gal — X-gal, also called BCIG, is a chromogenic substrate used to detect beta-galactosidase activity. In blue-white screening and lac-operon assays, it yields an intense blue product when cleaved.

  • creatine — Creatine is a nitrogenous compound synthesized from glycine and arginine, then modified by SAM-dependent enzymes, and it is taken up by muscle as a rapid energy reserve. Its kinase, creatine kinase, is used in diagnosing myocardial infarction and muscle disease, so elevated levels can point to muscular dystrophy or related myopathies.

  • Disulfide bonds
    • disulfide bonds or bridges — Disulfide bonds are covalent links formed when two cysteines are oxidized. They are common in proteins and are reduced before electrophoresis with reagents such as beta-mercaptoethanol.

  • DOPA — DOPA is the first product of tyrosine in the biosynthetic pathway that leads to dopamine and related compounds. Its L stereoisomer was used in the film Awakenings to treat Parkinson’s patients.

  • Electron carriers
    • NADPH — NADPH is the reduced phosphate-containing dinucleotide used as a major electron donor in biosynthesis. It is produced by the pentose phosphate pathway and powers processes such as fatty acid synthesis and the Calvin cycle.
  • Enzyme kinetics

    • Michaelis constant — The Michaelis constant is the substrate concentration at which an enzyme-catalyzed reaction reaches half of Vmax in the Michaelis-Menten model. It is commonly used to describe enzyme affinity in basic kinetics and can be treated as an apparent dissociation constant in some limits.

    • Michaelis-Menten hypothesis — The Michaelis-Menten hypothesis describes enzyme kinetics by relating reaction velocity to substrate concentration and active-site saturation. It is associated with the Lineweaver-Burk and Eadie-Hofstee plots used to analyze enzyme-catalyzed reactions.

  • Enzyme mechanisms
    • kinetic proofreading — Kinetic proofreading is an error-correcting biochemical mechanism that uses extra irreversible steps to improve the accuracy of substrate selection. Hopfield’s ratchet-like model increases discrimination between correct and incorrect products by spending free energy.

  • heme — Heme is the iron-porphyrin complex in hemoglobin and cytochrome P450 proteins, where it binds oxygen and participates in redox chemistry. Quizbowl clues often mention the Bohr effect, carbon monoxide binding, porphyrins, and the red blood cell protein that carries oxygen.

  • hydrophobic effect — The hydrophobic effect is the entropic tendency of nonpolar groups to cluster together in water. It helps explain the inward-facing tails of lipid bilayers and many protein-ligand interactions.

  • Metabolism
    • lactic acid — Lactic acid is the hydroxyacid with formula CH3CH(OH)COOH, produced when pyruvate is reduced in oxygen-limited muscle tissue during anaerobic metabolism. It is the compound associated with sour milk and lactate buildup in muscle, appears in Ringer’s solution and the Cori cycle, and gives its name to lactic acid bacteria. Its L-isomer is used to make biodegradable polymers, and it is also linked to the historical sour-milk health theories of Metchnikoff.
  • Nucleobases
    • Adenine, Thymine, Cytosine, Guanine — The four main nitrogenous bases of DNA are adenine, thymine, cytosine, and guanine. In DNA, adenine pairs with thymine and cytosine pairs with guanine.
  • Nutrients
    • iodine and tyrosine — Iodine and tyrosine are both required in the diet and are associated with thyroid hormone production. Tyrosine is an amino acid, while iodine is the element commonly added to table salt.

  • peptide bonds — Peptide bonds are the amide linkages that connect amino acids into proteins. They are planar because of resonance, are formed during translation and solid-phase synthesis, and are broken by proteases like trypsin and chymotrypsin.

  • Phosphocreatine
    • creatine phosphate — Creatine phosphate is the high-energy phosphate reserve in muscle and brain that can rapidly regenerate ATP. It is also used as an energy source in cell-free protein synthesis systems and is sometimes called phosphocreatine.
  • Pigments
    • carotenoids — Carotenoids are orange-yellow accessory pigments found in plants and algae that help absorb light and protect against damage from excess blue and violet light. Xanthophylls are one class of carotenoids, and some carotenoids can be converted into vitamin A.

      • Carotene — An orange carotenoid pigment and hydrocarbon found in carrots and other plants, with beta-carotene as a common form. It has the formula C40H56, contributes to autumn leaf color, and is best known as a provitamin A compound that the human body converts into vitamin A.

      • Carotenes
        • beta-carotene — Beta-carotene is an orange-red carotenoid that the body can convert into vitamin A. It is found in carrots and yams, can cause a harmless orange discoloration when eaten in excess, and is also used as a provitamin in enriched foods like golden rice.

      • lycopene — Lycopene is a red carotenoid pigment found in tomatoes, watermelon, grapefruit, guava, and papaya. It is fat-soluble and is often described as a powerful antioxidant.

      • xanthophyll — Xanthophyll is a yellow carotenoid pigment found in plants. It is associated with the yellow color of autumn leaves.
  • Polysaccharides
    • agar — A seaweed-derived gelling agent made from red algae such as Gracilaria and Gelidium. It is a gelatinous polysaccharide that gels as it cools after being dissolved in hot water, and is used to solidify microbiology culture media in Petri dishes, including standard streak plates and specialized media such as chocolate agar. It is also used in molecular gastronomy for spherification and other gel-based preparations.
  • sialic acids — Sialic acids are nine-carbon sugars that are often found at the ends of cell-surface glycans. They are recognized by proteins such as selectins and hemagglutinin, and include N-acetylneuraminic acid.
• Chirality
  • achiral — An achiral substance lacks handedness and is therefore optically inactive. Glycine is the standard amino acid that is achiral because its side chain is only a hydrogen atom.
• Metabolites
  • Amino acid derivatives
    • NAG — N-acetylglutamic acid, often abbreviated NAG, allosterically activates carbamoyl phosphate synthetase I in the urea cycle. The abbreviation NAG can also refer to N-acetylglucosamine, a sugar found in peptidoglycan and chitin.
• Tetrapyrroles
  • porphyrins — Porphyrins are ring-shaped compounds built from four pyrrole units and found in important biomolecules like heme and chlorophyll. Their biosynthesis is linked to sideroblastic anemia and to porphyrias, which can cause urine that darkens in sunlight.
• Vitamins — Vitamins are essential organic compounds needed in small amounts. Casimir Funk originally called them vital amines. They are commonly divided into water-soluble and fat-soluble groups; the B vitamins are water-soluble, while vitamin K is needed for clotting, vitamin D has sunlight-dependent synthesis, and vitamin C prevents scurvy. Deficiencies of some vitamins cause diseases such as beriberi and scurvy. Thiamine is one example of a B vitamin.
  • B vitamins — B vitamins are the family of water-soluble vitamins that includes thiamine, niacin, folate, riboflavin, pyridoxine, and cobalamin. They are tied to clues about beriberi, pellagra, NAD and FAD precursors, sulfa-drug-sensitive synthesis, and the only known eukaryotic riboswitch.
    • Folate — Vitamin B9, also called folate, folic acid, folacin, or pteroylglutamic acid, is a one-carbon carrier required for DNA synthesis and methionine production. Deficiency causes megaloblastic anemia and can contribute to neural tube defects such as spina bifida and anencephaly, so it is especially important in pregnancy. It is used in fortified foods, found in leafy green vegetables, helps lower homocysteine, and is targeted by methotrexate and related antifolate drugs; excess folate can mask vitamin B12 deficiency.

  • Pantothenic acid — Pantothenic acid is an essential B-complex vitamin required for coenzyme A synthesis, and thus for the metabolism of fats, carbohydrates, and amino acids. It is involved in fatty-acid synthesis.

  • Vitamin B6 — Vitamin B6 is the B vitamin that occurs as pyridoxine, pyridoxal, and pyridoxamine. Its active form, pyridoxal phosphate, helps stabilize intermediates in transamination and other amino acid reactions.

  • Vitamin D
    • vitamin D3 — Vitamin D3, or cholecalciferol, is the form of vitamin D synthesized in human skin from 7-dehydrocholesterol under ultraviolet light. Deficiency can lead to rickets and osteoporosis, and the molecule is further hydroxylated in the liver to form calcifediol.

Metabolism

Connect energy transfer to photosynthesis and cellular respiration.

Metabolism

Photosynthesis

• Overall equation — 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂.

• Light-dependent reactions — occur in the thylakoid membranes; capture light energy to produce ATP and NADPH; split H₂O, releasing O₂. Photosystem II (P680) and Photosystem I (P700); electron transport chain; chemiosmosis drives ATP synthase.

• Calvin cycle (light-independent reactions) — occurs in the chloroplast stroma. RuBisCO fixes CO₂ to RuBP; ATP and NADPH support reduction, and most G3P regenerates RuBP. Fixing three CO₂ in total gives one net three-carbon G3P and consumes nine ATP and six NADPH. “Light-independent” describes the reaction chemistry, not independence from the light reactions that supply ATP and NADPH.⁴
  • Calvin cycle
    • RuBP — RuBP, short for ribulose 1,5-bisphosphate, is the five-carbon sugar used by rubisco in the Calvin cycle. It starts the Calvin-Benson cycle’s carbon-fixation stage, where carbon dioxide combines with it to produce 3-phosphoglycerate. It is also commonly identified as ribulose bisphosphate.

• C3, C4, and CAM plants — C3 (first product is 3-carbon 3-PGA; most plants); C4 (spatial separation of CO₂ fixation and Calvin cycle; reduces photorespiration in hot climates; e.g., corn, sugarcane); CAM (temporal separation; stomata open at night; e.g., cacti, succulents).

• Chloroplast structures
  • pyrenoids — Pyrenoids are dense structures in algal chloroplasts that contain high concentrations of RuBisCo. In Euglena, they may be capped with paramylon.
• Chloroplasts

  • Granum — A granum is a stack of thylakoids inside a chloroplast; the plural is grana. These stacks are part of the photosynthetic membrane system in plant cells and house the light-dependent machinery associated with chlorophyll. Grana are found in chloroplasts of higher plants, and they are absent from the chloroplasts of C4 plants in the way noted in the source records.

  • thylakoid — A flattened membrane-bound sac inside chloroplasts where the light-dependent reactions of photosynthesis occur, converting light energy to chemical energy through photophosphorylation. Thylakoids are the membrane compartments that house the light-dependent machinery of photosynthesis, and they are often stacked into grana. Their interior lumen is the compartment targeted by plastocyanin and other proteins imported by specialized transport pathways.

  • Thylakoid membranes — Thylakoid membranes are the chloroplast membranes that contain the photosynthetic machinery for the light reactions. They enclose the thylakoid lumen and are suspended in the surrounding stroma.

  • thylakoids — Thylakoids are chlorophyll-containing membrane sacs inside chloroplasts where the light reactions of photosynthesis occur. They can be stacked into grana, and their membranes house photosystems and the electron transport machinery that drives ATP production.

• Photosynthetic pigments

  • chlorophyll — Chlorophylls are green photosynthetic pigments in the thylakoid membranes of chloroplasts that absorb light in plants and algae. They contain a magnesium-coordinated porphyrin or chlorin ring and a phytol chain, with major forms including chlorophyll a and b; special reaction-center pairs such as P680 and P700 drive electron flow in photosystems. Chlorophyll biosynthesis begins from 5-ALA, and breakdown produces pheophytin-related products. Newer forms such as chlorophyll f have been found in stromatolites.

  • Chlorophyll a — Chlorophyll a is the primary photosynthetic pigment in Photosystem II. It contains a magnesium atom in a porphyrin ring and absorbs red and blue light.

• Plant metabolism
  • photorespiration — Photorespiration is the wasteful plant process that begins when rubisco fixes oxygen instead of carbon dioxide. It produces phosphoglycolate and requires salvage pathways before carbon can return to the Calvin cycle.
    • 2-phosphoglycolate — This compound is formed when Rubisco acts as an oxygenase and is associated with photorespiration. It is 2-phosphoglycolate, a photosynthesis inhibitor salvaged through the peroxisome and mitochondria.

Cellular respiration

• Overall equation — C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~30–32 ATP (aerobic).
  • cellular respiration — Cellular respiration is the set of metabolic reactions that harvest energy from nutrients to make ATP. It includes glycolysis, the citric acid cycle, and oxidative phosphorylation, and it can proceed aerobically or anaerobically.
    • ATP synthase — ATP synthase is the rotary enzyme of oxidative phosphorylation that makes ATP from ADP and phosphate using a proton gradient. Clues include its F0 and F1 subunits, the Boyer binding-change model, and inhibition by oligomycin or bedaquiline in some organisms.
      • ATP Synthases — ATP synthases are membrane enzymes that make ATP by using an electrochemical gradient to drive rotation. They contain an F0 membrane sector and an F1 catalytic sector, and they are central to oxidative phosphorylation.
    • Electron carriers
      • FADH2 — FADH2 is the reduced electron-carrying form of flavin adenine dinucleotide. In cellular respiration, it is generated in reactions such as succinate-to-fumarate and by enzymes such as succinate dehydrogenase and fatty acyl-CoA dehydrogenase, then passes its electrons into the electron transport chain through ubiquinone.
    • Electron transport chain — The electron transport chain is the final stage of cellular respiration, a mitochondrial pathway of membrane complexes in the inner mitochondrial membrane that passes electrons from NADH and FADH2 through coenzyme Q and cytochrome complexes to cytochrome c oxidase and ultimately oxygen. Molecular oxygen serves as the terminal electron acceptor, combining with protons to form water. The electron flow generates a chemiosmotic proton gradient that powers ATP synthase by chemiosmosis, making it the last stage of oxidative phosphorylation and linking the process to ATP production.

      • coenzyme Q10 — A lipid-soluble electron carrier in the mitochondrial electron transport chain. It is the mobile carrier that accepts electrons from NADH and FADH2, carries them through the Q cycle, and shuttles them between Complex I or II and Complex III, helping drive proton pumping across the inner mitochondrial membrane.

      • Complex I — Complex I, also called NADH:ubiquinone oxidoreductase or NADH dehydrogenase, is the first enzyme complex of the electron transport chain. It accepts electrons from NADH, passes them through iron-sulfur clusters to ubiquinone, and pumps protons across the inner mitochondrial membrane.

      • cytochrome c — Cytochrome c is the small heme protein that carries electrons between complexes III and IV in the mitochondrial electron transport chain. It is also central to apoptosis, because its release from mitochondria helps form the apoptosome with Apaf-1, and its sequence is commonly used in molecular clock studies.

      • cytochrome c oxidase — Cytochrome c oxidase, also called Complex IV, is the final enzyme in the mitochondrial electron transport chain. It transfers electrons to oxygen to form water and is inhibited by cyanide. Its mitochondrial gene is often used in DNA barcoding.

      • cytochrome oxidase — Cytochrome oxidase is the terminal enzyme complex of the electron transport chain, where cytochrome c passes electrons onward to oxygen. It contains heme and copper centers and helps reduce oxygen at the end of aerobic respiration.

      • ferredoxin — Ferredoxin is an iron-containing electron carrier that receives electrons in photosynthetic electron transport and can pass them to NADP+ reductase to help form NADPH. It is associated with the thylakoid and can also feed electrons into cyclic electron flow through the cytochrome b6f complex.

      • Q cycle — The Q cycle is the electron-transfer mechanism in the mitochondrial and chloroplast electron transport chains that helps generate a proton motive force. It operates through redox cycling of ubiquinol and ubiquinone at complex III and contributes to the proton gradient used for ATP synthesis.

      • Rieske protein — The Rieske protein is an iron-sulfur subunit of electron transport complexes such as cytochrome bc1 and cytochrome b6f. It is unusual among its class because two histidines, rather than two cysteines, coordinate its central metal ion.

    • Krebs cycle — The Krebs cycle, also called the citric acid cycle, tricarboxylic acid cycle, or TCA cycle, is a central stage of cellular respiration in the mitochondrial matrix. It begins when acetyl-CoA condenses with oxaloacetate to form citrate, then proceeds through an eight-step enzyme-driven cycle that regenerates oxaloacetate. The pathway oxidizes acetyl-CoA to carbon dioxide and produces NADH, FADH2, and GTP or ATP, linking glycolysis to oxidative phosphorylation. It is named for Hans Adolf Krebs.

      • Citric acid — Citric acid, also called citrate, is a six-carbon tricarboxylic acid formed from acetyl-CoA and oxaloacetate at the start of the Krebs cycle. It is the first product of the citric acid cycle, which is also called the citric acid cycle or TCA cycle, and aconitase converts it into isocitrate. Citric acid is a key intermediate in cellular respiration, is involved in fatty acid synthesis, the glyoxylate cycle, and the methylcitrate pathway, and regulates metabolism by activating acetyl-CoA carboxylase and fructose-1,6-bisphosphatase while inhibiting phosphofructokinase-1. It is abundant in citrus fruits such as oranges, lemons, and limes, and is widely used as a food additive, in medicines, and in products like blueprint paper.

      • Krebs cycle or citric acid cycle — The Krebs cycle, or citric acid cycle, is the mitochondrial process that oxidizes acetyl units to generate NADH, FADH2, and GTP. Its sequence includes aconitase, succinyl-CoA synthetase, fumarase, and regeneration of oxaloacetate.

      • malate — Malate is a dicarboxylic acid and an intermediate in the citric acid cycle. It is formed from fumarate and converted to oxaloacetate by malate dehydrogenase; it also plays roles in C4 and CAM metabolism. In its protonated form, malate is malic acid, the compound that gives many sour candies and Warheads their tart taste.

      • oxaloacetate — Oxaloacetate is a four-carbon intermediate in the citric acid (Krebs/TCA) cycle that combines with acetyl-CoA to form citrate and is regenerated from malate at the end of the cycle. It also links gluconeogenesis, the malate-aspartate shuttle, and aspartate transamination, making it a central metabolic hub. It appears in the Hatch-Slack (C4) pathway and can be converted into pyruvate.

      • succinate — Succinate, or succinic acid, is a citric acid cycle intermediate oxidized to fumarate while reducing FAD to FADH2. It is the first symmetrical Krebs-cycle intermediate and the product of the step in which acetyl-CoA and oxaloacetate combine to form citrate.

    • oxidative phosphorylation — Oxidative phosphorylation is the mitochondrial process in which electron transport across the inner membrane builds a proton gradient that powers ATP synthase. It depends on oxygen as the final electron acceptor and is closely linked to the electron transport chain.
      • chemiosmotic synthesis — Chemiosmotic synthesis is the stage of aerobic respiration in which the electron transport chain creates a proton gradient that powers ATP formation by ATP synthase. Peter Mitchell proposed the process as the modern explanation for how oxidative phosphorylation works.
    • the electron transport chain — The electron transport chain is the final major stage of aerobic respiration, carried out in the inner mitochondrial membrane. Electrons from NADH and succinate move through carriers such as ubiquinone and cytochrome complexes to drive proton pumping and ATP production.
• Glycolysis — cytoplasm; splits glucose (6C) into 2 pyruvate (3C); net yield: 2 ATP, 2 NADH; no oxygen required. It is also known as the Embden-Meyerhof pathway and is regulated by key enzymes including hexokinase or glucokinase, phosphofructokinase-1, and pyruvate kinase. It begins with an energy investment phase and is the first stage of cellular respiration.

  • DHAP — DHAP is the three-carbon glycolytic intermediate that is interconverted with glyceraldehyde 3-phosphate by triose phosphate isomerase. It is produced when aldolase splits fructose 1,6-bisphosphate during glycolysis.

  • hexokinases — Hexokinases are the mammalian enzymes that catalyze the first step of glycolysis by phosphorylating six-carbon sugars. A specific member of the family is also called glucokinase.

  • Intermediates
    • phosphoenolpyruvate — Phosphoenolpyruvate is a high-energy glycolytic intermediate formed from 2-phosphoglycerate by enolase. It is converted to pyruvate by pyruvate kinase to generate ATP, and it also serves as a carbon acceptor in C4 and CAM carbon fixation and as a gluconeogenic intermediate for oxaloacetate formation.

  • phosphofructokinase — Phosphofructokinase is a key regulatory enzyme of glycolysis that phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate. It is allosterically controlled by cellular energy signals and is often discussed alongside its liver and muscle isoforms and the bifunctional PFK-2/FBPase-2 system.

  • phosphofructokinase-1 — Phosphofructokinase-1 is the major regulatory enzyme of glycolysis, catalyzing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. It is allosterically activated by AMP and inhibited by ATP, helping control the pathway’s rate-determining step.

• Pyruvate oxidation — pyruvate enters mitochondrial matrix; converted to acetyl-CoA + CO₂ + NADH by pyruvate dehydrogenase complex.

• Krebs cycle (citric acid cycle) — mitochondrial matrix; 2 turns per glucose; yields per turn: 1 ATP, 3 NADH, 1 FADH₂, 2 CO₂.

• Electron transport chain (ETC) — inner mitochondrial membrane; NADH and FADH₂ donate electrons; O₂ is the final electron acceptor; proton gradient drives ATP synthase (chemiosmosis); produces ~26–28 ATP.

• Fermentation — anaerobic; regenerates NAD⁺; lactic acid fermentation (animals, some bacteria) or alcoholic fermentation (yeast, some plants). Produces only 2 ATP net. It also refers to the microbial conversion of sugars to acids, gases, or alcohol and is used in breadmaking, cheesemaking, wine, beer, pickles, miso, soy sauce, kimchi, and many other foods.
  • Alcohol fermentation — Alcohol fermentation is an anaerobic pathway in which pyruvate is converted to acetaldehyde and then reduced to ethanol. It regenerates NAD+ so glycolysis can continue and is used by many bacteria and yeast-like organisms under oxygen-poor conditions.

• Gluconeogenesis — the synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, glucogenic amino acids, and TCA intermediates such as oxaloacetate); occurs mainly in the liver (and kidney) during fasting or starvation; essentially glycolysis run in reverse but with three irreversible steps bypassed by dedicated enzymes: pyruvate carboxylase (pyruvate → oxaloacetate) and PEP carboxykinase (oxaloacetate → phosphoenolpyruvate) replace pyruvate kinase, and glucose-6-phosphatase (on the luminal side of the ER) and fructose-1,6-bisphosphatase replace the kinases; muscle and brain cells lack glucose-6-phosphatase and so cannot release free glucose; the Cori cycle shuttles lactate from muscle to the liver for reconversion to glucose; reciprocally regulated with glycolysis by fructose-2,6-bisphosphate and transcriptionally upregulated by FOXO1 and PGC-1α.

• Glycolytic enzymes

  • GAPDH — GAPDH is a glycolytic housekeeping enzyme commonly used as a loading control in molecular biology. It catalyzes the oxidation of glyceraldehyde-3-phosphate and produces the only NADH generated in glycolysis.

  • enolase — Enolase is a glycolytic enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate. It is also known for neuron-specific forms used as cancer markers and for its TIM-barrel structure.

• Electron transport chain proteins
  • cytochromes — Cytochromes are heme-containing electron-transfer proteins found in mitochondria and chloroplasts that shuttle electrons in electron transport chains and related metabolic pathways. Examples include cytochrome c, which carries electrons between complexes III and IV and is implicated in apoptosis, the b6f complex in chloroplasts, and P450 enzymes involved in drug metabolism.
    • cytochrome — Cytochromes are heme proteins that transfer electrons in the electron transport chain. Cytochrome c is especially famous for its role in apoptosis after release from mitochondria, while cytochrome P450 enzymes act as monooxygenases.
• Citric acid cycle enzymes
  • aconitase — Aconitase is the iron-sulfur enzyme of the citric acid cycle that catalyzes the conversion of citrate to isocitrate. Because it acts in the early steps of the Krebs cycle, it is a standard clue for that pathway’s first rearrangement reaction.
• Glycolysis enzymes
  • pyruvate kinase — Pyruvate kinase is the final enzyme of glycolysis, converting phosphoenolpyruvate into pyruvate and generating ATP. Its low-activity M2 isozyme is associated with tumor metabolism, and deficiencies can cause hemolytic anemia.

Lipid, Amino-Acid, and Integrated Metabolism

• Cytochrome P450 enzymes

  • cytochrome P450 — Cytochrome P450 is a large family of heme-containing monooxygenases central to drug metabolism and steroid biosynthesis. These enzymes are named for their carbon-monoxide absorption peak and handle much of Phase I metabolism, including paracetamol processing and cholesterol side-chain cleavage. They also include enzymes such as CYP1A1 and are targeted by azole antifungals.

  • cytochrome P450 monooxygenases — Cytochrome P450 monooxygenases are heme-containing enzymes responsible for much of phase I drug metabolism. Their activity is affected by polymorphisms and by substances such as grapefruit juice and St. John’s wort.

• Fatty acid metabolism
  • beta-oxidation — Beta-oxidation is the metabolic pathway that breaks fatty acids down into acetyl-CoA, and it begins with the carnitine shuttle for long-chain substrates before proceeding through repeated cycles of oxidation and thiolase cleavage. It occurs in mitochondria and peroxisomes, produces propionyl-CoA from odd-chain substrates, and is disrupted by defects in enzymes such as enoyl isomerase or transport proteins involved in fatty-acid import.
• Metabolic pathways

  • shikimate pathway — The shikimate pathway is the biosynthetic route plants and bacteria use to make aromatic amino acids. It produces chorismate and related intermediates such as prephenate, and it is named for shikimic acid, a compound linked to oseltamivir synthesis.

  • Isoprenoid biosynthesis
    • mevalonate pathway — The mevalonate pathway produces the isoprenoid precursors IPP and DMAPP and is central to cholesterol and terpenoid biosynthesis. Its key early step depends on HMG-CoA reductase, the enzyme inhibited by statins, and it proceeds through intermediates such as mevalonate and squalene.
  • pentose phosphate pathway — An alternative route for glucose-6-phosphate metabolism that generates NADPH and five-carbon sugars, including ribose-related intermediates. It has oxidative and nonoxidative phases that produce intermediates used in glycolysis and gluconeogenesis; the nonoxidative carbon rearrangements are handled by transketolase and transaldolase, and the pathway involves enzymes such as G6PD.

• acetyl-CoA — A central metabolic carrier of acetyl groups linked by a thioester bond. It is produced from pyruvate by pyruvate dehydrogenase or from fatty acid beta-oxidation, then combines with oxaloacetate to form citrate and enter the Krebs/TCA cycle. It also feeds fatty-acid synthesis and oxidation, ketone body formation, the glyoxylate cycle, cholesterol synthesis via the mevalonate pathway, gibberellin synthesis, and melatonin biosynthesis.

• anabolism — Anabolism is the constructive side of metabolism, the process that builds larger molecules from simpler ones. It is the opposite of catabolism and often requires energy input such as ATP.

• anaerobic conditions — Anaerobic conditions are environments lacking oxygen. They are central to fermentation, swamp-gas production, and processes such as denitrification and anaerobic digestion.

• Carbohydrate metabolism

  • Entner-Doudoroff pathway — The Entner-Doudoroff pathway is an alternate prokaryotic route for catabolizing glucose to pyruvate. It is less ATP-efficient than glycolysis and begins with the formation of KDPG.

  • Triose phosphates
    • glyceraldehyde 3-phosphate — A three-carbon sugar phosphate formed in glycolysis and also produced in the Calvin cycle. It is made alongside dihydroxyacetone phosphate by aldolase, can be interconverted with DHAP, and is used in carbon fixation and carbohydrate metabolism.

  • glucose 6-phosphate — Glucose 6-phosphate is the first glycolytic intermediate formed when hexokinase traps glucose inside the cell. It is also the starting substrate for the pentose phosphate pathway, where G6PD acts on it to begin NADPH production.

  • Pyruvate — Pyruvate, or pyruvic acid, is the three-carbon end product of glycolysis and the simplest keto acid. Its ionized form is converted to acetyl-CoA by pyruvate dehydrogenase for entry into the Krebs/citric acid cycle, or to oxaloacetate by pyruvate carboxylase; under anaerobic conditions it is reduced to lactate, and it can also be fermented toward ethanol.

• Catabolism — Catabolism is the breakdown phase of metabolism, the opposite of anabolism. It breaks complex molecules into simpler ones and releases energy, such as when fats, proteins, and carbohydrates are degraded for use by the cell.

• Chemosynthesis — Chemosynthesis is the process by which organisms, especially bacteria near deep-sea vents or seeps, make food without sunlight. It uses chemical energy from compounds such as methane or hydrogen sulfide rather than photosynthesis.

• Citric acid cycle variants
  • glyoxylate cycle — The glyoxylate cycle is a variation of the citric acid cycle that lets plants, fungi, and some bacteria convert acetyl-CoA into compounds for gluconeogenesis. It bypasses carbon-loss steps by using isocitrate lyase and malate synthase, and is especially important in oil-rich seeds.
• Cori cycle — The Cori cycle is the metabolic pathway in which lactate produced by muscles during anaerobic activity is sent to the liver and converted back into glucose. It links muscle glycolysis with hepatic gluconeogenesis after exercise. It is named for Carl and Gerty Cori, the husband-and-wife scientists who studied the recycling of lactate and glucose.
  • the Cori cycle — The Cori cycle links anaerobic muscle metabolism and liver gluconeogenesis by converting lactate back into glucose. It operates as a closed metabolic loop at an ATP cost to the organism.
• Energy carriers
  • adenosine triphosphate — Adenosine triphosphate is the cell’s main energy currency and the main energy-carrying molecule produced in cellular respiration. It consists of adenine, ribose, and three phosphate groups, with two high-energy phosphate bonds. It is generated in glycolysis and oxidative phosphorylation, including by the F0-F1 ATP synthase complex using a proton gradient in chemiosmosis, and is synthesized in mitochondria and chloroplasts. Its breakdown to ADP releases usable energy for active transport, many metabolic reactions, muscle contraction, and glycolysis regulation, and it is also a precursor of cyclic AMP.

• ketone bodies — Ketone bodies are energy-rich molecules such as acetoacetate and beta-hydroxybutyrate made by the liver during fasting or starvation. They accumulate in diabetic ketoacidosis and can be used as fuel by peripheral tissues and the brain.

• Ketone body metabolism
  • ketogenesis — Ketogenesis is the metabolic process that produces the ketone bodies acetoacetate, beta-hydroxybutyrate, and acetone during low-carbohydrate states such as fasting. It is associated with ketosis and ketoacidosis, and severe activation can lead to Kussmaul breathing and positive nitroprusside urine tests.

• lactate — Lactate is the ionized form of lactic acid and the product of anaerobic fermentation of pyruvate in animal cells and many microbes. It can accumulate in muscle during strenuous exercise and in tumors, is associated with fatigue, soreness, and metabolic acidosis, and is a key intermediate in the Cori cycle, where it can be converted back to glucose in the liver.

• Metabolic rate — Metabolic rate is the rate at which an organism converts chemical energy into heat and expends energy. Its basal form is abbreviated BMR, and it is often discussed with scaling laws such as Kleiber’s law.

• urea cycle — The urea cycle disposes of toxic ammonia by converting it into urea for excretion. It occurs largely in the liver and features ornithine, citrulline, carbamoyl phosphate, and the rate-limiting enzyme CPS1.

• Nicotinamide adenine dinucleotides
  • NADH — NADH is the reduced form of nicotinamide adenine dinucleotide and a major electron carrier in metabolism. It is produced in glycolysis, beta-oxidation, and the Krebs cycle, then feeds electrons into complex I of the electron transport chain. Related clues may also point to NAD+ and niacin.
• Oxidative stress
  • reactive oxygen species — Reactive oxygen species are unstable oxygen-containing compounds, including oxygen-based radicals and peroxides such as superoxide, that damage cells through oxidative stress. They are produced during the neutrophil respiratory burst, are implicated in aging, DNA damage, and toxic side effects of metabolic reactions, and are deficient in chronic granulomatous disease.
• Purine metabolism
  • uric acid — Uric acid is the end product of purine metabolism in humans and the main compound implicated in gout. Because many birds and reptiles excrete nitrogen in this form, it is also an important waste product in comparative physiology.
• Tryptophan metabolism
  • kynurenine pathway — The kynurenine pathway is a tryptophan catabolic route that produces NAD+ in mammals. Its metabolites include quinolinic acid and kynurenine, and the pathway has been linked to inflammation and mood regulation.

Molecular Biology and Genetics

Follow genetic information from inheritance to molecular expression.

Molecular Genetics

DNA structure and replication

• Double helix — antiparallel strands; sugar-phosphate backbone; complementary base pairing (A=T via 2 H-bonds; G≡C via 3 H-bonds). Structure determined by Watson, Crick, and Franklin (1953); Franklin’s X-ray Photo 51 was critical. The double helix is the twisted ladder shape of DNA, with two antiparallel strands held together by complementary base pairing. Watson and Crick identified it, and Rosalind Franklin’s Photo 51 provided crucial X-ray evidence.

• Semi-conservative replication — each daughter DNA molecule has one original strand and one new strand; confirmed by Meselson-Stahl experiment (1958).

• Key enzymes — helicase (unwinds double helix), DNA polymerase (synthesizes new strand 5’→3’, requires primer), primase (lays RNA primer), DNA ligase (seals Okazaki fragments on the lagging strand), topoisomerase (relieves torsional stress).

• Leading vs lagging strand — leading strand synthesized continuously toward the replication fork; lagging strand synthesized in fragments (Okazaki fragments) away from the fork.

• Telomeres — repetitive sequences at chromosome ends; shorten with each division; maintained by telomerase (an RNA-containing enzyme) in germ cells and most cancer cells. Their maintenance is tied to the shelterin complex, and their loss is linked to cellular senescence, the Hayflick limit, and aging.

  • telomere — A telomere is the repetitive DNA at the end of a chromosome. It helps protect chromosome integrity, forms T-loops, and shortens with cell division, making it central to studies of ageing and replicative limits.

  • TTAGGG — TTAGGG is the repeating six-base sequence found in human telomeres. It is the canonical telomeric repeat in humans.

• Semiconservative replication — each daughter DNA double helix retains one parental strand; the alternative models (conservative: both parental strands stay together; dispersive: parental and new DNA mix throughout) were ruled out by Meselson-Stahl.
  • semiconservative — Semiconservative DNA replication produces daughter molecules that each contain one parental strand and one newly synthesized strand. The Meselson-Stahl experiment established this mode of replication.

• Okazaki fragments — short DNA fragments (100–200 nt in eukaryotes, ~1000–2000 nt in prokaryotes) synthesized discontinuously on the lagging strand of replication; each requires a separate RNA primer; ligated into a continuous strand by DNA ligase; discovered by Reiji and Tsuneko Okazaki. They illustrate discontinuous lagging-strand synthesis because DNA polymerases can extend only in the 5’ to 3’ direction; the fragments are later joined after primer removal.

• Chargaff’s rules — Erwin Chargaff found that in any DNA sample, [A]=[T] and [G]=[C] (base ratios are 1:1); the ratio (A+T)/(G+C) varies between species; these rules were key constraints that helped Watson and Crick deduce base pairing.

• Telomerase — a reverse-transcriptase-containing ribonucleoprotein that extends telomeres using an internal RNA template; active in germ cells, stem cells, and most cancer cells; its absence in somatic cells contributes to cellular senescence; discovered by Elizabeth Blackburn, Carol Greider, and Jack Szostak (2009 Nobel Prize). It adds TTAGGG repeats from the RNA template. Blackburn and Greider first characterized it in Tetrahymena, and its activity helps cells evade the Hayflick limit.

• DNA conformations
  • Z-DNA — Z-DNA is the left-handed DNA conformation with a zig-zag backbone and about 12 base pairs per turn. It has no major groove and is favored by alternating purine-pyrimidine sequences under negative supercoiling.
• Nucleic acid base pairing
  • Non-Watson–Crick base pairing
    • Hoogsteen base pairing — Hoogsteen base pairing is a non-Watson–Crick arrangement in which a purine rotates about 180 degrees relative to its usual position. It can help stabilize triple helices and G-quadruplexes. Reversed Hoogsteen pairing appears in some unusual nucleic acid structures, including hairpins and higher-order helices. It is a standard clue in questions about alternative DNA and RNA base pairing.
• DNA metabolism
  • DNA repair — DNA repair is the set of cellular pathways that fix damage to DNA, including base excision repair, mismatch repair, nucleotide excision repair, and non-homologous end joining. It is tied to UV damage, Lynch syndrome, and famous Nobel-recognized work by Lindahl, Modrich, and Sancar.

    • BRCA1 — BRCA1 is a tumor-suppressor gene and protein involved in DNA repair, especially double-strand break repair, and genome surveillance. It is classically linked to hereditary breast cancer risk and works with repair partners such as Rad51.

    • DNA damage — DNA damage refers to lesions, breaks, or chemical alterations to DNA caused by agents such as UV light, intercalators, or genome-editing nucleases. It can trigger pathways like SOS response, mismatch repair, nonhomologous end joining, and HDR, and is often assayed with markers such as comet formation or gamma-H2AX.

      • double-strand breaks — Double-strand breaks are DNA lesions in which both strands of the double helix are severed. Cells repair them primarily by homologous recombination or nonhomologous end joining.

      • thymine dimers — Thymine dimers are UV-induced DNA lesions in which adjacent pyrimidines form a cyclobutane ring. They are repaired by nucleotide excision repair, and defects in that pathway are associated with xeroderma pigmentosum.

      • UV damage

        • pyrimidine dimers — Pyrimidine dimers are UV-induced DNA lesions formed when adjacent pyrimidines bond to one another. They are repaired by nucleotide excision repair and accumulate in xeroderma pigmentosum, where they raise skin-cancer risk.

    • DNA glycosylase — DNA glycosylases are repair enzymes that recognize damaged or altered bases in DNA and remove them. They are important components of base-excision repair.

    • microsatellite instability — Microsatellite instability is a hypermutable state caused by defects in DNA mismatch repair. It is associated with Lynch syndrome and produces changes in short tandem repeat sequences during replication.

    • mismatch repair — Mismatch repair is a DNA repair pathway that fixes replication errors such as mispaired bases and small insertion-deletion loops. Its failure causes microsatellite instability and is linked to Lynch syndrome, with key proteins including MLH1, MSH2, MutS, and MutL.

    • non-homologous end-joining — Non-homologous end-joining repairs double-strand DNA breaks when Ku proteins bind the broken ends, recruit DNA-PKcs, and help complete end processing and ligation. The pathway also acts on shortened telomeres and is important in V(D)J recombination.

    • Nucleotide Excision Repair — Nucleotide excision repair is the DNA repair pathway that removes bulky, helix-distorting lesions such as thymine dimers and 6-4 photoproducts caused by ultraviolet light. It is defective in xeroderma pigmentosum and Cockayne syndrome, and in bacteria it begins with UvrABC recognizing the damage and cutting out the segment.
• DNA repair genes
  • BRCA1 or BRCA2 — BRCA1 and BRCA2 are tumor-suppressor genes involved in homology-directed DNA repair. Mutations in either gene are linked to hereditary breast cancer and can make PARP inhibitor therapies effective.
• DNA sequence features
  • GC content — GC content is the proportion of guanine and cytosine in a DNA sequence. It affects melting temperature and PCR annealing, and genome regions with uniform values of it are called isochores.
• DNA structure

  • Francis Crick and James Watson — Francis Crick and James Watson were the Cambridge scientists who proposed the double-helix structure of DNA in 1953. Their model of DNA replication was later confirmed by the Meselson-Stahl experiment.

  • James D. Watson and Francis Crick — James D. Watson and Francis Crick are the scientists credited with the double-helical structure of DNA. Their model drew on Rosalind Franklin’s X-ray data and later became the standard account of DNA replication.

  • major groove — The major groove is the wider groove of DNA, a common binding site for proteins that read the base sequence. Restriction enzymes and DNA methyltransferases often recognize sequences there, and EcoRV binding bends the helix while altering the groove shape.

  • minor groove — The minor groove is the narrower, shallower groove of the DNA double helix, especially in B-form DNA. Many DNA-binding proteins, including TATA-binding protein, recognize or bend DNA by interacting with this groove.

  • supercoiling — Supercoiling is the twisting of DNA beyond its relaxed state, producing extra turns in circular or constrained molecules. It commonly builds up ahead of replication forks and can be relaxed by enzymes such as DNA gyrase and other topoisomerases.

    • DNA supercoiling — DNA supercoiling is the overwinding or underwinding of DNA, described by changes in the linking number. It is altered by topoisomerases and can be negative in most organisms or positive during replication.
• DNA topology
  • linking number — The linking number of a DNA molecule is the topological quantity equal to twist plus writhe. It stays constant unless the DNA is cut, so enzymes like topoisomerases and gyrase change it by altering DNA supercoiling.
• DNA repair proteins
  • RecA — RecA is the bacterial ATPase that promotes homologous recombination by pairing single-stranded DNA with a matching duplex and catalyzes the SOS response through LexA cleavage.
• DNA replication enzymes

  • DNA primase — DNA primase is the enzyme that lays down the short RNA primer needed before DNA polymerase can extend a new strand. It is used once on the leading strand but repeatedly on the lagging strand to start each Okazaki fragment.

  • Primase — Primase is the enzyme that synthesizes short RNA primers on single-stranded DNA. Those primers provide the 3’ OH needed to start DNA replication and to form Okazaki fragments.

• DNA repair responses
  • SOS response — The SOS response is the bacterial DNA-damage response controlled by LexA and RecA. It shuts down cell division, induces repair pathways such as nucleotide excision repair, and can also promote error-prone mutagenesis.

Transcription and translation (Central Dogma)

• Central dogma — DNA → RNA → Protein (Crick, 1958). Reverse transcriptase (in retroviruses) allows RNA → DNA, an exception. The term is commonly stated as the usual flow of genetic information in molecular biology or molecular genetics, and Francis Crick’s formulation excluded information transfer from protein back to nucleic acid or from protein to protein; prions are often noted in that context.

• Transcription — DNA template strand read 3’→5’ by RNA polymerase, producing mRNA 5’→3’; occurs in the nucleus (eukaryotes). Prokaryotes use a single RNA polymerase; eukaryotes use RNA Pol I (rRNA), Pol II (mRNA), Pol III (tRNA, 5S rRNA). Transcription is the cellular process of copying genetic information from DNA into RNA. It begins at promoter regions such as the Pribnow box and TATA box and ends through rho-dependent or rho-independent termination. RNA polymerase reads the DNA template strand 3’→5’ to produce RNA 5’→3’; in eukaryotes this occurs in the nucleus. Prokaryotes use a single RNA polymerase, while eukaryotes use RNA polymerase I for rRNA, polymerase II for mRNA, and polymerase III for tRNA and 5S rRNA.
  • promoters — Promoters are DNA regions where RNA polymerase and transcription factors bind to start transcription, and in bacteria they include the -10 and -35 elements recognized by sigma factors. They are especially important in operons and expression vectors, with common examples drawn from viral and mammalian sequences.

    • Pribnow box — The Pribnow box is the bacterial promoter element around the minus-10 region that helps RNA polymerase initiate transcription. Its consensus sequence is usually TATAAT, making it the prokaryotic analogue of the eukaryotic TATA box. It is a standard clue in questions about promoter architecture and sigma factor recognition.

    • TATA box — The TATA box is a short promoter DNA sequence recognized during initiation of eukaryotic transcription. It is named for its thymine-adenine repeat, is bound by TFIID through TBP, helps recruit RNA polymerase II near the transcription start site, and is often contrasted with the prokaryotic Pribnow box.

  • RNA Polymerase II — RNA polymerase II is the eukaryotic enzyme that transcribes mRNA, snRNA, and microRNA from DNA. Its C-terminal domain is rich in heptapeptide repeats that are phosphorylated during transcription, and it is famously inhibited by alpha-amanitin.

  • RNA polymerases — RNA polymerases are the enzymes that transcribe DNA into RNA, and the eukaryotic forms include the major polymerases recruited to promoters during transcription. Their C-terminal domain is marked by different phosphorylation states across initiation and elongation, and alpha-amanitin is a classic inhibitor of one of them.

  • RNA transcription — RNA transcription is the process by which RNA polymerase copies DNA into RNA, producing transcripts that can be capped and polyadenylated in eukaryotes. It begins at promoters with factors such as TBP and sigma factor and can end through rho-dependent or stem-loop mechanisms.

  • sigma factors — Sigma factors are bacterial proteins that guide RNA polymerase to promoters and initiate transcription. The best-known example is the housekeeping sigma factor, which is about 70 kilodaltons and recognizes the Pribnow box and -10 and -35 promoter elements.

• mRNA processing (eukaryotes) — 5’ cap added, 3’ poly-A tail added, introns removed by spliceosomes (leaving exons). Mature mRNA exported to cytoplasm.

• Spliceosome — large ribonucleoprotein complex (snRNPs) that removes introns from pre-mRNA and joins exons; composed of five snRNAs (U1, U2, U4, U5, U6) and associated proteins; alternative splicing allows one gene to encode multiple protein isoforms. It catalyzes splicing through two transesterification steps.

• Ribosome — molecular machine for translation; prokaryotic 70S (50S + 30S); eukaryotic 80S (60S + 40S); the large subunit catalyzes peptide bond formation (peptidyl transferase activity, actually rRNA-based — the ribosome is a ribozyme); mitochondrial/chloroplast ribosomes are 70S (consistent with endosymbiotic origin). Ribosomes are the molecular machines that carry out translation, using large and small subunits to read mRNA and assemble polypeptides. They are targeted by many antibiotics and other translation inhibitors.
  • Translation sites
    • P site — The P site is the ribosomal site that holds the growing polypeptide chain. tRNA usually moves from the A site into the P site, and initiator tRNA can enter there directly during translation.
• Genetic code — triplet codons (64 total); 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA); start codon AUG codes for methionine; code is degenerate (multiple codons per amino acid) and universal.
  • codon — A codon is a three-nucleotide sequence in messenger RNA that specifies an amino acid during translation or signals stop. It is a basic unit of the genetic code; because of genetic code degeneracy, 64 codons map onto 20 amino acids. AUG is the start codon, and codons were famously decoded by Marshall Nirenberg. The least-used stop codon can be reassigned in amber suppression to incorporate unnatural amino acids.
    • AUG — AUG is the mRNA start codon that initiates translation and specifies methionine.

  • codon degeneracy — Codon degeneracy is the redundancy of the genetic code, where multiple codons specify the same amino acid. This is why some mutations are synonymous and why wobble effects help explain codon usage patterns.

  • stop codons — The RNA stop codons are UAA, UAG, and UGA; in DNA these correspond to TAA, TAG, and TGA. They signal termination of translation and recruit release factors. The standard names amber, ochre, and opal are often used for the three signals. Stop codons are central to nonsense mutations and to techniques that suppress or recode translation for special amino acids such as selenocysteine, sometimes involving SECIS elements, suppressor strains, or engineered E. coli lacking rare stop signals.

    • stop codon — An mRNA nucleotide triplet that signals the end of translation by recruiting release factors. The standard stop codons are UAG, UAA, and UGA; they are also called termination codons, with amber, ochre, and opal as the classic names for the three standard examples. UGA can sometimes be recoded for selenocysteine when a SECIS element is present.

    • Uracil-Adenine-Adenine — UAA is one of the three stop codons in RNA translation, signaling termination of peptide synthesis. It is read using RNA bases as uracil-adenine-adenine.

  • wobble base pairing — Wobble base pairing is the nonstandard pairing that helps explain the degeneracy of the genetic code during translation. It allows some tRNAs to recognize more than one codon, often through flexible pairing at the third codon position.
    • wobble base pair — A wobble base pair is a nonstandard pairing in RNA, especially important in translation. Guanine-uracil pairing is the classic example, and it allows one tRNA anticodon to recognize more than one codon.
• Translation — ribosome reads mRNA 5’→3’; tRNA anticodons carry amino acids; A site (aminoacyl — incoming tRNA), P site (peptidyl — growing chain), E site (exit). Elongation: peptidyl transferase forms peptide bonds. Ends at a stop codon (release factor). It begins with initiation at the ribosome, commonly at an AUG start codon; in bacteria the first amino acid is typically formylmethionine. Its major stages are initiation, elongation, and termination, and it is the step of protein synthesis that converts the genetic code into a polypeptide.
  • Initiation factors
    • eukaryotic initiation factors — Eukaryotic initiation factors are the proteins that help start translation by recruiting ribosomal subunits and initiation complexes. Familiar examples include eIF2 and eIF4A, and defects or phosphorylation of these factors can shut down protein synthesis.

  • protein synthesis from mRNA — This function of the ribosome is the synthesis of a protein from an mRNA template. It is the central process of translation.

  • Wobble — Wobble is the relaxed base-pairing rule that lets a tRNA anticodon recognize more than one codon. It explains why the third codon position is often less specific during translation.

  • wobble hypothesis — The wobble hypothesis is Francis Crick’s explanation for how one tRNA can recognize multiple codons. It holds that pairing at the third codon position is less stringent than at the first two positions.

• Alternative splicing — the spliceosome can include or exclude different exons from the same pre-mRNA, generating multiple protein isoforms from a single gene; estimates suggest ~95% of human multi-exon genes are alternatively spliced; enables proteome diversity far exceeding the ~20,000 gene count. It can involve exon skipping, altered splice sites, or intron retention, and it is regulated in examples such as Drosophila sex determination and the LMNA gene.

• RNA polymerase, Schwann cells, and gradient — RNA polymerase is the enzyme that synthesizes messenger RNA. Schwann cells make the myelin sheath around peripheral nerves, and a gradient is a directional drive caused by differences in energy, temperature, or concentration.

• Transcription complexes
  • Mediator — Mediator is a transcriptional coactivator complex that helps connect transcription factors to RNA polymerase II. It has been associated with older names such as DRIP and TRAP and is important in chromatin looping during transcription.
• Nucleic acid secondary structure
  • stem-loop — A stem-loop, or hairpin loop, is a nucleic-acid secondary structure in which a strand folds back on itself to form a paired stem and an unpaired loop. It is common in RNA and can help form larger structures such as pseudoknots. In bacteria, SECIS elements use this structure to recode UGA as selenocysteine, and intrinsic transcription terminators also form stem-loops.
• RNA processing

  • 5′ cap — The 5′ cap is a 7-methylguanosine structure attached to the 5′ end, or front end, of eukaryotic pre-mRNA and mature mRNA. It helps protect transcripts from degradation, participates in splicing, and promotes translation initiation by serving as a ribosome-binding feature; it is also recognized by proteins associated with mature transcripts and RNA silencing pathways. Influenza targets the cap during cap snatching.

  • mRNA splicing — mRNA splicing is the post-transcriptional process that removes introns from pre-mRNA and ligates exons together. It involves the spliceosome, snRNPs, and lariat formation, and it can occur in alternative forms to produce different protein isoforms.

  • polyadenylation — Polyadenylation is the post-transcriptional process that adds a poly-A tail to the 3-prime end of mRNA. It is coordinated by CPSF, CstF, and PAP and matters for alternative processing and antibody expression.
    • poly-A tail — A stretch of adenine nucleotides added to the 3’ end of mature eukaryotic mRNA during RNA processing. It helps stabilize the transcript, protect it from degradation, aid nuclear export, and support translation; microRNAs can recruit decay machinery that shortens it during mRNA turnover.

  • RNA editing — RNA editing is a post-transcriptional process that changes RNA sequences by insertion, substitution, or deamination. It includes A-to-I and C-to-U editing, the kinetoplast guide-RNA system in trypanosomes, and the editing that produces distinct ApoB isoforms.

  • RNA splicing — RNA splicing is the post-transcriptional process that removes introns from pre-mRNA and joins exons together. Its alternative form can produce multiple transcripts from one gene, and it is carried out by the spliceosome, whose snRNPs create a lariat intermediate at a branch-point adenine.

    • lariat — A lariat is the looped RNA intermediate formed during spliceosome-mediated intron removal. It is created when the intron’s branch-point adenine attacks the 5’ splice site.

    • Spliceosome components
      • snRNPs — snRNPs, or small nuclear ribonucleoproteins, are the RNA-protein particles that make up the spliceosome. They include U1, U2, and U6, which are involved in recognizing splice sites and catalyzing intron removal.

    • spliceosomes — Spliceosomes are large RNA-protein complexes that remove introns from pre-mRNA. They are built from snRNPs and other proteins, and their catalytic action produces a lariat-shaped intron intermediate.

    • splicing — Splicing is the RNA-processing step that removes introns from pre-mRNA by two transesterification reactions that form a lariat intermediate. It is carried out by the spliceosome and can occur co-transcriptionally or post-transcriptionally to generate different protein isoforms.
• Translation machinery
  • Ribosomes — Ribosomes are the cellular machines that synthesize proteins by translating mRNA. They contain rRNA and protein, with peptidyl transferase activity supplied by rRNA in the large subunit, and have large and small subunits with A, P, and E sites for tRNA binding. They occur free in the cytosol or bound to rough ER, are measured in Svedberg units, and can be inhibited by antibiotics such as chloramphenicol and erythromycin in bacteria or mitochondria.
    • peptidyl transferase — Peptidyl transferase is the ribosomal activity that catalyzes peptide-bond formation during translation. It acts in the large ribosomal subunit between amino acids attached to tRNAs at the A and P sites.
• Protein synthesis
  • Translation factors
    • elongation factors — Elongation factors are translation proteins that help tRNA interact with the ribosome and advance through the elongation stage of protein synthesis. They include factors that deliver tRNA to the ribosome and factors that translocate the growing peptide chain and mRNA.
  • Translation initiation
    • fMet — fMet, or N-formylmethionine, is the modified methionine used to initiate protein synthesis in prokaryotes. It is delivered to the ribosomal P site by initiation factors rather than by EF-Tu.
• Translation initiation

  • Kozak consensus sequence — The Kozak consensus sequence is the eukaryotic mRNA context surrounding the AUG start codon that helps ribosomes initiate translation efficiently. Strong Kozak sequences have a purine at -3 and a G at +4, while weak ones can lead to leaky scanning.

  • Shine-Dalgarno sequence — The Shine-Dalgarno sequence is a prokaryotic ribosome-binding site in mRNA, usually located just upstream of the start codon. Its consensus sequence helps the ribosome initiate translation. Its name comes from John Shine and Lynn Dalgarno, and it is the bacterial analogue of the Kozak sequence in eukaryotes.

  • Shine-Delgarno sequence — The Shine-Dalgarno sequence is the purine-rich mRNA element used in prokaryotic translation initiation. It lies just upstream of the start codon and pairs with 16S rRNA to position the ribosome correctly.

• Nucleic acid polymerases
  • Polymerases — Polymerases are enzymes that synthesize DNA or RNA by adding nucleotides to a growing strand. They include DNA polymerases, RNA polymerases, reverse transcriptase, and the thermostable Taq polymerase used in PCR.
    • Polymerase — An enzyme that synthesizes nucleic acid chains by joining nucleotides together. It is central to PCR, DNA replication, and rolling-circle replication, with common examples including Taq polymerase, DNA polymerase, and RNA polymerase.
• RNA modifications
  • pseudouridine — Pseudouridine is the most abundant modified nucleoside in RNA and contains an unusual carbon-carbon glycosyl bond. It helps stabilize the TΨC loop of tRNA and is a hallmark modification in processed RNA.
• Ribozymes

  • Ribozyme — An RNA molecule that catalyzes chemical reactions, including self-cleavage, splicing, and other reactions. Ribozymes were first identified in studies of self-splicing introns in Tetrahymena and are often cited in support of the RNA world hypothesis. Classic examples include the hammerhead ribozyme, hairpin ribozyme, and group I and group II introns; some experimentally engineered RNAs are also ribozymes. The work on catalytic RNA helped support the RNA world hypothesis and won Sidney Altman and Thomas Cech a Nobel Prize.

  • Hammerhead ribozymes

    • MC Hammerhead ribozyme — The hammerhead ribozyme is a small catalytic RNA motif named here by combining the rapper MC Hammer with the ribozyme’s shape. It is an example of an RNA molecule capable of self-cleavage.
• RNA structure
  • Secondary structures
    • pseudoknot — A pseudoknot is an RNA structure in which part of one stem-loop folds into another, creating a crossing interaction. It appears in telomerase RNA and was first noted in turnip yellow mosaic virus.
• RNase III enzymes
  • Dicer — Dicer is an RNase III-family nuclease that processes double-stranded RNA into miRNA and siRNA precursors. It works in RNA interference, typically in the cytoplasm, and helps generate short RNA fragments that can be loaded into RISC.

Gene regulation

• Gene regulation

  • Enhancer — An enhancer is a regulatory DNA sequence that can boost transcription from far away from a gene. Activator proteins bind these elements to help recruit the transcription machinery, and rearrangements of enhancers can contribute to oncogene overexpression such as myc.

  • enhancers — Enhancers are cis-regulatory DNA elements that increase transcription, often by binding transcription factors far from a promoter. They can loop to interact with promoters and are discussed in developmental genetics, chromatin conformation, and gene regulation.

  • François Jacob and Jacques Monod — François Jacob and Jacques Monod were French biologists and scientists who helped formulate the operon model of gene regulation, first postulated it, and are associated with the lac operon and the regulation of gene expression in bacteria. They also helped introduce the term messenger RNA and shared the 1965 Nobel Prize in Physiology or Medicine with André Lwoff.

  • lac operon — The lac operon is Jacob and Monod’s classic model of gene regulation in E. coli, a prokaryotic system controlling lactose metabolism and the production of beta-galactosidase. It is the standard example of an inducible operon in molecular biology. The operon includes the structural genes lacZ, lacY, and lacA, along with an operator, promoter, and the lacI repressor that shuts transcription off when lactose is absent. It is induced by allolactose or IPTG and is repressed when glucose is abundant.

  • Operon — An operon is a bacterial or other prokaryotic gene-regulation unit in which multiple related genes are controlled by a single promoter and operator and transcribed together as a single polycistronic mRNA. Classic examples include the lac operon, which governs lactose metabolism in E. coli, and the trp operon. Operons are regulated by repressors, inducers, activators such as CAP bound to cAMP, and the concept was worked out by Jacob and Monod in E. coli.

    • Inducible — An inducible operon is usually off but can be switched on when a small molecule interacts with a regulatory protein. The lac operon is the standard example of this type of gene regulation.

    • Jacob-Monod theory — The Jacob-Monod theory is the classic model of prokaryotic gene regulation centered on the operon. It explains how repressors and operators control transcription, and it is associated with the Nobel-winning work of Jacob and Monod.

    • operator — In genetics, an operator is the DNA sequence in an operon where a repressor binds to block transcription. It is also the regulatory site sometimes called the O locus.

    • operon model — The operon model is the Jacob and Monod model of gene regulation, developed to explain bacterial enzyme synthesis such as beta-galactosidase production. It separates structural genes from regulatory elements that control their expression.

    • trp operon — The trp operon is a repressible, attenuated gene cluster that controls tryptophan biosynthesis in bacteria. Unlike the lac operon, it is turned off when tryptophan is present. Its regulation depends on leader-peptide translation and alternative RNA hairpins that determine whether transcription continues.

  • Repressors
    • repressor — A repressor is a regulatory protein that binds DNA to inhibit transcription. In gene control, repressors often act at operator or promoter regions to block RNA polymerase and shut down expression.
  • RNA elements
    • riboswitches — Riboswitches are cis-acting RNA elements, usually in the 5-prime untranslated region, that regulate gene expression by changing shape when they bind a small molecule. They can control transcription or translation by forming terminator stem-loops or by sequestering Shine-Dalgarno sequences.
      • riboswitch — A riboswitch is a regulatory RNA element, usually in the 5’ UTR, whose aptamer binds a metabolite and changes gene expression. It can control transcription or translation by forming alternative RNA structures that expose or hide the relevant signals.
  • transcription factors — Transcription factors are proteins that bind specific DNA sequences to regulate gene expression and the conversion of DNA into mRNA by RNA polymerase. They can activate or repress transcription by interacting with promoters, enhancers, and the transcriptional machinery. General transcription factors recruit RNA polymerase at promoter regions such as the TATA box and help assemble the pre-initiation complex, while specific transcription factors tune gene expression. Many have DNA-binding domains such as helix-turn-helix or zinc finger motifs.
    • AP-1
      • c-Jun — c-Jun is an AP-1 transcription factor subunit and oncogene that pairs with c-Fos through a leucine zipper. It is activated by JNK phosphorylation and helps regulate cell-cycle genes such as cyclin D1 and cyclin A2.

    • Foxp3 — Foxp3 is a transcription factor required for regulatory T-cell differentiation and is also known as scurfin. Mutation or deficiency of Foxp3 causes IPEX syndrome, an X-linked disorder marked by immune dysregulation, polyendocrinopathy, and enteropathy.

    • NF-kappa-B — NF-kappa-B is a family of transcription factors central to inflammatory signaling and immune responses. It was originally identified through binding to an enhancer in the immunoglobulin light-chain locus, and proteins such as IKK activate it by freeing it from inhibitory proteins.

    • POU transcription factors
      • Oct — Oct factors are a family of POU-domain transcription factors that bind the sequence ATGCAAAT. They include members involved in the fibroblast growth factor 4 enhancer and in the Yamanaka factors.
    • SOX family
      • Sox — SOX is a family of transcription factors named for the SRY-related HMG box domain. SOX proteins are associated with cell fate control and reprogramming, including the Yamanaka-factor work on iPSCs. The other clue sense points to baseball teams such as the White Sox, Red Sox, and the Black Sox scandal.

• Operon (prokaryotes) — a cluster of co-regulated genes; the lac operon (E. coli) is induced by allolactose when glucose is absent (negative control by lac repressor) and by CAP-cAMP (positive control when glucose is absent); the trp operon is repressed when tryptophan is abundant (tryptophan acts as corepressor).

• Eukaryotic regulation — more complex; involves transcription factors, enhancers, silencers, chromatin remodeling (histone acetylation opens chromatin; methylation often silences), and non-coding RNAs (miRNA, siRNA).

• Epigenetics — heritable changes in gene expression without changes to DNA sequence; mechanisms include DNA methylation and histone modification. Typical examples include DNA methylation and histone modification; the field is also associated with gene silencing and Waddington’s epigenetic landscape.
  • chromatin immunoprecipitation — Chromatin immunoprecipitation, or ChIP, is a method for capturing DNA fragments bound to specific proteins with antibodies. It is used to map protein-DNA interactions and chromatin states, and can be paired with microarrays or sequencing in ChIP-chip and ChIP-seq experiments after crosslinking and shearing the chromatin.

    • ChIP-seq — ChIP-seq is a chromatin immunoprecipitation method used to map DNA bound to a particular protein. It crosslinks protein-DNA complexes, shears chromatin, pulls down the target with an antibody, and then identifies the recovered fragments by sequencing.

    • chromatin immunoprecipitation or ChIP — Chromatin immunoprecipitation, or ChIP, is a method for isolating DNA bound to proteins such as transcription factors or modified histones. After cross-linking and shearing, antibodies pull down the complexes so the associated DNA can be analyzed, often by sequencing.

  • DNA methylation — DNA methylation is an epigenetic modification in which methyl groups are added to DNA, usually at cytosines in CpG islands. It is important in imprinting, X-inactivation, and gene silencing, and it can be detected by bisulfite-based methods.

  • histone acetylation — Histone acetylation is a post-translational modification in which acetyl groups are added to lysine residues on histones. By weakening histone-DNA interactions, it generally opens chromatin and increases transcription, and it is reversed by histone deacetylases.

  • methylation — Methylation is the addition of a methyl group, often to DNA or histone tails, and it can activate or silence gene expression. It is central to CpG island silencing, genomic imprinting, and DNA mismatch repair, which distinguishes old from newly replicated strands by their methylation state.

  • X-inactivation — X-inactivation is the epigenetic silencing of one X chromosome in female mammals, producing Barr bodies and helping equalize gene dosage. It is controlled by XIST and Tsix and can be detected indirectly with assays such as HUMARA-based PCR tests.
    • Lyon hypothesis — The Lyon hypothesis proposes that in cells with multiple X chromosomes, all but one are inactivated during embryonic development. The inactive X chromosomes condense into Barr bodies, making the idea a standard clue in genetics questions about X-chromosome dosage compensation.
• DNA regulatory elements
  • promoter — A promoter is the DNA region where RNA polymerase and transcription factors bind to initiate transcription. Promoters in prokaryotes include the minus-10 and minus-35 elements, while eukaryotic promoters often contain a TATA box.
• DNA-binding domains
  • zinc fingers — Zinc fingers are DNA-binding motifs stabilized by a zinc ion coordinated by cysteines and histidines. They appear in zinc finger nucleases and in proteins such as PRDM9.
• Histone-modifying enzymes
  • histone acetyltransferases — Histone acetyltransferases are enzymes that acetylate lysine residues on histones, reducing their positive charge and loosening chromatin. They include GNAT and MYST family members and act oppositely to HDACs.
• RNA interference — RNA interference, or RNAi, is a post-transcriptional gene-silencing pathway in which double-stranded RNA is processed by Dicer, then small RNA guides are loaded into RISC with Argonaute to target complementary messenger RNA for destruction or suppression. It was first elucidated in C. elegans by Fire and Mello, whose Nobel-winning work established the mechanism. RNAi is widely used in research to knock down gene expression and in pooled genetic screens, and it is also associated with factors such as Drosha and the therapeutic drug patisiran.
  • Argonaute proteins
    • argonaute — Argonaute proteins are catalytic components of the RISC complex involved in RNA interference and gene silencing. They bind small RNAs and help direct the destruction or suppression of complementary RNA targets.
  • Small interfering RNA — Small interfering RNA is a double-stranded RNA that functions in RNA interference. It helps silence gene expression and is thought to have evolved as a defense against RNA viruses.
• Chromatin regulators
  • Histone deacetylases — Histone deacetylases remove acetyl groups from histone lysines and generally promote chromatin compaction and reduced transcription. Their activity is opposed by histone acetyltransferases, and some members are the NAD+-dependent sirtuins.

Mutations

• Point mutations — substitutions: missense (change amino acid), nonsense (create stop codon), silent (synonymous, no amino acid change). They are changes in DNA at a single nucleotide position, and can also involve insertion or deletion of one base; such changes may be frame-shifting. Sickle-cell anemia is a classic missense example.

  • point mutation — A point mutation is a change in a single nucleotide or base pair of DNA. Such mutations can produce missense or nonsense changes and are the class of mutations detected by tests like Ames screening.

  • silent mutation — A silent mutation is a point mutation that changes the DNA sequence without altering the amino acid sequence of the encoded protein. It is also called a synonymous mutation and often occurs in exons.

• Frameshift mutations — insertions or deletions shift the reading frame; generally severe. They result from insertions or deletions of nucleotides that are not in multiples of three, shifting the reading frame. They often have severe effects on coding sequences and can arise from slippage or error-prone repair.
  • frameshift mutation — A frameshift mutation results from an insertion or deletion of nucleotides in a number not divisible by three, shifting the reading frame of a gene and altering every downstream codon. The resulting protein is usually severely disrupted or nonfunctional. These mutations disrupt the triplet codon system and are commonly contrasted with substitutions; they can arise from replication errors or mutagenic damage such as UV radiation.

• Chromosome mutations — deletion, duplication, inversion, translocation (e.g., Philadelphia chromosome in CML from a translocation between chromosomes 9 and 22).

• Mutagens — UV radiation (pyrimidine dimers), X-rays (double-strand breaks), chemical mutagens (base analogs, alkylating agents).

  • intercalating agents — Intercalating agents are planar mutagens that slip between DNA base pairs and distort the double helix. Classic examples include ethidium bromide, acridine orange, and doxorubicin, all of which can promote frameshift mutations by unwinding DNA.

  • Mutagen — A mutagen is an agent that causes changes in DNA and can lead to substitutions, frameshifts, or deletions. Examples include ethidium bromide, 5-bromouracil, ultraviolet light, X-rays, and mustard gas.

• Gene mutations
  • Deletion and insertion — Deletion and insertion are types of mutation in which nucleotide pairs are lost or added to a gene. They are common examples of small-scale changes that can alter a DNA sequence.
• Chromosomal mutations

  • chromosomal inversion — A chromosomal inversion is a rearrangement in which a chromosome segment flips end for end. These mutations may be paracentric or pericentric, and they can suppress recombination by forcing a loop during meiosis.

  • translocation — A translocation is a chromosomal rearrangement in which material moves between chromosomes, and it may be balanced or unbalanced. Quizbowl clues often use the Philadelphia chromosome, Burkitt lymphoma, and Robertsonian translocations as examples.

• Chromosomal rearrangements
  • Inversions
    • chromosomal inversions — Chromosomal inversions are large-scale rearrangements in which a chromosome segment is reversed. They can be paracentric or pericentric, and they are often discussed for their effects on recombination and local adaptation.
• Chromosomal translocations
  • Robertsonian translocation — A Robertsonian translocation is a whole-arm chromosomal fusion in which the long arms of two acrocentric chromosomes join and the short arms are often lost. It can cause some cases of Down syndrome, especially when chromosome 21 fuses with chromosome 14.
• Mutants
  • auxotrophs — Auxotrophs are mutant organisms, often bacteria, that cannot synthesize an essential nutrient and therefore require it in the growth medium. They are used in experiments such as the Ames test and were important to Beadle and Tatum’s one gene-one enzyme work.
    • auxotroph — An auxotroph is a mutant organism that cannot synthesize a required organic compound such as an amino acid, nucleotide, or vitamin. Because of that defect, it must obtain the missing compound from the growth medium.

Related Concepts and Applications

• Gene structure
  • exons — Exons are the coding sequences of genes that remain in mRNA after splicing removes introns. They are central to topics like exon skipping in dystrophin disorders and the exon junction complex.
• Molecular biology
  • Blotting techniques

    • dot blot — A dot blot is a hybridization technique used to detect or estimate specific DNA or RNA in a sample. The sample is spotted directly onto a membrane and probed, rather than being separated by electrophoresis first.

    • Southern blotting — Southern blotting detects specific DNA fragments by separating them on a gel, transferring them to a membrane, and probing them with labeled nucleic acid by hybridization. The technique is named for Edwin Southern, and its relatives include Northern and Western blotting.

  • central dogma of molecular biology — The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein. Francis Crick proposed it as a one-way model of information transfer, even though edge cases like reverse transcription complicate the slogan version.

  • chromatin — Chromatin is the DNA-protein material of the nucleus that packages interphase DNA into nucleosomes and eventually chromosomes. It appears in loosely packed euchromatin and densely packed heterochromatin forms, and its structure is remodeled by complexes such as SWI/SNF, INO80, and NuRD. Techniques like ChIP and Hi-C are used to study its organization.

    • chromatin accessibility — Chromatin accessibility is the property of DNA being open or exposed enough for binding, cleavage, or transposase insertion. It is measured by methods such as DNase I hypersensitivity assays, ATAC-seq, and related chromatin-profiling protocols.

    • Chromatin remodeling — Chromatin remodeling is the ATP-dependent alteration of nucleosome position or composition to regulate transcription. Complexes such as SWI/SNF, Polycomb-related factors, and Brahma can open or reposition chromatin so DNA becomes more or less accessible.

    • heterochromatin — Heterochromatin is the tightly packed, transcriptionally silent form of chromatin. It is associated with condensed regions of the genome that are often inaccessible for gene expression.

    • Histone readers
      • bromodomains — Bromodomains are protein domains that recognize acetylated lysine residues on histone tails. They are common targets of BET inhibitors and are contrasted with chromodomains, which can bind methylated histones.
    • Histones — Histones are the small, highly basic proteins around which eukaryotic DNA wraps to form nucleosomes and chromatin. They are rich in lysine and arginine, and their N-terminal tails are major targets of acetylation, methylation, phosphorylation, and other epigenetic modifications that help regulate gene expression and DNA repair. The major classes are H1, H2A, H2B, H3, and H4.

      • H1 — Histone H1 is the linker histone that sits on the outside of the nucleosome. It helps stabilize DNA packaging by holding neighboring nucleosomes together.

      • Histone — Histones are the basic proteins around which DNA is wrapped to form nucleosomes. Their chemical modification, including acetylation and related changes, helps regulate chromatin structure and gene transcription.

    • Nucleosomes — Nucleosomes are the basic units of chromatin, made of about 150 base pairs of DNA wrapped around a histone octamer. They are often described as beads on a string, and in archaea clues may refer to hypernucleosomes or other specialized chromatin-like structures.

  • chromosome conformation capture — Chromosome conformation capture is a family of methods used to detect long-range interactions between regions of chromatin, such as promoter-enhancer looping. Variants include 3C, 4C, 5C, Hi-C, Capture-C, and related interaction-mapping techniques.

  • Cloning vectors
    • Cosmid — A cosmid is an artificial cloning vector that combines plasmid features with lambda phage cos sites. Because it can be packaged into phage particles, it is useful for cloning relatively large DNA fragments.
  • DNA replication — DNA replication is the semiconservative copying of genetic material during S phase, carried out by DNA polymerase with helicase, primase, and ligase. Standard clues include the replication fork, Okazaki fragments on the lagging strand, and the Meselson-Stahl experiment proving semiconservative copying.

    • 5 prime to 3 prime — DNA synthesis proceeds in the 5 prime to 3 prime direction. Replication and polymerase extension add nucleotides to the 3-prime end, so growth is conventionally described from 5-prime to 3-prime.

    • DNA polymerase — DNA polymerases are the enzymes that synthesize new DNA strands during replication. Clues include Arthur Kornberg’s discovery, the Klenow fragment, Taq polymerase in PCR, and specialized translesion forms used after DNA damage.

    • DNA polymerase III — DNA polymerase III is the bacterial enzyme that adds nucleotides to a growing DNA strand during replication. It synthesizes continuously on the leading strand and in Okazaki fragments on the lagging strand.

    • lagging strand — The lagging strand is the DNA strand synthesized discontinuously in short Okazaki fragments during replication. DNA polymerase must build it away from the replication fork in the 5’ to 3’ direction, and ligase later joins the fragments.

    • Matthew Meselson and Frank Stahl — Matthew Meselson and Frank Stahl are the biologists known for the classic experiment supporting semiconservative DNA replication. Using density-gradient centrifugation, they showed that after replication each daughter molecule contains one old strand and one newly synthesized strand.

    • origins of replication — Origins of replication are DNA sites where replication begins, with helicases unwinding the duplex to start fork movement. In eukaryotes they are licensed by the origin recognition complex, while bacterial plasmids and chromosomes often contain specific ori sequences such as oriC.

    • primosome — A primosome is the protein complex that helps start DNA replication by making RNA primers. It includes primase and helicase components and is used repeatedly to initiate Okazaki fragments on the lagging strand.

    • replication fork — A replication fork is the Y-shaped structure where DNA unwinds and the leading and lagging strands are synthesized. Helicases open it, and Okazaki fragments are made on the lagging strand.

    • Replication strands
      • leading strand and lagging strand — During DNA replication, the leading strand is synthesized continuously while the lagging strand is built discontinuously in Okazaki fragments. The two strands are distinguished by the direction in which the replication machinery can copy the template.

    • RNA primers — RNA primers are short RNA segments laid down to give DNA polymerase a starting point for DNA synthesis. They anneal to the template strand and are later removed and replaced with DNA during replication.

    • rolling circle replication — Rolling circle replication is a DNA replication mechanism used by some circular plasmids and bacteriophages. A nick exposes a free 3’ end that is extended around the template, producing long strands that can be copied into multiple genome equivalents.

    • semiconservative DNA replication — Semiconservative DNA replication is the model in which each daughter double helix contains one old strand and one newly synthesized strand. It was confirmed by the Meselson-Stahl experiment, which used nitrogen isotopes and a cesium chloride gradient to distinguish the replicated DNA.

    • origin of replication — The origin of replication is the DNA sequence where replication begins and where initiator proteins such as DnaA bind in bacteria and plasmids. Plasmids need an ori to be copied, and it helps determine plasmid copy number.
  • DNA sequencing
    • sequencing DNA — Sequencing DNA is the process of determining the order of nucleotides in a DNA molecule, often extended to whole genomes. Methods associated with it include Sanger sequencing, shotgun sequencing, and modern next-generation techniques.
  • DNA structure
    • Maurice (Hugh Frederick) Wilkins — Maurice Hugh Frederick Wilkins was a scientist whose X-ray diffraction work helped reveal DNA’s structure. He shared the Nobel Prize with Watson and Crick for the discovery of DNA’s double helix.
  • homologous recombination — Homologous recombination is the DNA repair and exchange process in which similar strands swap genetic material, including the crossover events seen in meiotic prophase I. It also underlies tools such as gene knockout, site-specific recombination systems, and homology-directed repair in genome editing.
    • Holliday junctions — Holliday junctions are four-stranded DNA intermediates that form during homologous recombination and repair. They are resolved by helicases and resolvases to produce crossover or noncrossover products. Their branch migration and stacked or open conformations make them central to meiotic recombination and DNA repair.
      • Holliday junction — A Holliday junction is the four-stranded, cross-shaped DNA intermediate formed during homologous recombination, where strands exchange partners at a branch point. It can undergo branch migration and be resolved by enzymes such as RuvC, producing either crossover or noncrossover products.
  • Hybridization techniques
    • in situ hybridization — In situ hybridization is a lab technique that uses labeled DNA or RNA probes to bind complementary sequences in chromosomes or tissue. Its fluorescent form, FISH, is used to detect specific genes and chromosome abnormalities such as the BCR-ABL fusion.

  • melting temperature — Melting temperature, or Tm, is the temperature at which half of the hydrogen bonds between DNA strands break and double-stranded DNA begins to denature. It matters in PCR and qPCR because primers must anneal below it, and GC-rich sequences have higher values.

  • Mobile genetic elements
    • long terminal repeats — Long terminal repeats are repeated sequences found at both ends of retroviral genomes and LTR retrotransposons. They are associated with elements such as HIV and ERV retrotransposons, helping define major classes of mobile DNA.
  • mRNA processing
    • poly(A) tail — The poly-A tail is the stretch of adenosines added to the 3’ end of most mature mRNAs to increase stability and aid translation. Its length is regulated by processing factors such as CPSF, CstF, PABP, TRAMP, and PARN.
  • Nucleic acid structures
    • hairpins — Hairpins are looped structures that can appear in nucleic acids and are important in processes such as viral fusion and rolling-hairpin replication. Short hairpin RNA is processed by Dicer into siRNA.
  • PCR

    • primers — Primers are short nucleotide sequences that start DNA replication and provide the starting point for PCR. They must be designed to avoid self-binding and other secondary structure, which is why software tools optimize their melting behavior before use. The word also names old elementary-school textbooks such as the New England Primer and McGuffey primers.

    • reverse transcription PCR — Reverse transcription PCR, or RT-PCR, is a PCR method used to detect and quantify RNA by first converting it into DNA. It is a standard technique in molecular biology for working with RNA viruses and gene-expression samples.

  • primers — Short nucleic acid sequences that provide a starting point for DNA synthesis. In PCR, short single-stranded DNA oligonucleotide primers are used in pairs to define the target region for amplification; they anneal to template DNA so a polymerase such as Taq can extend from them. Primers can form primer-dimers, and in DNA replication they also initiate synthesis on Okazaki fragments.

  • proteomics — Proteomics is the study of the full set of proteins an organism can produce, analogous to genomics for DNA. It is often approached with top-down or bottom-up mass spectrometry and techniques such as 2-D gel electrophoresis.

  • Restriction digestion
    • DNA restriction digest — A DNA restriction digest is the cutting of DNA by restriction endonucleases such as EcoRI and HindIII. It creates blunt or sticky ends for cloning, and the products can be analyzed by RFLP.
  • restriction enzymes — Bacterial endonucleases that cut double-stranded DNA at specific recognition sites, often palindromic sequences, producing sticky or blunt ends. They originally functioned as a defense against invading DNA or phages and became foundational tools in recombinant DNA work, cloning, plasmid construction, DNA mapping, and DNA fingerprinting/RFLP analysis. Examples include EcoRI, HindIII, BamHI, and NotI. Their activity can be affected by methylation or star activity under poor reaction conditions; they are associated with Arber, Nathans, and Smith.

    • EcoRI — EcoRI is a restriction enzyme first isolated from E. coli. It recognizes the sequence GAATTC and cuts it to produce sticky ends.

    • restriction enzymes or endonucleases — Restriction enzymes, or restriction endonucleases, are bacterial proteins that cut DNA at specific recognition sequences. They are used to make blunt or sticky ends for cloning and recombinant DNA work, with EcoRI and HindIII among the classic examples.

    • star activity — Star activity is the loss of specificity in a restriction enzyme digest, causing cleavage at sequences similar to the intended recognition site. It is promoted by nonoptimal reaction conditions such as unusual ions, high pH, or other deviations from standard buffer conditions.

  • reverse transcription — Reverse transcription is the process of synthesizing DNA from an RNA template. It is carried out by reverse transcriptase in retroviruses such as HIV and is often described as reversing the central dogma.

  • Transfection — The introduction of foreign nucleic acids into eukaryotic cells, especially mammalian cells. Common methods include electroporation, calcium phosphate or calcium chloride methods, liposome fusion, and other laser- or pore-based techniques.

  • Vectors
    • cosmids — Cosmids are cloning vectors that combine plasmid features with lambda phage packaging signals. They can carry relatively large DNA inserts and are useful for constructing genomic libraries and chromosome walking.
• restriction fragment length polymorphism — Restriction fragment length polymorphism is a DNA analysis method and marker based on comparing fragment lengths after restriction enzyme digestion. It reflects variation in restriction-enzyme cut sites, including sequence changes such as single-nucleotide polymorphisms, and can be detected by Southern blotting. It was once used in genetic fingerprinting, paternity testing, disease testing, and other Southern blot analyses.

Molecular Mechanisms in Detail

• Reverse transcriptase — enzyme that synthesizes DNA from an RNA template; discovered by Howard Temin and David Baltimore (1970; 1975 Nobel Prize); present in retroviruses (e.g., HIV); the canonical exception to the central dogma (RNA → DNA); also underlies LINE retrotransposons. It is used to make cDNA libraries and is targeted by antiviral drugs such as AZT.

• Molecular replication

  • self-replication — Self-replication is the process by which a system or molecule makes copies of itself. It is central to discussions of the RNA world, hypercycles, and major transitions in evolution.

Mendelian and Population Genetics

Mendelian genetics

• Law of Segregation — alleles separate during gamete formation; each gamete carries one allele per gene. It is one of Mendel’s foundational laws of inheritance and is confirmed by meiosis.

• Law of Independent Assortment — alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes or far apart on the same chromosome). Mendel’s law of independent assortment states that alleles for different traits are passed into gametes independently. Dihybrid-cross ratios such as 9:3:3:1 rely on this principle, and linked genes are the classic exception. It is Mendel’s second law.

• Dominance — complete dominance (one allele fully masks the other); incomplete dominance (heterozygote is intermediate); codominance (both alleles expressed equally, e.g., AB blood type). The term also includes dominant-negative effects.

  • codominance — Codominance is the genetic relationship in which both alleles are fully expressed in the heterozygote. The ABO blood group system is a classic example, since the A and B alleles can both appear in the phenotype.

  • incomplete dominance — Incomplete dominance is an inheritance pattern in which heterozygotes show an intermediate phenotype rather than a fully dominant one. It is commonly illustrated by pink snapdragons, pink four o’clocks, and other traits that blend the parental forms.

• Dihybrid cross — two heterozygous parents yield 9:3:3:1 phenotypic ratio among offspring. A dihybrid cross tracks two traits at once, usually between heterozygous parents, and is commonly represented with a 16-box Punnett square.

• Testcross — crossing an unknown genotype with a homozygous recessive individual to determine the unknown genotype.

• Linked genes — genes on the same chromosome violate independent assortment; recombination frequency is used to map them; 1 map unit (centimorgan) = 1% recombination frequency. The closer two genes are on the same chromosome, the more likely they are to be inherited together. Linkage can be analyzed with recombination frequency, linkage maps, and LOD scores.

• Chi-square test — statistical test used in genetics to determine whether observed ratios deviate significantly from expected Mendelian ratios; χ² = Σ[(observed − expected)²/expected]; compared to critical value at appropriate degrees of freedom. It is attributed to Karl Pearson and is also used for goodness-of-fit tests and for testing independence in contingency tables, comparing observed counts with expected values to judge whether deviations are likely due to chance.

• Sex-linked traits — genes on X chromosome (X-linked); males (XY) are hemizygous; examples: red-green color blindness, hemophilia A.
  • sex-linked — Sex-linked traits are controlled by genes on the sex chromosomes, especially the X chromosome. Classic examples include color blindness and hemophilia, and related disorders such as Turner syndrome and Klinefelter syndrome reflect abnormalities of the sex chromosomes.
    • Sex-linked trait — A sex-linked trait is determined by an allele on a sex chromosome, often the X chromosome, and is therefore expressed more often in males when recessive. Classic examples include red-green color blindness and hemophilia.
  • Sex-linked inheritance
    • X-linked recessive — X-linked recessive inheritance is a sex-linked pattern in which affected males are much more common than affected females. It is seen in disorders such as hemophilia and Duchenne muscular dystrophy, and it produces the classic inheritance pattern where unaffected parents can have affected sons but unaffected daughters.

• Morgan and Drosophila — Thomas Hunt Morgan used Drosophila melanogaster to establish that genes reside on chromosomes; discovered sex-linkage (white eye mutation, 1910), genetic linkage, and crossing over; his lab created the first genetic maps.

• Pedigrees
  • pedigree — A pedigree is a genetic family tree used to trace inheritance patterns and family relationships. Standard symbols include circles for females, squares for males, shaded figures for affected individuals, and special marks for miscarriages or consanguinity.
• Sex chromosomes

  • X chromosome — The X chromosome is the sex chromosome whose mutations cause conditions such as fragile X syndrome, hemophilia, and red-green color blindness. In females, one X is inactivated by lyonization, producing Barr bodies; Turner syndrome results from monosomy X, while Klinefelter syndrome includes an extra X.

  • X chromosomes — X chromosomes are the human sex chromosomes that determine biological sex along with the Y chromosome. They are the ones subject to X-inactivation, producing Barr bodies, and extra or missing copies are associated with disorders such as Klinefelter syndrome and Turner syndrome. Many sex-linked traits, including red-green color blindness and hemophilia, are carried on them.

Genetic disorders

• Genetic disorders

  • Albinism — Albinism is a hereditary genetic condition in which the body produces little or no melanin, causing very pale skin, hair, and eyes that may appear pink. It is often caused by defects in tyrosinase or other pigment-pathway genes. It is associated with Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, and Waardenburg syndrome. The term is also used for animals and plants with a damaging lack of pigment.

  • polydactyly — Polydactyly is the condition of having extra fingers or toes. It appears in literature and folklore as a striking inherited trait and is also seen in some cats. The condition is especially memorable in characters such as Count Tyrone Rugen in The Princess Bride.

• Cystic fibrosis — autosomal recessive; caused by mutations in the CFTR gene (chromosome 7, most common allele ΔF508); CFTR encodes a chloride ion channel; loss of function causes thick, sticky mucus in the lungs, pancreas, and intestines; most common life-shortening autosomal recessive disorder in people of European descent. It is often diagnosed by a positive sweat test and can cause recurrent lung infections, pancreatic insufficiency, meconium ileus, poor vitamin absorption, and male infertility from congenital absence of the vas deferens. CFTR modulators include ivacaftor and the lumacaftor/tezacaftor/elexacaftor combination.

• Sickle-cell disease — autosomal recessive; single point mutation in the HBB gene (Glu→Val at position 6); causes RBCs to sickle under low oxygen; classic example of pleiotropy and heterozygote advantage (sickle-cell trait confers malaria resistance). It can cause vaso-occlusive pain crises, hemolytic anemia, organ damage, jaundice, gallstones, autosplenectomy, acute chest syndrome, and Salmonella osteomyelitis; it is often associated with African ancestry and may be treated with hydroxyurea.

• Tay-Sachs disease — autosomal recessive; deficiency of hexosaminidase A; GM2 ganglioside accumulates in neurons; progressive neurodegeneration; fatal in early childhood; high carrier frequency in Ashkenazi Jewish, French-Canadian, and Cajun populations. It is an inherited lysosomal storage disorder caused by HEXA mutations and is classically associated with a cherry-red macular spot.

• Huntington’s disease — autosomal dominant; caused by a CAG trinucleotide repeat expansion (>36 repeats) in the HTT gene; encodes a toxic polyglutamine (polyQ) protein; progressive neurodegeneration; onset typically 30–50 years; full penetrance. It affects the striatum, especially the caudate and putamen, and commonly causes chorea, psychiatric changes, and dementia/cognitive decline. The gene is on chromosome 4, and anticipation is characteristic, with repeat length often increasing in later generations.

• Down syndrome (Trisomy 21) — presence of a third copy of chromosome 21; most often due to non-disjunction during maternal meiosis I; risk increases with maternal age; features include intellectual disability, characteristic facial features, and increased risk of congenital heart defects and leukemia. It is the most common chromosomal aneuploidy compatible with life and is associated with increased Alzheimer’s risk.

• Klinefelter syndrome (47,XXY) — sex chromosome aneuploidy in males; extra X chromosome; features include tall stature, small testes, infertility, and sometimes mild learning differences; most common sex chromosome aneuploidy.

• Turner syndrome (45,X) — sex chromosome monosomy in females; features include short stature, gonadal dysgenesis, infertility, and cardiac defects (coarctation of the aorta); the only viable monosomy in humans.

• Hemoglobin disorders
  • sickle cell trait — Sickle cell trait is the carrier state for an altered HBB gene, usually with one mutated copy rather than two. It is often noted for partial protection against malaria, while the two-copy form produces sickle cell disease or sickle cell anemia.
• Chromosomal abnormalities
  • aneuploidy — Aneuploidy is the presence of an abnormal number of chromosomes, such as monosomy or trisomy. It can be detected by karyotyping and is associated with disorders including Turner syndrome, Klinefelter syndrome, and Down syndrome.
• Chromosomal disorders
  • Edwards syndrome — A severe chromosomal disorder caused by trisomy 18, associated with severe developmental abnormalities and very high infant mortality. Common findings include clenched fists with overlapping fingers, heart and kidney defects, and rocker-bottom feet.
• Chromosomal conditions
  • Sex chromosome aneuploidies
    • double Y syndrome — Double Y syndrome, or XYY, is a chromosomal condition affecting males who have an extra Y chromosome. It is associated with tall stature and was once incorrectly linked to violence, though later studies found only mild cognitive effects on average.

Extensions of Mendelian genetics

• Polygenic inheritance — multiple genes affect a single trait (e.g., skin color, height); produces a continuous distribution.
  • polygenic — Polygenic describes traits influenced by many genes rather than a single locus. The term also appears in genetics for risk scores and for continuous traits such as height, hair color, and blood pressure.

• Pleiotropy — one gene affects multiple phenotypic traits (e.g., sickle-cell mutation affects RBC shape, oxygen carrying, and malaria resistance). It is illustrated by disorders such as Marfan syndrome and phenylketonuria.

• Epistasis — one gene masks or modifies the expression of another; example: coat color in Labrador retrievers (B/b locus determines color; E/e locus determines whether pigment is deposited at all). It explains deviations from Mendelian ratios such as 9:3:3:1 and can be described in terms of suppressor or reciprocal effects.
  • epistatic gene — An epistatic gene is one that masks or suppresses the expression of another gene’s phenotype. Classic examples include albinism, where one gene can prevent pigment-related traits from appearing even when other loci are present.

• ABO blood groups — controlled by three alleles (IA, IB, i); IA and IB are codominant; i is recessive; illustrates multiple alleles at one locus.

• Genomic imprinting — expression of a gene depends on which parent it came from; imprinted genes carry methylation marks; disruption causes Prader-Willi syndrome (paternal copy missing) and Angelman syndrome (maternal copy missing), both involving the same region on chromosome 15. It is an epigenetic phenomenon in which gene expression depends on whether the allele came from the mother or the father. It is classically associated with Prader-Willi and Angelman syndromes and with DNA methylation marks that silence one parental copy.

• Polyploidy
  • autopolyploid — An autopolyploid is an organism with more than two chromosome sets derived from the same species.
• Quantitative genetics
  • additive genetic variance — Additive genetic variance is the heritable component of phenotype due to the average effects of alleles, excluding dominance and epistasis. It is the key quantity in Fisher’s fundamental theorem and in discussions of the G-matrix and the lek paradox.
• ABO blood group system
  • type AB blood — Type AB blood has both A and B antigens and no anti-A or anti-B antibodies, making it the universal recipient. It is relatively uncommon, and a person with type O blood cannot have a child with it.
• Blood groups
  • ABO blood group system
    • Bombay phenotype — The Bombay phenotype is a rare blood type in which the H antigen is absent, preventing formation of the usual A and B antigens. As a result, people with this phenotype can appear to be type O even when they carry A or B alleles.
• Blood types
  • ABO blood group
    • blood type AB — Blood type AB is the blood type that results when one parent contributes an A allele and the other contributes a B allele. It is the universal recipient in the ABO system, with both A and B antigens and no usual anti-A or anti-B antibodies.

Population genetics

• Population genetics

  • allele frequency — Allele frequency is the proportion of a population carrying a particular genetic variant. It appears in Hardy-Weinberg equilibrium, the Wahlund effect, and statistics such as the D-statistic and F4-statistic.

  • effective population size — Effective population size, usually written Ne, is the breeding population size that governs genetic drift and coalescence in population genetics. It is often much smaller than the census population size and drops sharply after bottlenecks or founder events.

  • Fisher’s fundamental theorem of natural selection — Fisher’s fundamental theorem of natural selection says the rate of increase of fitness in a population is proportional to the genetic variance in fitness. It is often paired with adaptive landscapes and with Sewall Wright’s contrasting work on rugged fitness landscapes.

  • founder effect — The founder effect is a form of genetic drift in which a new population is started by a very small number of individuals, or a small group splits off from a larger population, causing reduced genetic variation compared with the source population. Because the founders carry only a sample of the original gene pool, rare alleles can become unusually common in the new group. Common examples include the Amish, Martha’s Vineyard, the Pennsylvania Dutch, and other isolated island or isolate populations.
    • the founder effect — The founder effect is a genetic pattern in which a small group splits off from a larger population and carries only part of its genetic variation. It is often associated with inbreeding, reduced diversity, and unusual frequencies of inherited traits in isolated communities such as island populations or the Amish.

  • gene pool — The gene pool is the total collection of genetic information, or alleles, present in a population of sexually reproducing organisms at a given time. It is the population-wide set of genes that biologists discuss in genetics and evolution.

  • genetic load — Genetic load is the reduction in a population’s fitness caused by harmful mutations or other unfavorable genetic differences. It is measured by comparing average fitness with the maximum possible fitness and is often discussed alongside Muller’s ratchet in asexual populations.

  • Hardy-Weinberg equation — The Hardy-Weinberg equation describes genetic equilibrium in a large randomly mating population with no mutation, selection, migration, or drift. Its standard form is p squared plus 2pq plus q squared equals one, and it is used to model allele and genotype frequencies.

  • Hardy-Weinberg law or principle — The Hardy-Weinberg law describes genetic equilibrium in a large, randomly mating population with no mutation, migration, or selection. Under those conditions, allele and genotype frequencies remain constant from generation to generation.

  • inbreeding — Inbreeding is mating between genetically related individuals, producing increased homozygosity and often inbreeding depression. Geneticists use measures such as the coefficient of inbreeding and F-statistics to track it, and plant breeding often avoids it through self-incompatibility and enforced outcrossing.

  • inbreeding depression — Inbreeding depression is the loss of vigor or fitness that occurs when closely related individuals reproduce. It is often explained by the unmasking of harmful recessive alleles and is especially discussed in small or bottlenecked populations.

  • Molecular evolution tests
    • McDonald-Kreitman test — The McDonald-Kreitman test compares synonymous and nonsynonymous variation within a species against fixed differences between species. It is used to test neutral molecular evolution and infer positive or negative selection.

  • Muller’s ratchet — Muller’s ratchet is the process by which asexual populations irreversibly accumulate harmful mutations over time. It is named for Hermann J. Muller and is often used to explain the advantage of sex and recombination over purely clonal reproduction.

  • Wahlund effect — The Wahlund effect is the reduction in observed heterozygosity that occurs when subpopulations with different allele frequencies are pooled together. Even if each subpopulation is in Hardy-Weinberg equilibrium, the combined population can show a deficit of heterozygotes.

  • Wright-Fisher model — The Wright-Fisher model is a simple population-genetics model of genetic drift with non-overlapping generations and constant population size. It is usually discussed as a foundational model later extended by the Moran and Cannings models.

  • allele frequencies — Allele frequencies are the proportions of different gene variants in a population. They are central to Hardy-Weinberg equilibrium, genetic drift, and population-genetic measures such as Fst and the Wright-Fisher model.
    • p and q — p and q are the standard symbols for allele frequencies in the Hardy-Weinberg equation, where they sum to 1. They can also denote the short and long arms of a chromosome in another common biology clue set.

  • coalescent theory — Coalescent theory is a retrospective model in population genetics that traces alleles backward to their most recent common ancestor. It is associated with John Kingman and with parameters such as theta in mutation-and-population-size calculations.

  • De Finetti diagrams — De Finetti diagrams are triangular plots used to represent genotype frequencies in diploid populations. They are used to visualize Hardy-Weinberg equilibrium, with equilibrium points lying on a symmetric parabola.

  • Tajima’s D — Tajima’s D is a population-genetics statistic used to compare measures of nucleotide diversity and identify departures from neutral evolution. Negative values suggest an excess of rare alleles, while positive values can indicate balancing selection or population structure.

  • population bottleneck — A sudden sharp decline in population size, often caused by disasters or disease, that leaves the surviving population with a nonrepresentative gene pool. It reduces genetic diversity and increases genetic drift and inbreeding; often discussed alongside the founder effect and other causes of changes in allele frequencies.
• Hardy-Weinberg equilibrium — in a large, randomly mating population with no mutation, migration, or selection, allele frequencies remain constant. Equation: p² + 2pq + q² = 1 (where p = dominant allele freq, q = recessive allele freq). Five conditions required: large population, random mating, no mutation, no migration, no natural selection. Also treated as the null model of population genetics, it is used as a baseline for measuring deviation from random mating and evolutionary change.

  • Godfrey Hardy and Wilhelm Weinberg — Godfrey Hardy and Wilhelm Weinberg are the population geneticists associated with the Hardy-Weinberg equilibrium. Their model states that allele frequencies in an ideal population remain constant unless acted on by evolutionary forces.

  • Hardy-Weinberg — The Hardy-Weinberg principle describes the ideal genetic equilibrium of a population under random mating, a large population, no mutation, migration, or selection, and equal reproductive success. Derived independently in 1908, it is the standard law used to predict stable allele frequencies.

  • Hardy-Weinberg Equilibrium or Law — The Hardy-Weinberg equilibrium law gives the allele-frequency relation p2 + 2pq + q2 = 1 in a large, randomly mating population. It describes a population free of evolutionary pressure at a single locus.

  • Hardy-Weinberg formula — The Hardy-Weinberg formula gives the expected genotype frequencies p^2, 2pq, and q^2 in a randomly mating population. It was developed in 1908 by G. H. Hardy and Wilhelm Weinberg to model population genetics under equilibrium conditions.

  • Hardy-Weinberg principle or equilibrium — The Hardy-Weinberg principle, or equilibrium, states that allele frequencies in a sexually reproducing population remain constant when no outside forces act. It is usually expressed by the equation p^2 + 2pq + q^2 = 1.

• Genetic drift — random changes in allele frequency; stronger in small populations; includes founder effect (new population founded by few individuals, e.g., Amish; high frequency of rare alleles) and bottleneck effect (severe population crash reduces diversity; e.g., cheetahs, elephant seals). Also called allelic drift or the Sewall Wright effect; it results from sampling error rather than selection and can reduce heterozygosity. It is associated with Sewall Wright’s Fisher–Wright model, the shifting-balance theory, and Kimura’s neutral theory.

• Gene flow — movement of alleles between populations via migration; tends to homogenize populations; counteracts divergence caused by drift and selection.
  • introgression — Introgression is the movement of genes from one related species into the gene pool of another through repeated hybridization and backcrossing. It is used in plant breeding and in techniques for introducing traits such as disease resistance or tolerance from wild relatives.
• Natural selection — differential reproductive success based on heritable traits; alters allele frequencies directionally. It underlies Darwinian evolution and is contrasted with genetic drift; classic examples include peppered moth coloration and finch beak variation.

  • directional selection — Directional selection is a type of natural selection in which one extreme phenotype is favored over the others. It shifts allele frequencies in a consistent direction and is the opposite of stabilizing selection. A classic example is the rise of dark peppered moths in polluted industrial England.

  • Disruptive selection — Disruptive selection is the form of natural selection that favors both extremes of a trait while selecting against intermediates. It can produce two distinct phenotypes, as in finches with either small or large beaks, and is contrasted with stabilizing selection.

  • stabilizing selection — Stabilizing selection favors intermediate trait values and reduces variation around the mean. Human birth weight is a classic example, since extreme values on either side are selected against.

  • survival of the fittest — “Survival of the fittest” is Herbert Spencer’s phrase for the differential success of organisms in natural selection. Charles Darwin later adopted it as a shorthand for evolutionary competition, though the wording was controversial because of its Social Darwinist misuse.

• Mutation — the ultimate source of new alleles. It is a change in DNA sequence that can result from replication errors, ultraviolet light, or chemical mutagens; types include point mutations, frameshifts, and nonsense changes.

  • genetic mutations — Genetic mutations are changes in DNA sequence that can alter traits, from point mutations to frameshifts. They are a basic source of evolutionary variation and were central to the Luria-Delbrück experiment on bacterial mutation rates.

  • missense mutation — A missense mutation changes one nucleotide so that a codon specifies a different amino acid. The resulting protein may still be produced, but it can lose normal function, as in sickle-cell disease.

  • mutation rate — Mutation rate, often symbolized by mu, is the frequency at which new genetic changes arise in a genome. It is central to molecular clock dating, appears in neutral theory and population-genetic estimators, and can be increased by mutagens or error-prone replication.

• Molecular clock — assumption that DNA (or protein) sequences accumulate mutations at a roughly constant rate over time; used to date divergence events by comparing sequence differences; calibrated against the fossil record; rates vary across genes and lineages.
  • Molecular clock hypothesis — The molecular clock hypothesis says genetic sequences change at an approximately steady rate over time, so scientists can use them to estimate when species diverged from a common ancestor. A relaxed molecular clock lets mutation rates vary across branches, and the idea is often linked to work on DNA and protein evolution.

Related Concepts and Applications

• Genetic mapping units
  • centimorgan — A centimorgan, or map unit, measures genetic linkage distance and corresponds to a 1% recombination frequency between two genes. It is the standard unit used in linkage analysis and is often discussed alongside LOD scores and Sturtevant’s early gene maps.
• genetics — Genetics is the science of heredity, studying how traits pass from parents to offspring through DNA and related material. It also underlies modern ancestry testing and genome-wide association studies such as those used by consumer DNA services.
  • alleles — Alleles are alternative forms of a gene that can produce different traits, such as dominant or recessive versions. Their frequencies are central to Hardy-Weinberg genetics, genetic drift, and Mendelian inheritance.
    • Dominance relationships
      • Recessive alleles — Recessive alleles are forms of a gene whose effects are masked by dominant alleles when both are present. They are expressed when an organism inherits two copies, a basic idea associated with Mendelian inheritance.
    • Dominant alleles
      • dominant allele — A dominant allele is one whose effect is expressed phenotypically in a heterozygote. In Punnett squares, it is usually represented by a capital letter. Genetic examples include Mendel’s round pea trait and dominant markers used in organisms such as C. elegans.
  • Association studies
    • genome-wide association studies — Genome-wide association studies examine large sets of single nucleotide polymorphisms to identify variants linked to disease or other traits. They are often abbreviated GWAS, and model organisms such as Xenopus can be used in them to study congenital heart disease.
  • Autosomal dominant inheritance
    • autosomal dominant — An autosomal dominant trait is inherited from a gene on a non-sex chromosome, and one copy of the allele is enough to express the condition. Huntington’s disease and Marfan syndrome are classic examples of this inheritance pattern.
  • Blood Group Systems
    • blood type — Blood type is the classification of erythrocytes by surface antigens such as A, B, AB, O, and Rh factor, with mismatches causing agglutination or hemolytic disease. The trait shows codominance, includes rarer phenotypes like Bombay, and is the basis for routine transfusion compatibility testing.
  • Cancer genes
    • tumor-suppressor gene — Tumor-suppressor genes help prevent cancer by restraining cell growth or preserving DNA repair pathways. Knudson’s two-hit hypothesis explains why both copies often must be inactivated, as in RB1, BRCA-related cancers, or TP53-associated Li-Fraumeni syndrome.

  • carriers — Carriers are companies that provide the telecommunications infrastructure needed to send a message, and the term also names people who carry recessive traits without showing them. It can also refer to laborers or porters in East African history.

  • Chromosomal abnormalities
    • trisomy — Trisomy is a chromosomal abnormality in which a cell has three copies of a chromosome. Common examples include Down syndrome, as well as Patau and Edwards syndromes, all caused by aneuploidy.
  • chromosome — Chromosomes are DNA-containing, histone-packaged structures in the nucleus that carry genes and are visible in a karyotype. In humans they normally occur as 23 homologous pairs, or 46 total, and crossing over between homologs during meiosis helps create genetic variation. Abnormal copies can cause disorders such as trisomy 21 (Down syndrome). Special forms include the Philadelphia chromosome, fragile X-associated chromosomes, and B chromosomes.
    • Autosomes — Autosomes are the chromosomes that are not sex chromosomes. In humans, they make up the first 22 pairs of chromosomes and are shared similarly by males and females.
      • autosome — An autosome is any chromosome that is not a sex chromosome. The term is used in genetics to distinguish ordinary chromosomes from the X and Y chromosomes.

    • Balancer chromosome — A balancer chromosome is a specially engineered chromosome used in genetics to maintain mutations in stock without recombination. Its many inversions suppress crossing over and it often carries recessive lethal alleles.

    • balancer chromosomes — Balancer chromosomes are specially engineered chromosomes used in Drosophila to maintain recessive lethal or deleterious mutations in stock. They usually carry multiple inversions, dominant markers, and recessive lethals so crossing-over is suppressed and only heterozygotes survive. Common examples include CyO and Tm3.

    • centromere — The centromere is the constricted chromosomal region where spindle fibers attach during cell division and where kinetochore assembly occurs. It links sister chromatids, can appear diffuse in some organisms, and is associated with compact point and regional forms in karyotypes.

    • centromeres — Centromeres are the condensed chromosome regions where kinetochores assemble and sister chromatids are held together. They are marked by CENP-A and are a standard clue in genetics questions about chromosome structure and segregation.

    • chromatids — Chromatids are the duplicated halves of a chromosome, including sister chromatids that separate during anaphase. They are the structures regulated by cohesin and studied as DNA-bearing units in cell division.

    • Chromatin structure
      • TADs — TADs, or topologically associating domains, are chromatin regions that interact more with themselves than with neighboring regions. They are commonly mapped with Hi-C and related chromosome conformation capture methods, and their boundaries are often marked by CTCF.

    • chromosome 21 — Chromosome 21 is the smallest human autosome and the chromosome whose trisomy causes Down syndrome. It carries genes such as APP and SOD1 and is often mentioned in connection with translocations and other chromosomal abnormalities.

    • Chromosome 22 — Chromosome 22 was the first human chromosome reported as fully sequenced by the Human Genome Project. It is associated with disorders such as DiGeorge syndrome, cat eye syndrome, neurofibromatosis 2, and the 22q deletion linked to developmental problems.

    • cohesin — Cohesin is the protein complex that holds sister chromatids together before anaphase. It is implicated in cohesinopathies such as Cornelia de Lange syndrome, and its regulation involves factors like shugoshin, PP2A, and separase.

    • Homologous chromosomes — Homologous chromosomes are chromosome pairs with the same overall structure and gene sequence positions, though the alleles on them may differ. In diploid nuclei, they pair up at the beginning of meiosis.

    • kinetochores — Kinetochores are disc-shaped protein complexes on chromosomes that mediate attachment to the mitotic spindle during cell division. They assemble at centromeres and are monitored by checkpoint proteins until sister chromatids can be pulled apart.

    • polytene chromosomes — Polytene chromosomes are giant chromosomes formed by repeated DNA replication without cell division. They are especially associated with Drosophila salivary glands, where puffing marks active transcription and has made them useful for chromosome mapping.

    • Sex chromosomes

      • The X chromosome — The X chromosome is one of the two human sex chromosomes and is present in two copies in normal female cells. Extra copies are usually inactivated by X-chromosome methylation, and abnormalities in it can produce fragile-X syndrome.

      • Y chromosome — The human male sex chromosome, carrying the SRY region that helps determine male development and other male-determining regions such as AZF. It is largely unable to recombine and has undergone heavy degeneration; associated disorders and karyotypes include Swyer syndrome and XYY syndrome. The shrinking theory is often mentioned in connection with it.

        • Y — Y is the letter used for the sex chromosome that determines human male sex and for related biological abbreviations such as Y RNA and neuropeptide Y. It can also denote the pyrimidine base uracil-thymine ambiguity in nucleotide notation. In mathematics and science it may also label a coordinate axis or other variable, but the main giveaway here is the Y chromosome.
  • Chromosome theory
    • Walter Stanborough Sutton — Walter Sutton was the American biologist who helped show that chromosomes carry hereditary units in distinct pairs. His work is a foundational part of chromosome theory of inheritance.
  • cloning — Cloning is the production of a genetically identical copy of an organism, cell, or DNA sequence. The clues point to both molecular cloning and famous animal examples such as Dolly the sheep and the Pyrenean ibex.

    • clones — Clones are genetically copied organisms, cells, or people created from another organism’s DNA rather than mere look-alikes. They are a common science-fiction premise, appearing in works such as The Prestige, Star Wars, Never Let Me Go, and Marvel stories involving X-23 and Ben Reilly.

    • Ian Wilmut — Ian Wilmut is the Scottish geneticist best known for helping clone Dolly the sheep at the Roslin Institute. His work on reproductive biology and embryo preservation made him a major figure in late twentieth-century cloning research.

    • molecular cloning — Molecular cloning is the laboratory process of inserting DNA into a vector and propagating recombinant DNA in a host. Clues point to plasmids, restriction enzymes, selectable markers, Gibson assembly, Golden Gate, and related vector-based methods.

      • Gibson assembly — Gibson assembly is a one-tube molecular cloning method that joins DNA fragments with overlapping ends using an exonuclease, DNA polymerase, and DNA ligase. It was developed for building large synthetic DNA molecules and is an alternative to restriction-site-based cloning methods such as Golden Gate assembly.
  • complementation — Complementation is the genetic phenomenon in which two mutants restore a wild-type phenotype when crossed, usually because the mutations affect different genes. The same word also appears in set theory for taking the complement of a set.
    • complementation testing — Complementation testing crosses two recessive mutants with the same phenotype to determine whether the mutations lie in the same gene. If the offspring are wild type, the mutations complement; if not, they are in the same complementation group.

  • Complementation test — The complementation test is a genetics assay used to determine whether two recessive mutations lie in the same gene. If crossing the mutant strains yields a wild-type phenotype, the mutations complement and are in different genes.

  • DNA structure
    • Erwin Chargaff — Erwin Chargaff was the scientist whose rules established that DNA contains equal amounts of adenine and thymine, and of cytosine and guanine. His work helped show that DNA, not protein, carries genetic information and became foundational for later studies of the double helix.
  • DNA variants
    • Single-nucleotide polymorphisms — Single-nucleotide polymorphisms are common one-base DNA sequence variants in which one nucleotide differs from another genome at the same position. A SNP is typically defined as a variant occurring at a frequency of at least 1 percent in a population. They are abundant in the genome and are widely used as genetic markers in linkage studies, DNA fingerprinting, genome-wide association studies, Manhattan plots, fine-mapping, and studies of traits such as sickle cell disease and altitude adaptation. Tag SNPs and haplotypes are used to track linked variation across the genome.
  • Dominance patterns
    • codominant — Codominance is a pattern of inheritance in which both alleles in a pair are fully expressed in the phenotype. A classic example is the expression of both A and B blood-group alleles in AB blood.
  • Dominance relationships

    • co-dominance — Co-dominance is a genetic pattern in which both alleles at a locus are fully expressed in the phenotype. Classic examples include ABO blood types and roan coat color in horses.

    • codominance — The described inheritance pattern is codominance, where the heterozygote shows an intermediate phenotype between the two homozygotes.

  • Dosage compensation
    • X chromosome inactivation — This mammalian dosage-compensation process randomly inactivates one X chromosome in female cells, producing a Barr body. It helps explain calico cat coat patterns and is regulated by Xist RNA and polycomb complexes; related terms include XIST/Tsix.

      • Barr bodies — Barr bodies are condensed, inactivated X chromosomes formed by lyonization for dosage compensation. They are absent in Turner syndrome and appear in extra copies in Klinefelter syndrome, helping explain calico coat patterns in cats.

      • Barr body — A Barr body is the inactive X chromosome found in female somatic cells after lyonization. It is a condensed heterochromatin structure associated with XIST expression, calico cats, and disorders such as Turner and Klinefelter syndromes.

      • Xist — A long noncoding RNA gene on the X chromosome that initiates X-chromosome inactivation and helps form a Barr body. It coats the chromosome to be silenced, is associated with the X-inactivation center, and is opposed by tsix.

  • Experiments
    • Beadle-Tatum experiment — The Beadle-Tatum experiment used X-rayed Neurospora crassa mutants to connect genes with specific biochemical functions. It led to the one gene, one enzyme hypothesis and helped bridge genetics and biochemistry.

  • gene — A gene is a basic unit of heredity, typically a DNA sequence that helps determine traits and can encode a protein or part of one. Genes are the units inherited from one generation to the next and are often discussed through Punnett squares and other genetics tools.

  • gene duplication — Gene duplication is the copying of a DNA segment, increasing copy number and often producing paralogs or pseudogenes. It can happen through unequal crossover, tandem duplication, or larger chromosomal rearrangements, and it is central to models of subfunctionalization and neofunctionalization. Ohno treated it as a major engine of evolution.
    • Paralogous genes — Paralogs are genes or proteins that arose by duplication within a lineage rather than by speciation. The alpha and beta hemoglobin chains are a classic example of paralogous genes.
  • gene expression — Gene expression is the biological process by which the information in a gene is used to produce a functional product. It is commonly measured with tools such as DNA microarrays and Northern blots, and projects like GTEx study it across human tissues.

    • central dogma of biology — The central dogma of biology describes the flow of genetic information from DNA to RNA to protein. It is the standard framework for gene expression, even though retroviruses such as HIV reverse part of that flow.

    • constitutive expression — Constitutive expression describes a gene that is transcribed continuously rather than only in response to a regulatory signal. Housekeeping genes such as GAPDH are classic examples. In operon language, a defective repressor or operator can make the lac operon behave as though it is constitutively expressed.

    • DNA microarray — A DNA microarray is a glass slide or chip covered with many oligonucleotide probes used to measure the expression of many genes at once. Fluorescently labeled cDNA is hybridized to it so the signal from each spot reports how much of a gene is present.

    • DNA transcription — DNA transcription is the process of synthesizing RNA, usually mRNA, from a DNA template. It involves promoter binding by RNA polymerase and can be followed by capping and poly-A tail addition in eukaryotes.

    • Polycistronic mRNA
      • polycistronic — Polycistronic describes an mRNA transcript that encodes more than one protein. The term is often applied to prokaryotic operons, where a single strand can be translated into multiple polypeptides.

    • Promoter Region — A promoter region is the DNA segment upstream of a gene where transcription machinery binds to begin RNA synthesis. Its motifs include the TATA box in eukaryotes and the -10 and -35 consensus sequences in prokaryotes.

    • RNA surveillance
      • nonsense-mediated decay — Nonsense-mediated decay is an mRNA surveillance pathway that destroys transcripts containing premature stop codons. It uses factors such as UPF proteins, the exon junction complex, and the SURF complex to prevent translation of truncated proteins.
    • transcription termination — Transcription termination is the stage of transcription at which RNA polymerase stops and releases the RNA transcript. In bacteria it can be rho-dependent or rho-independent, while in eukaryotes related pathways include the torpedo model.
      • Rho factor — Rho factor is a bacterial transcription-termination protein that can end transcription in a rho-dependent pathway. It is a hexameric helicase that unwinds the RNA-DNA hybrid in the transcription bubble when termination is not triggered by a simple U-rich sequence.
  • Gene variants
    • allele — An allele is one of the alternative forms of a gene at a particular locus. In basic genetics, different alleles can produce traits such as ABO blood types or wild-type versus mutant variants.
  • genes — Genes are the basic units of heredity and DNA that encode traits through alleles and can be altered by mutation. They are the targets of CRISPR editing and can also appear as transposons or “jumping genes.”
    • Introns — Introns are noncoding sequences found within eukaryotic genes between exons, transcribed into pre-mRNA and removed during RNA splicing by spliceosomes made from snRNPs. They form lariats during spliceosome-mediated processing, and some rare introns are self-splicing or mobile.
      • Intron — Introns are noncoding sections of a gene that are transcribed but removed from the RNA before translation. They are cut out by spliceosomes so that the exons can be joined together in the mature mRNA.
    • pseudogenes — Pseudogenes are nonfunctional gene copies that can no longer code for proteins. Common examples include the GULO pseudogene in primates and globin-related pseudogenes in icefish.
  • Genes and Transcripts
    • exon — An exon is the coding segment of a gene or transcript that remains after RNA processing and can be translated into protein. It contrasts with an intron, which is removed during splicing.
  • Genetic anticipation
    • anticipation — Anticipation is the genetic phenomenon in which symptoms of a repeat-expansion disorder appear earlier and often more severely in successive generations. It is classically associated with Huntington’s disease, myotonic dystrophy, and fragile X syndrome. The term also names a non-chord tone in music when a note of the next harmony sounds early.

  • genetic linkage — The tendency of genes close together on the same chromosome to be inherited together. It is the basis for linkage analysis, map distances in centimorgans, recombination-based genetic mapping, LOD scores, and genetic hitchhiking.

  • Genetic mapping

    • centimorgans — Centimorgans are units of genetic map distance used in molecular biology. They were named by Sturtevant in honor of Thomas Hunt Morgan, the fly geneticist.

    • haplotype map — A haplotype map is a genome map that divides chromosomes into blocks and identifies common patterns of genetic variation within each block. It is used to summarize linked variation patterns across the genome.

  • genetic recombination — Genetic recombination is the DNA break-and-rejoin process that creates new genetic combinations, including homologous recombination and meiotic crossing over. Common clues include Holliday junctions, synapsis in prophase I, PRDM9, Spo11, Cre-lox, and V(D)J recombination.

  • Genetic variation
    • copy number variation — Copy number variation is a genomic structural change in which the number of copies of a gene or region differs because of duplication or deletion. It is commonly studied with array CGH, SNP arrays, FISH, and related software such as PennCNV and CNVnator.
  • Geneticists
    • Lederberg — The Lederberg surname refers to Joshua Lederberg and Esther Lederberg, two geneticists central to phage and plasmid research. Joshua Lederberg is associated with generalized transduction and the term plasmid, while Esther Lederberg discovered lambda phage and the fertility factor.

  • genome — A genome is the complete set of genetic material in an organism, whether stored in DNA or, in some viruses, RNA. It is a standard biology answer tied to the Human Genome Project, genome assembly, and comparative genomics.

  • genotype — A genotype is an organism’s allele makeup, or genetic constitution at a locus, as opposed to its phenotype. It is the basis of heritability calculations in genetics and twin studies.

  • Genotypes
    • heterozygous — An organism or individual carrying two different alleles at a locus, or at one or more loci. It is the standard contrast to homozygous in genetics and heredity, appears in the Hardy-Weinberg equation as the 2pq term, and can be associated with heterozygote advantage in some populations. The term is also used for knockouts affecting only one allele and for hybrid organisms such as ligers and mules in some contexts. In Mendelian genetics, a heterozygous parent often produces a 3:1 dominant-to-recessive phenotype ratio in the F1 generation.

  • haplodiploidy — Haplodiploidy is the sex-determination system in which males develop from unfertilized eggs and females from fertilized eggs. It is common in bees and ants and is often discussed in explanations of eusociality and kin selection.

  • Haploinsufficiency — Haploinsufficiency is the condition in which one functional copy of a gene does not produce enough gene product for normal function. It can make a loss-of-function mutation appear dominant because a single wild-type allele is not sufficient.

  • haplotype — A haplotype is a set of alleles at different loci inherited together from one parent. It is the genetic unit whose frequencies are analyzed in linkage studies and mapped in large genome projects.

  • haplotypes — Haplotypes are sets of linked alleles or SNPs transmitted together on a chromosome. They are the focus of mapping projects that chart patterns of inherited variation across populations.

  • heritability — Heritability measures how much phenotypic variation in a population is attributable to genetic variation, often written as narrow-sense or broad-sense h-squared. It is commonly estimated from twin studies and can be expressed as the response to selection divided by the selection differential in artificial selection experiments.

  • heterozygosity — Heterozygosity is the condition of having two different alleles for a gene. It is measured in Hardy-Weinberg genetics by the 2pq term and is connected to balancing selection and heterozygote advantage.

  • heterozygote advantage — Heterozygote advantage is the phenomenon in which individuals with two different alleles have higher fitness than either homozygote. It is also called heterosis or hybrid vigor, and is often discussed as the opposite of inbreeding depression. Classic examples include the sickle-cell trait’s protection against malaria and other cases where hybrid offspring or carriers show greater vigor than either parent line, such as mixed-breed dogs.

  • heterozygotes — Heterozygotes are organisms with two different alleles for a gene, usually written with one uppercase and one lowercase letter. In a monohybrid cross between two heterozygotes, the expected phenotypic ratio is 3:1.

  • Homozygous — An organism is homozygous when it has two identical alleles at a gene locus. The term is often contrasted with heterozygous and is used in Mendelian genetics.

  • horizontal gene transfer — Horizontal gene transfer is the movement of genetic material between organisms other than by parent-to-offspring inheritance. Common mechanisms include transformation, transduction, and conjugation.
    • bacterial conjugation — Bacterial conjugation is the direct cell-to-cell transfer of DNA between bacteria, usually through a sex pilus and an F plasmid or Hfr strain. It is a classic mechanism of horizontal gene transfer and a major way antibiotic-resistance R factors spread.

      • Hfr — Hfr is the bacterial strain type in which the F factor is integrated into the chromosome. These cells can transfer chromosomal DNA to an F-negative recipient through high-frequency recombination.

      • sex pilus — A sex pilus is the thin bacterial appendage used during conjugation to bring donor and recipient cells together. It is encoded by the F plasmid, and the related F pilus or conjugation tube transfers DNA between bacteria.

  • Inheritance patterns
    • X-linked recessive inheritance — X-linked recessive inheritance is the pattern behind hemophilia A and B and red-green color blindness. Because males have only one X chromosome, the condition usually appears far more often in males and cannot pass directly from father to son.

  • James D. Watson — James D. Watson is the American biologist who co-discovered the double-helix structure of DNA. He is also known for work on the Human Genome Project and for the competitive race against Linus Pauling’s proposed DNA model.

  • lethality — Lethality is the phenotype of causing death, whether in genetics, toxicology, or dosage testing. The clue set points to embryo-lethal mutations, balancer chromosomes, and the LD50 measure of fatal effect.

  • linkage disequilibrium — Linkage disequilibrium is the nonrandom association of alleles at different loci. It is commonly summarized with measures such as Lewontin’s D prime and is an important source of clustered associations in genetic studies.

  • maternal effect — A maternal effect is a genetic phenomenon in which the mother’s genotype determines the offspring’s phenotype. It often happens because the mother supplies proteins or mRNA to the egg, as in the bicoid and nanos genes.

  • maternal effect genes — Maternal effect genes produce an offspring phenotype according to the mother’s genotype or deposited products in the egg. They help establish early developmental patterns such as anterior-posterior axis formation in Drosophila.

  • Mendelian inheritance

    • Mendel’s Law of Segregation — Mendel’s law of segregation states that hereditary traits are controlled by two alleles that separate into different gametes. It is one of the foundational principles of classical genetics associated with Gregor Mendel’s pea experiments.

    • Punnett square — A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from a genetic cross. In its simplest form, it is a two-by-two matrix for Mendelian inheritance. It is named for Reginald Punnett and is a standard tool for visualizing Mendelian inheritance, especially monohybrid and dihybrid crosses, where it displays possible allele combinations from known parents. It is used to visualize dominant and recessive inheritance patterns, and the forked-line method can serve as an alternative.

    • Test cross — A test cross is a genetic cross used to determine the genotype of an organism that shows the dominant phenotype but may be either homozygous or heterozygous. It is performed by crossing the unknown individual with a homozygous recessive partner.

    • The law of Segregation — The law of segregation, Mendel’s first law, states that the two alleles of a gene separate into different gametes during gamete formation. It explains how paired hereditary factors are divided before fertilization.

  • mitochondrial DNA — Mitochondrial DNA is the small circular genome found in mitochondria and is usually inherited maternally. It encodes proteins involved in respiration and the electron transport chain, as well as several tRNAs and rRNAs, and is much smaller than nuclear DNA. It has been used in ancestry, haplogroup, and evolutionary relationship studies, including genome sequencing as the first major non-nuclear genetic material sequenced in the Human Genome Project. Mutations in mtDNA can cause disorders such as MELAS, Leber hereditary optic neuropathy (LHON), and Kearns-Sayre syndrome.
    • D-loop — The D-loop is a DNA region in mitochondria and chloroplasts where a third strand displaces one of the original strands. In human mitochondrial DNA, it is the most polymorphic segment and contains key transcriptional origins.

  • mosaicism — Mosaicism is a genetic condition in which different cells in the same individual have different genotypes, usually arising from mutation, nondisjunction, or anaphase lag after development begins. It is distinct from chimerism because it can arise from a single zygote.

  • mutations — Mutations are permanent changes in DNA, including point mutations, insertions and deletions, and frameshift or nonsense changes. They are tested by the Ames test, studied in the Luria-Delbrück experiment, and are one source of heritable variation on which evolution acts.

    • Deletion — A deletion is a mutation in which part of a chromosome or DNA sequence is lost. In genetics, deletions can remove base pairs or larger segments, including breaks that fail to rejoin.

    • DNA mutations — DNA mutations are permanent changes in genetic material, including point mutations, missense and nonsense changes, and frameshifts. They can arise spontaneously or be induced by agents such as radiation or chemicals like EMS.

    • dominant negative mutations — Dominant negative mutations produce mutant gene products that antagonize the normal protein made by the wild-type allele. They are classically discussed in Marfan syndrome and in p53 mutants that form inactive dimers or heterotetramers.

    • Luria-Delbruck experiment — The Luria-Delbrück experiment showed that bacterial mutations arise spontaneously rather than being induced by selection. Using bacteriophage pressure in E. coli, it became a classic test of mutation rates and random fluctuation in microbial populations.

    • mutagenesis — Mutagenesis is the deliberate introduction of mutations into DNA or an organism’s genome. It includes random, insertional, and site-directed methods, and it is the process tested by the Ames assay and commonly used in reverse genetics.

    • mutation(s) — Mutations are changes in genetic material such as point, frameshift, nonsense, and silent mutations. They were central to the Luria-Delbrück fluctuation test and later work on mutation rates in DNA.

    • nonsense mutations — Nonsense mutations are stop-gain mutations that introduce a premature termination codon into a gene. They can be suppressed by amber, opal, or ochre readthrough mutations, and they may trigger nonsense-mediated decay.

    • transversions — Transversions are nucleotide substitutions that swap a pyrimidine for a purine, or the reverse. They are less common than transitions and are highlighted by mutation models that distinguish the two kinds of base changes.

  • nonsense mutation — A nonsense mutation is a point mutation that changes a codon into a stop codon. Because it truncates translation early, it usually produces a shortened, nonfunctional protein.

  • open reading frames — Open reading frames are stretches of DNA from a start codon to a stop codon that can potentially encode proteins. Genome analysis tools such as ORF Finder identify them, and many viruses and other organisms are described in terms of their ORFs.

  • Overdominance — Overdominance is the genetic situation in which a heterozygote has higher fitness than either homozygote. It is often illustrated by the sickle-cell trait’s advantage against malaria.

  • penetrance — Penetrance is the proportion of individuals with a genotype who actually express the associated phenotype. A trait with low penetrance can be common in carriers but appear in only some of them.

  • phenotype — A phenotype is the observable set of characteristics expressed by an organism, as opposed to its genotype. It can be influenced by effects such as epistasis, pleiotropy, maternal effects, and position effects in genetics.

  • phenotypes — Phenotypes are the observable physical traits or outward expressions produced by an organism’s genotype. They include classic Mendelian traits such as dwarfness, color, and wrinkling in pea plants, and can show effects like incomplete dominance.

  • Population genetics
    • allele fixation — Allele fixation is the process by which one allele reaches 100 percent frequency in a population. In finite populations, genetic drift can eventually fix an allele, making it the sole version present for a trait.

  • R.C. Punnett — R. C. Punnett was a British geneticist and Cambridge professor best known for Punnett squares, the diagrams that show the genetic outcomes of allele combinations. He also published Mendelism in 1905.

  • Recessive traits
    • recessive — A recessive trait appears only when an organism inherits two copies of the relevant allele. Recessive traits are typically masked by dominant alleles and are often written in lowercase in genetics notation.
  • Repeat expansions
    • trinucleotide repeat expansion — A trinucleotide repeat expansion is the growth of repeated three-base sequences, often causing genetic anticipation. It underlies disorders such as Huntington’s disease, fragile X syndrome, and Friedreich’s ataxia.
  • Repetitive DNA
    • microsatellites — Microsatellites are short repeated DNA sequences, usually two to six base pairs long, that are highly polymorphic because replication slippage changes their length. They are important in DNA fingerprinting, mismatch-repair defects, Lynch syndrome, and repeat-expansion disorders such as Huntington’s disease.
  • reverse genetics — Reverse genetics is a research approach that starts with a known gene or DNA sequence, introduces mutations, and then observes the resulting phenotype. It is often used with gene knockouts, site-directed mutagenesis, and studies of MHC-linked disease associations.
    • TILLING — TILLING is a reverse-genetics technique that uses chemical mutagenesis and high-throughput screening to identify mutations in a chosen gene. It works by creating a DNA heteroduplex that is cut by a single-strand nuclease to reveal the desired lesions.
  • Sex-determining genes
    • SRY gene — The SRY gene is the Y-chromosome gene that produces the testis-determining factor, launching male embryonic sex differentiation. Its activity triggers development of the testes in the embryo and is a standard clue in questions about sex chromosomes.
  • Strains
    • wild type strain — A wild-type strain is the nonmutant form used as the normal or standard reference in genetics. It represents the most common allele or phenotype found in nature.
  • Synthetic lethality
    • synthetic lethal — Synthetic lethal describes a gene pair in which either deletion alone is survivable, but losing both genes causes death. The term is used in genetics and cancer biology to describe interactions that reveal hidden essentiality.
  • Transgenic organisms
    • transgenic — A transgenic organism contains foreign DNA inserted into its genome, often through recombinant DNA methods. The term was used for animals carrying chimeric or human genes, such as sheep engineered with factor IX or related constructs.
  • Transposable elements

    • Alu element — Alu elements are the most common transposable elements in the human genome. These primate SINEs are about 300 base pairs long and derive from 7SL RNA.

    • retrotransposons — Retrotransposons are mobile genetic elements that copy themselves through an RNA intermediate before reinserting into the genome. They include LTR elements, LINEs, and SINEs, and they make up a large fraction of the human genome.

    • Transposons — Transposons are mobile DNA elements, or transposable elements, that can move within genomes and sometimes disrupt open reading frames. Barbara McClintock identified them in maize. Major classes include Class I retrotransposons, which move through an RNA intermediate using reverse transcriptase, and Class II DNA transposons. Named examples include the Ac/Ds system, P elements, Spm, Alu sequences, LINEs, SINEs, and engineered forms such as Sleeping Beauty.

      • Insertion Sequences — Insertion sequences are the simplest bacterial transposons, consisting only of the DNA needed for transposition. They typically encode only a transposase gene and are flanked by inverted repeats.
  • Tumor suppressors — Tumor suppressors are proteins that help prevent cancer by slowing cell division, promoting apoptosis, or otherwise restraining growth. Common examples include p53 and BRCA-related genes, and they are contrasted with oncogenes.

    • p53 — p53 is a tumor suppressor transcription factor encoded by TP53 and commonly called the guardian of the genome. It responds to DNA damage and cellular stress by halting the cell cycle, especially at the G1/S checkpoint, activating DNA repair pathways, or triggering apoptosis through targets such as p21, Bax, and PUMA. It is negatively regulated by MDM2, is central to ATM-mediated DNA-damage signaling, and is frequently mutated in human cancers; loss of its function is associated with Li-Fraumeni syndrome.

    • Rb — Rb is the retinoblastoma tumor suppressor pathway that inhibits E2F and helps keep cells in G0. When phosphorylated by cyclins, Rb releases control and allows cells to reenter the cell cycle.

  • whole genome duplication — Whole genome duplication is the process in which an organism’s genomic content doubles, increasing genome size in a single event. It is contrasted with allopolyploidy, which increases genome size through hybridization. The duplication of vertebrate genomes and the later teleost and salmonid rounds are classic examples, and the resulting duplicate genes can be described by the DDC model.

  • wobble base — The wobble base is the third position of a codon, which is often more variable because the genetic code is degenerate.

  • Zygosity
    • hemizygous — Hemizygous describes a condition in which a gene is present in only one copy, most often for genes on the X chromosome in males. Clues often point to sex-linked traits such as colorblindness, hemophilia, and white eyes in fruit flies.

Genetics and Genomics in Detail

• Transposable elements (transposons) — mobile DNA sequences that can move within a genome; Class I (retrotransposons, copy-and-paste via RNA intermediate; e.g., LINEs, SINEs); Class II (DNA transposons, cut-and-paste; e.g., Ac/Ds in maize); discovered by Barbara McClintock; comprise ~45% of the human genome.

• Imprinting — see Genomic imprinting under Extensions of Mendelian genetics; key canonical examples at quizbowl level: Prader-Willi (paternal 15q11-q13 deletion or maternal UPD) vs Angelman (maternal deletion or paternal UPD of same region; different gene, UBE3A).

• Complementation test (cis-trans test) — determines whether two recessive mutations are in the same gene; if two homozygous mutant strains are crossed and the F1 is mutant, the mutations fail to complement (same gene); if F1 is wild-type, they complement (different genes); developed in phage genetics by Benzer (rII locus of T4).

• Epistasis types — recessive epistasis (homozygous recessive at one locus masks expression of another; produces 9:3:4 ratio); dominant epistasis (dominant allele at one locus masks another; 12:3:1 ratio); duplicate dominant epistasis (15:1 ratio).

• Next-generation sequencing (NGS) — high-throughput DNA sequencing platforms (Illumina, Oxford Nanopore, PacBio) that sequence millions of fragments in parallel; enabled the “$1,000 genome”; basis of RNA-seq, ChIP-seq, ATAC-seq, and clinical genomics.

• Repetitive DNA
  • Alu elements — Alu elements are short primate SINE retrotransposons derived from 7SL RNA. They are the most common short interspersed elements and among the most abundant repetitive elements in the human genome. They are transcribed by RNA polymerase III and are often used as genetic markers, including for tracing primate evolution, and are noted as disease-associated primate-specific repeats.
• Transposable elements

  • P elements — P elements are transposable elements in Drosophila melanogaster that can cause hybrid dysgenesis. They are widely used in mutagenesis and insertional genetics, especially in classic fruit-fly transformation work.

  • transposase — A transposase is an enzyme that mediates the movement of transposable DNA elements and is used in ATAC-seq and Nextera library preparation. Tn5 and Sleeping Beauty are familiar examples, and these enzymes are related to recombinases and integrases.

  • transposases — Transposases are enzymes that cut and move transposable DNA elements, and their DDE catalytic motif makes them useful in ATAC-seq and related chromatin-accessibility methods. They are activated in engineered form for genome insertion and tagging applications.

• Trinucleotide repeats
  • CAG — CAG is the trinucleotide repeat that codes for glutamine and is associated with Huntington disease and other polyglutamine disorders. When expanded in the HTT gene, this repeat can drive earlier onset and more severe neurodegeneration.
• Biotechnology
  • GMOs — Genetically modified organisms are plants or animals whose DNA has been altered to add traits such as pest, disease, or drought resistance. Examples include golden rice, Flavr Savr tomatoes, and Roundup Ready crops. They are often discussed in connection with controversies over labeling, trade regulation, patents, monoculture, agricultural biotechnology, and public suspicion.
• Cancer genetics

  • loss of heterozygosity — Loss of heterozygosity occurs when a diploid cell loses its remaining functional allele of a gene. It is a classic second hit in Knudson’s model of tumor suppressor inactivation and often results from large deletions or genomic instability.

  • two-hit hypothesis — The two-hit hypothesis says that both alleles of a tumor suppressor gene must be inactivated before a cancerous phenotype appears. It was developed from studies of retinoblastoma and is often associated with Alfred Knudson.

• Human cloning
  • Richard Seed — Richard Seed was a Chicago physicist who became notorious in the late 1990s for proposing human cloning. He was identified as holding a Harvard PhD in nuclear physics. The cloning controversy also involved his wife Gloria and discussion of creating embryos with donor eggs.
• Mitochondrial genetics
  • heteroplasmy — Heteroplasmy is the presence of more than one mitochondrial genome within a single cell or person. It helps explain variable severity in mitochondrial disease and can involve mixtures of normal and mutant mtDNA.
• Molecular interactions
  • protein-protein interactions — Protein-protein interactions are the binding events in which polypeptides associate to form complexes, whether transient or stable. They are studied with methods such as co-immunoprecipitation, yeast two-hybrid screening, fluorescence complementation, and large interaction maps like STRING and BioPlex.
• Noncoding RNAs
  • long noncoding RNAs — Long noncoding RNAs are a large class of mostly nuclear RNAs that often show strong developmental specificity. They are implicated in chromatin regulation and transcriptional control, with XIST as a classic example involved in X inactivation.
• Ribonucleoprotein particles
  • vaults — Vaults are conserved cytoplasmic ribonucleoprotein particles with a barrel-like shell built from many copies of the major vault protein. They are associated with RNA, can move along microtubules, and have been linked to transport and possible drug resistance roles in cancer cells.
• Secretases
  • beta-secretase 1 — Beta-secretase 1, also called BACE1, is an aspartic protease involved in amyloid precursor protein processing. It cleaves APP to produce the C99 fragment, which can then be processed by gamma-secretase to form amyloid plaques.

Development, Patterning, and Regeneration

• Development
  • growth — Growth is the process of increasing size, number of cells, or length in organisms. In plants it includes primary and secondary growth, tropic growth, and other forms regulated by hormones such as auxins and gibberellins.
• Developmental biology

  • Christiane Nusslein-Volhard — Christiane Nusslein-Volhard is a German developmental biologist who shared the Nobel Prize in Physiology or Medicine for work on embryonic development. She helped identify genes controlling Drosophila development through large-scale mutant screens.

  • embryo — An embryo is the early developmental stage that forms from the zygote before the fetus. It is the stage in which germ layers form and from which pluripotent stem cells can be derived.
    • blastocyst — A blastocyst is an early embryonic stage that forms from the morula as a hollow, fluid-filled ball of cells. It contains the inner cell mass used for embryonic stem cell extraction, and in many zoology contexts it is treated as the mammal-specific form of the blastula.
  • embryology — Embryology is the study of the development of an organism from the fertilized egg through birth or hatching. It concerns the early layers of development such as the ectoderm, mesoderm, and endoderm.

    • Archenteron — The archenteron is the primitive gut cavity formed during gastrulation, and its opening is the blastopore. In deuterostomes, later digestive structures develop from this embryonic cavity.

    • blastocoel — The blastocoel is the fluid-filled central cavity of a blastula or blastocyst. During gastrulation, its cells rearrange and eliminate this space.

    • blastula — A blastula is the early embryonic stage that precedes gastrulation and forms a hollow sphere around the blastocoel. In mammals, the comparable stage is often called a blastocyst, and its inner cell mass is the source of embryonic stem cells.

    • Cleavage — Cleavage is rapid early embryonic cell division that increases cell number without increasing overall volume. The term also refers to the way a mineral splits along flat planes, as in perfect cleavage in feldspar or micas.

    • Cleavage cells
      • Blastomeres — Blastomeres are the cells produced by the earliest cleavages of a zygote. These cells divide to form the morula during early embryonic development.
    • Embryonic stages
      • morula — A morula is an early embryonic stage made of roughly 16 to 32 tightly compacted blastomeres. It resembles a little raspberry and develops into the blastocyst after cleavage divisions.

    • Embryos — An embryo is the early multicellular stage formed from a dividing zygote before the fetal stage begins. In humans it implants in the endometrium after the blastocyst forms, and by the later embryonic period limb buds appear and the head dominates the body plan. In biology, the term also appears in plant development, where embryos pass through globular, heart, torpedo, and maturation stages. The same word can also point to embryogenesis, the process by which a zygote develops into a fetus.

    • Extraembryonic membranes
      • amnion — The amnion is the thin extraembryonic membrane that surrounds the embryo or fetus in a fluid-filled sac. It occurs in reptiles, birds, and mammals as part of the group of amniotes.

    • gastrula — The gastrula is the embryonic stage that follows the blastula. It is marked by gastrulation, formation of the three germ layers, and development of the archenteron and blastopore.

    • Germ layers

      • ectoderm — The ectoderm is the outer germ layer of the embryo. It gives rise to the nervous system, epidermis, and neural crest cells.

      • mesoderm — Mesoderm is the middle germ layer formed in gastrulation, between ectoderm and endoderm. It gives rise to muscles, bones, dermis, and much of the coelom and connective tissues in animals.

    • Hans Spemann — Hans Spemann won the 1935 Nobel Prize in physiology or medicine for work on embryonic development. He is best known for the organizer effect, showing that the dorsal blastopore lip can direct the organization of the rest of the embryo.

    • Karl Ernst von Baer — Prussian-Estonian naturalist and embryologist born in 1792, known for laws of embryology, the discovery of the mammalian ovum, and as a founder of comparative embryology. He also identified the blastula stage and the notochord, and is associated with criticism of recapitulation theory in embryology.

    • neural crest — The neural crest is a migratory embryonic tissue that forms at the border of the neural plate and later gives rise to diverse structures, including melanocytes, parts of the autonomic nervous system, and craniofacial tissues. It is classically studied through quail-chick chimera experiments. Defects in neural crest migration are linked to conditions such as Hirschsprung’s disease, DiGeorge syndrome, and Treacher-Collins syndrome.

    • Organizers
      • Spemann-Mangold organizer — The Spemann-Mangold organizer is a group of embryonic cells that directs gastrulation in amphibians. It was identified through dorsal lip transplant experiments in newts and is known for inducing development by blocking BMP signals.

    • pharyngeal arches — Pharyngeal arches are embryonic ridges that give rise to bones such as the incus, malleus, and stapes. In chordates, the same structures are linked to pharyngeal slits and gill-like anatomy in fish.

    • Placenta
      • trophoblast — The trophoblast is the outer layer of the blastocyst and a major precursor of the placenta. Its invasive forms secrete hCG, fuse to form chorionic villi, and can be involved in gestational trophoblastic disease.
    • somites — Somites are paired embryonic blocks of paraxial mesoderm that segment the body along the anterior-posterior axis. They give rise to the dermatome, sclerotome, and myotome, which later form skin, vertebrae, and skeletal muscle.

  • Eric Wieschaus — Eric Wieschaus is a developmental biologist who won the 1995 Nobel Prize in Physiology or Medicine with Christiane Nüsslein-Volhard. His work helped establish genetic screens for patterning mutants in Drosophila embryos.

  • invagination — Invagination is the folding inward of a cell sheet or tissue layer. The term is used in developmental biology to describe curvature changes in embryonic structures.

  • morphogens — Morphogens are signaling molecules that pattern developing tissues by producing different effects at different concentrations. They are central to the French flag model of embryonic development.
    • morphogen — A morphogen is a diffusable developmental signal that patterns tissues by acting in concentration gradients. Examples include members of the TGF-beta family and Sonic Hedgehog, which provide positional information during embryonic development.

  • neoteny — Neoteny is a form of paedomorphosis in which somatic development slows while reproductive maturity is retained or reached. It is associated with adult forms that retain juvenile traits, such as the axolotl keeping its gills.

  • ontogeny — This term refers to the development of an individual organism. In biology it names the course of growth from conception onward, as opposed to evolutionary history.

  • paedomorphosis — Paedomorphosis is the retention of juvenile traits into adulthood, and neoteny is a common example of it. The axolotl is a standard clue for this term because it keeps larval features such as gills as an adult.

  • preformation — Preformation was an early theory of development that held a fully formed organism already existed in either the egg or the sperm. It was opposed by epigenesis and was associated with ideas such as homunculi in sperm, as discussed in the work of Leeuwenhoek and Hartsoeker.

  • Puberty — Puberty is the biological maturation process marked by the onset of sexual development and associated physical changes. It can be delayed by puberty blockers, and the evidence mentions the gene Gpr54 as essential for initiating it.

  • segmentation — Segmentation is the division of an organism or body plan into repeated units, as in insect head, thorax, and abdomen organization. The word also appears in business for dividing customers into subgroups and in computer science as a memory-access fault.
• Developmental genes

  • Gap genes — Gap genes are early embryonic patterning genes in Drosophila that divide the embryo into broad regions. Mutations in genes such as hunchback, Krüppel, and knirps can eliminate whole body segments and affect downstream pair-rule genes.

  • Homeotic genes — Homeotic genes are developmental transcription factors that tell body segments and organs where to form, and Hox genes are a major subset of them. They are central to animal segmentation, flower patterning in plants, and patterning of structures such as the vertebrate body plan.

  • Segmentation genes

    • pair-rule genes — Pair-rule genes are developmental patterning genes in Drosophila that act after gap genes and before segmental identity is fully established. Examples include fushi tarazu, hairy, and even-skipped.
• Developmental signaling
  • Hedgehog signaling
    • sonic hedgehog protein — Sonic hedgehog protein is a developmental signaling molecule that helps pattern the embryonic limb along the anterior-posterior axis. It is one of the hedgehog family proteins named for the spiky mutant phenotype seen in fruit flies.
• Developmental stages
  • fetuses — Fetuses are unborn offspring nourished by the placenta during pregnancy. They have fetal hemoglobin with gamma subunits, and their circulation includes the foramen ovale to bypass the lungs. The same developmental stage is the one affected by fetal alcohol syndrome and related prenatal conditions.
• Embryonic axes
  • anterior-posterior axis — The anterior-posterior axis is the main head-to-tail developmental axis in Drosophila embryos. It is established by gradients involving Bicoid, Nanos, Hunchback, and Caudal, with segment polarity genes refining the pattern later.
• Embryonic cells
  • Neural crest cells — Neural crest cells are pluripotent embryonic cells that arise from the border of the neural tube. They migrate to form diverse tissues, including melanocytes, Schwann cells, cartilage, and parts of the craniofacial skeleton and adrenal medulla.
• Histology — Histology is the microscopic study of the structure of plant and animal tissues. It is a core technique in biology and medicine for examining tissue organization under the microscope.
  • fixation — Fixation is the histology process that preserves tissue by stabilizing it against decay, often with formaldehyde or related reagents. In biology, it also means the point at which an allele has taken over a population.
• regeneration — Regeneration is the biological process by which organisms regrow lost or damaged body parts, such as tails, limbs, or even internal tissues. It is especially associated with salamanders, starfish, and some other animals that can replace structures after injury.
  • limb regeneration — Limb regeneration is the process by which organisms regrow lost body parts such as limbs, tails, or other structures. It is studied in animals like axolotls and in planarians, where it depends on specialized cells and signaling that rebuild a blastema.

Protein Folding, Modification, and Turnover

• Post-translational modification
  • glycosylation — Glycosylation is the post-translational attachment of sugar chains to proteins and lipids, occurring in O-linked and N-linked forms in the ER and Golgi. It is used in protein trafficking and recognition, and defects in the process are associated with diseases such as CDG-Ia and Walker-Warburg syndrome.
    • N-linked glycosylation — N-linked glycosylation is a cotranslational form of protein glycosylation in which a preassembled carbohydrate is attached to asparagine residues. Its products include high-mannose, hybrid, and complex glycans, and it is a major target of tunicamycin.
• Protein degradation
  • proteasomes — Proteasomes are protein complexes that degrade unwanted or misfolded proteins by proteolysis. They recognize polyubiquitinated substrates, use a 20S core with regulatory caps, and are important in cell-cycle control and antigen processing.
• Protein degradation complexes
  • proteasome — The proteasome is the large protein complex that degrades ubiquitinated proteins into peptides. It has a 20S core particle with regulatory caps, and drugs such as bortezomib inhibit it in the treatment of multiple myeloma.
• Protein modification
  • ubiquitination — Ubiquitination is the process of attaching ubiquitin to proteins, usually to mark them for degradation by the proteasome. It is carried out through E1, E2, and E3 enzymes and is used by complexes such as SCF and APC/C.
    • E1 enzymes — E1 enzymes are ubiquitin-activating enzymes in the ubiquitination pathway. They adenylate ubiquitin and hand it off to E2 enzymes through a thioester-linked transfer step.
• Post-translational modifications

  • acetylation — Acetylation is a post-translational modification that adds an acetyl group, often to lysine residues on histones. In chromatin, histone acetylation is associated with open euchromatin and increased transcription, and bromodomains recognize the modified lysines.

  • phosphorylation — Phosphorylation is the kinase-driven addition of a phosphate group to a target molecule, most often on serine, threonine, or tyrosine residues. It is often detected with radioactive phosphorus labeling, phospho-specific stains, or by the binding of SH2 and 14-3-3 domains to modified proteins.

  • prenylation — Prenylation is a post-translational modification that attaches a lipid group such as a farnesyl or geranylgeranyl chain to a protein, usually at a C-terminal cysteine in a CAAX motif. It helps anchor proteins like Ras to membranes and is implicated in disorders such as progeria. Drugs like lonafarnib can inhibit the process.

• Post-translational protein processing
  • inteins — Inteins are self-splicing protein segments that excise themselves from a host polypeptide after translation. Some inteins act as homing endonucleases and can insert their own coding sequence at intragenic sites, including in RecA and Cas9-related systems. Split inteins are encoded by more than one gene and together assemble into a splicing unit.
• Chaperone proteins
  • calnexin — Calnexin is an endoplasmic reticulum membrane chaperone that binds incompletely folded glycoproteins and keeps them from leaving the ER. It recognizes N-linked oligosaccharides with a single remaining glucose as part of protein-quality control.
• Protein aggregates
  • amyloid — Amyloid is the name for fibrous protein aggregates implicated in disorders such as Alzheimer’s disease and several prion diseases. These deposits are stable, cross-beta-sheet structures that can form plaques in diseases like kuru and fatal familial insomnia.

Microbes, Viruses, and Experimental Tools

Distinguish microbes and viruses, then connect them to experimental methods.

Virology and Microbial Genetics

Viruses

• Viruses
  • Adenoviruses — Adenoviruses are double-stranded DNA viruses first isolated from human adenoids. They are known as respiratory pathogens and have also been used as vectors in gene therapy and some vaccines; the family name Adenoviridae comes from the same adenoid origin.
    • adenovirus — Adenoviruses are DNA viruses first discovered in adenoid tissue and known for their fiber proteins and use in gene-therapy vectors. They can cause common-cold symptoms in humans and include strains associated with avian disease.
  • Bacteriophage — Viruses that infect bacteria. They were central to classic molecular biology and genetics, including the Hershey-Chase experiment, the Luria-Delbrück experiment, and the work of Frederick Twort and the American Phage Group. Common examples include T1, T2, T4, lambda, phi X 174, and phi29. Many phages alternate between lytic and lysogenic cycles, and their structures often have an icosahedral head and a tail used to inject genetic material into the bacterial host. They are also used in phage display and in studies of CRISPR defense, where bacteria recognize phage DNA through PAM sequences. They are studied as possible antibacterial agents.

    • Enterobacteria phage T4 — Enterobacteria phage T4 is a virulent bacteriophage that infects E. coli and reproduces only through the lytic cycle. It is a classic model phage with an icosahedral head and a tail structure. It is also associated with the T4 DNA ligase commonly used in molecular biology.

    • lambda phage — Lambda phage is the temperate bacteriophage that infects E. coli and is famous for the lysogenic-lytic switch controlled by cI and Cro. It is also a classic cloning vector and model organism in molecular genetics, identified by its cos sites and its use of the lamB receptor.

    • prophage — A prophage is viral genetic material integrated into a bacterium’s chromosome during the lysogenic cycle. It is the dormant form of a bacteriophage genome while it remains embedded in the host.

    • Salvador Luria — Salvador Luria was an Italian-born microbiologist and Nobel laureate who studied bacteriophages with Max Delbrück and Hershey. He is also associated with the culture medium sometimes called Luria broth.

    • T-even bacteriophages
      • T4 bacteriophage — T4 bacteriophage is an E. coli virus that undergoes only the lytic cycle. It is a standard example in molecular biology because its DNA replication and genome packaging have been widely studied.

    • T4 phage — T4 phage is a bacteriophage that infects E. coli and became famous in classic genetics experiments. Seymour Benzer used its rI, rII, and rIII mutants to study fine-structure recombination and complementation.

    • T7 RNA polymerase — T7 RNA polymerase is the phage enzyme commonly used for in vitro transcription because of its strong promoter specificity. In E. coli DE3 strains, the polymerase is often used to drive recombinant protein expression, and T7 infection begins by synthesizing this protein to activate the rest of the phage genome.

  • capsids — A capsid is the protein shell of a virus, often described as icosahedral or helical and used in classifying viruses. In HIV, p24 is a capsid protein, and the term can refer to the outer protein coat surrounding viral genetic material.

  • coronaviruses — Coronaviruses are an enveloped family of RNA viruses named for the crown-like spikes on their surfaces. They include the agents of SARS, MERS, and COVID-19, and are also a common cause of the cold.

    • coronavirus — Coronaviruses are enveloped positive-sense RNA viruses named for their crownlike spikes. The family includes SARS and MERS, and SARS-CoV-2 caused COVID-19; these clues often mention ACE2 binding and the club-shaped surface peplomers.

    • SARS-CoV-2 spike protein — The SARS-CoV-2 spike protein is the viral surface protein that binds ACE2 to attach the coronavirus to host cells. It is notable for its S1 and S2 subunits, a furin cleavage site, and its role in vaccine and antibody research.

  • DNA viruses
    • papilloma viruses — Papilloma viruses are a family of viruses that cause warts and tumors in humans. They are especially associated with plantar and genital warts and with cervical cancer.

  • Ebolavirus — Ebolavirus is the genus that includes the pathogens responsible for Ebola hemorrhagic fever, including the Zaire and Reston strains. Its outbreaks in West Africa and its bat reservoir are central clues, along with the distinctive severe hemorrhagic symptoms and high fatality risk.

  • Enteroviruses

    • polio virus — The polio virus is the agent responsible for poliomyelitis, a disease once treated with iron lungs and later controlled by vaccines from Salk and Sabin. It attacks the anterior horn of the spinal cord and was the virus synthesized from scratch by Eckard Wimmer in 2002.

    • rhinovirus — Rhinovirus is a single-stranded RNA virus responsible for the common cold. It is an enterovirus that binds ICAM-1, also known as CD54, to enter epithelial cells.

  • filovirus — A family of filamentous negative-sense RNA viruses that includes Ebola, Marburg, and Reston virus. They are named for their U- and 6-shaped particles and are known for causing severe hemorrhagic fevers with high mortality rates.

    • Ebola virus — Ebola virus is a filovirus that causes severe hemorrhagic fever and is named for the Ebola River in the Democratic Republic of the Congo. It spread in major outbreaks beginning in 1976 and again in the 2014 West African epidemic. Quizbowl clues often mention ZMapp, VP24, and its relation to Marburg virus.

    • Marburg virus — Marburg virus is an African filovirus that causes hemorrhagic fever and is named for the German city where it was first identified in 1967. Outbreaks have been linked to caves and mines in Uganda, Angola, and the Democratic Republic of the Congo, with bats often implicated in transmission.

  • Hantaviruses — Hantaviruses are a genus of viruses named for the Hantan River in Korea. In the Americas they can cause hantavirus pulmonary syndrome, a severe and often fatal lung disease.

  • lysogeny — Lysogeny is the viral state in which a phage genome integrates into a host cell and remains dormant as a prophage. It can later be induced into the lytic cycle, and related biology appears in terms like lysogenic conversion and lysogeny broth.

  • Orthopoxviruses
    • Smallpox virus — The smallpox virus is the virus responsible for the eradicated disease smallpox. It is infamous for the 1978 Janet Parker laboratory accident and for being the only human virus officially declared eradicated.

  • papillomaviruses — Papillomaviruses are DNA viruses that infect epithelial tissue and can cause warts. Some human papillomavirus strains are also linked to cervical cancer.

  • Parvoviruses
    • adeno-associated virus — Adeno-associated virus is a nonpathogenic parvovirus widely used as a gene therapy vector and in optogenetics. Its wild-type genome can integrate into chromosome 19, and the self-complementary form is designed to speed transduction by bypassing second-strand synthesis.
  • Plant viruses
    • Tobacco mosaic virus — Tobacco mosaic virus is a single-stranded RNA virus that infects tobacco plants and causes a mottled leaf pattern. It was the first virus to be isolated and identified, the first virus discovered, and one of the first to be crystallized by Wendell Stanley. Its rigid rod-shaped particles were later studied by Rosalind Franklin and made it useful in early nanotech experiments.
  • retroviruses — Retroviruses are RNA viruses that use reverse transcriptase to make DNA and integrate into host genomes. HIV is the best-known example, and these viruses belong to Baltimore class VI.
    • Oncoviruses
      • Rous sarcoma virus — Rous sarcoma virus is an RNA retrovirus first discovered as a tumor virus in chickens. It causes connective-tissue tumors and was the first virus shown to cause cancer. The viral src oncogene gave rise to the famous v-src proto-oncogene studied in cancer biology. It is named for Peyton Rous.
    • SIV — SIV is a simian retrovirus that causes AIDS-like immunodeficiency in monkeys and is considered an ancestor of HIV. Different primate strains of SIV are associated with cross-species transmission from nonhuman primates to humans.
  • Rhabdoviruses
    • vesicular stomatitis virus — Vesicular stomatitis virus is a model rhabdovirus whose G glycoprotein has been used widely in molecular biology. It played a role in studies of ER-Golgi trafficking and in pseudotyping lentiviruses.
  • Viral envelopes
    • viral envelope — A viral envelope is the host-derived lipid bilayer that surrounds some viruses outside the nucleocapsid. It carries viral glycoproteins such as HIV gp120 and mediates membrane fusion with target cells.
  • Viral structure
    • capsid — A capsid is the protein shell that encloses a virus’s genome or genetic material. Viral capsids are often icosahedral and are built from repeating protein subunits arranged around the genome, sometimes inside an envelope.
  • virus — Viruses are infectious agents that can replicate only inside living host cells. They have a capsid and sometimes a lipid envelope, and their genomes may be DNA or RNA. Examples include tobacco mosaic virus, HIV, herpesviruses, bacteriophages, and retroviruses such as HIV; bacteriophages may follow lytic or lysogenic cycles. They are obligate intracellular and are classified by the Baltimore system.

• Virus structure — a virus consists of a protein coat (capsid) surrounding a nucleic acid genome (DNA or RNA, single- or double-stranded); some are further enclosed in a lipid envelope derived from the host membrane. Viruses are obligate intracellular parasites; they are not cells and lack ribosomes.

• Lytic cycle — virus injects its genome, hijacks host machinery to replicate, lyses (bursts) the host cell to release hundreds of new virions. The path taken by virulent phages. It is the rapid phage cycle that produces new virus particles and ruptures the host cell. The alternative is the lysogenic cycle, in which viral DNA remains integrated or inactive in the host.

• Lysogenic cycle — viral genome integrates into the host chromosome as a prophage (bacteria) or provirus (eukaryotes); replicates silently with the host; can be induced to enter the lytic cycle by environmental stress. A lysogenic cycle is a viral reproductive strategy in which the genome integrates into the host chromosome and can remain dormant as the host continues dividing. In bacteriophages, this produces a prophage and is contrasted with the faster lytic cycle.

• Bacteriophage — virus that infects bacteria; the lambda phage (λ) is the classic lytic/lysogenic example; the T4 phage is a canonical lytic phage used to demonstrate DNA as genetic material (Hershey-Chase 1952). They were discovered by Frederick Twort and Félix d’Herelle and are important in transduction and lysogenic conversion. They were also central to genetics and molecular biology experiments, including the Luria-Delbrück experiment and phage-display biotechnology.

• Retrovirus — RNA virus (e.g., HIV) that uses reverse transcriptase to convert its RNA genome into DNA, which then integrates into the host genome as a provirus; requires reverse transcriptase and integrase. The RNA-to-DNA direction is the canonical exception to the central dogma. HIV is the best-known example, and retroviral biology also includes proviruses, long terminal repeats, and oncogenic animal viruses; murine leukemia virus is another classic example.

• Viroid — the simplest known pathogen: a small, circular, single-stranded RNA molecule with no capsid; infects plants; mechanism involves RNA interference-like silencing. They are naked, usually circular single-stranded RNA molecules with no protein coat. They infect plants, including potato tubers, and replicate using host RNA polymerase.

• Prion — infectious misfolded protein (PrP^Sc); contains no nucleic acid; causes disease by inducing normal prion proteins to misfold; responsible for bovine spongiform encephalopathy (BSE/mad cow disease), scrapie, and Creutzfeldt-Jakob disease (CJD) in humans. Also called a proteinaceous infectious particle. Prions are unusually rich in beta sheets, resist protease digestion, and can occur in yeast as self-propagating conformations of proteins such as Sup35p and Ure2p. The prion hypothesis was developed into a major scientific theory by Stanley Prusiner, and prions are also associated with kuru.

• Enteroviruses
  • Polioviruses
    • poliovirus — Poliovirus is the positive-sense RNA enterovirus that causes poliomyelitis and can damage anterior horn motor neurons, producing paralysis. It was targeted by the Salk and Sabin vaccines, became famous through iron-lung epidemics, and was central to both the Cutter incident and later eradication efforts.
• Herpesviridae — Herpesviridae is a family of double-stranded DNA viruses that establish latency and can reactivate. It includes herpes simplex viruses, varicella-zoster virus, Epstein-Barr virus, cytomegalovirus, and Kaposi’s sarcoma-associated herpesvirus. Members cause cold sores, genital lesions, shingles, roseola, mononucleosis, and other persistent infections, and are often treated with acyclovir. They may produce multinucleated giant cells on Tzanck smear and are associated with Burkitt lymphoma and Kaposi’s sarcoma.

  • Cytomegalovirus — Cytomegalovirus is a herpesvirus named for the enlarged cells it produces during infection. It often causes a mild mononucleosis-like illness in healthy adults, but it can cause congenital defects, deafness, blindness, and severe disease in immunocompromised patients. It produces owl-eye inclusion bodies and can cause retinitis, esophagitis, and severe liver disease. It is also used in biotechnology promoters such as CAG and pcDNA vectors.

  • Epstein-Barr virus — Epstein-Barr virus (EBV), also called human herpesvirus 4, is a human herpesvirus spread through saliva. It infects B cells/B lymphocytes, epithelial cells, and salivary gland cells, and establishes lifelong latency. It is the major cause of infectious mononucleosis and is associated with Burkitt lymphoma, nasopharyngeal carcinoma, oral hairy leukoplakia, other cancers, and the heterophile antibody test. Many infections are asymptomatic.

  • Varicella-zoster virus — Varicella-zoster virus, also known as human herpesvirus 3, causes chickenpox on initial infection and shingles upon reactivation. It can remain dormant in nerve ganglia after the rash described as a dewdrop on a rose petal.

• Influenza viruses

  • hemagglutinin — Hemagglutinin is the influenza surface protein abbreviated H in H1N1, where it binds sialic acid on host cells to initiate infection. It is also the viral glycoprotein responsible for red blood cell clumping and is paired in strain names with neuraminidase.

  • neuraminidase — Neuraminidase is an influenza surface enzyme that cleaves sialic acid residues so newly formed virions can escape host cells. Along with hemagglutinin, it helps define influenza subtypes, and drugs such as oseltamivir and zanamivir inhibit it.

• Polyomaviruses
  • SV40 — SV40 is a polyomavirus first noticed as a contaminant in polio vaccines and known for its large T antigen. Its large T antigen is used in studies of nuclear localization and in cell transformation because it interacts with key tumor-suppressor pathways.
• DNA viruses
  • poxviruses — Poxviruses are large double-stranded DNA viruses that replicate in the cytoplasm of host cells. The family includes variola, vaccinia, molluscum contagiosum, and myxoma virus.
    • cowpox — Cowpox is a zoonotic poxvirus related to vaccinia and variola. Edward Jenner used it in developing the first vaccine against smallpox.
• RNA viruses
  • Picornaviruses — Picornaviruses are a family of small RNA viruses with a single-stranded RNA genome enclosed in an icosahedral protein shell. They include poliovirus, rhinovirus, and the virus that causes foot-and-mouth disease.
• Plant pathogens
  • viroids — Viroids are the smallest known pathogens and consist of a short, naked loop of RNA. They infect plants only, including potatoes, and their study has been tied to the RNA world hypothesis.
    • Potato spindle tuber viroid — Potato spindle tuber viroid was the first viroid discovered. It infects plants such as tomatoes and potatoes, where it causes characteristic stunting and spindle-tuber symptoms.
• Diseases
  • Prion diseases
    • Bovine Spongiform Encephalopathy — Bovine spongiform encephalopathy (mad cow disease) is a prion disease of cattle that causes sponge-like degeneration and destruction of the brain and spinal cord. It became notorious after contaminated feed spread it through British herds, leading to major British food-safety fears in the 1990s, wide restrictions on beef exports, and mass cattle slaughter. It has also been linked to variant Creutzfeldt-Jakob disease in humans.
• Prions

  • prion protein — The prion protein is a normal cell-surface protein that can misfold into the infectious scrapie form responsible for diseases such as Creutzfeldt-Jakob disease and mad cow disease. Its cellular and scrapie isoforms are usually abbreviated PrP^C and PrP^Sc, and the protein contains octapeptide repeats that help form a metal-binding region.

  • PrP — PrP is the key protein associated with prions, the infectious agents that cause diseases such as scrapie and Creutzfeldt-Jakob disease. Its abnormal form is rich in beta sheet and resists normal enzymatic breakdown.

• HIV genes
  • gag — In HIV, gag is the gene that encodes structural proteins such as p17 and p24, including the capsid and matrix components. It comes before pol and env in the viral genome and is often tested through antibody detection of its proteins.
• HIV proteins
  • Envelope glycoproteins
    • HIV env — This HIV surface protein is the envelope trimer made of gp41 and gp120. Its binding to CD4 and the CCR5 or CXCR4 co-receptor triggers viral entry, and vaccine design targets its variable epitopes.
  • HIV-1 protease — HIV-1 protease is the viral enzyme that cleaves the Gag and Pol polyproteins during maturation of the AIDS virus. It is a dimeric aspartyl protease targeted by early antiretroviral drugs such as saquinavir.
• Virology

  • antigenic drift — Antigenic drift is the gradual accumulation of mutations in viral surface antigens such as hemagglutinin and neuraminidase. It is why influenza strains change from year to year, helping the virus evade immunity from prior infections and vaccines.

  • Virus classification systems

    • Baltimore classification — The Baltimore classification is the standard seven-group system for organizing viruses by genome type and how they make mRNA. It includes the positive-sense and negative-sense RNA groups, and it is named for David Baltimore, a Nobel laureate known for reverse transcriptase.

Bacterial genetics

• Transformation — uptake of naked DNA from the environment by a competent bacterium; demonstrated by Griffith (1928) and confirmed as DNA by Avery et al. (1944); basis of modern cloning protocols. It is a form of horizontal gene transfer in which a competent cell takes up foreign or naked DNA from the environment. Griffith demonstrated the phenomenon in 1928, and Avery, MacLeod, and McCarty later showed that DNA was the transforming material in 1944. It underlies laboratory methods such as heat shock, electroporation, and transfection used in cloning and genetic engineering.
  • bacterial transformation — Bacterial transformation is the uptake of foreign DNA by bacteria, often after treatment with calcium chloride and heat shock. Griffith’s experiments first pointed to the process, and the Avery-MacLeod-McCarty work showed that DNA was the transforming principle.
    • genetic competence — In microbiology, competence is the bacterial state that allows uptake of naked DNA from the environment through transformation. It is induced naturally in some species and can also be produced in the lab by calcium chloride treatment or electroporation so cells can absorb plasmids.
      • Competent — Competent bacteria are able to take up free DNA from their environment and undergo transformation. In genetics and microbiology, this term marks cells that can incorporate external genetic material and thereby acquire new traits.
    • Griffith’s Experiment — Griffith’s experiment showed that harmless bacteria could be transformed into virulent forms by material from heat-killed pneumococci. It provided early evidence for bacterial transformation and helped establish that hereditary information could pass between cells.
• Transduction — transfer of bacterial DNA from one cell to another via a bacteriophage acting as a vector; generalized transduction carries random host DNA fragments, specialized transduction carries specific genes adjacent to the prophage insertion site. A phage can package bacterial DNA and carry it into another bacterium, where it may recombine with the recipient genome.
  • A bacteriophage injects DNA into a bacterial cell — This describes transduction, the transfer of bacterial DNA by a bacteriophage injecting genetic material into a bacterial cell. It is one of the major mechanisms of horizontal gene transfer in bacteria.

• Conjugation — direct cell-to-cell transfer of DNA through a pilus connection; typically involves plasmid transfer (F factor in E. coli); the closest bacterial analog to sexual reproduction.

• Plasmid — small, circular, extrachromosomal DNA element that replicates independently in bacteria; often carries antibiotic resistance genes; the core tool of recombinant DNA technology. Plasmids are major vehicles for horizontal gene transfer and are widely used as cloning vectors in recombinant DNA technology. Examples include the F factor, R plasmids, and Ti plasmids, and they can move between cells by conjugation or be isolated in minipreps.

  • episomes — An episome is a plasmid that can exist independently in a cell or integrate into the bacterial chromosome. In bacteria, the term is used for plasmids capable of inserting into the genome in the nucleoid region.

  • F plasmid — The F plasmid, or fertility factor, is a bacterial plasmid that carries the tra region and genes needed for conjugation, including formation of the sex pilus. It is transferred during bacterial conjugation and enables DNA transfer between bacteria. When integrated into the chromosome, it produces Hfr cells and can carry adjacent bacterial DNA during transfer, helping spread genetic traits. It is also called the sex factor or F plasmid.

  • Ti plasmid — The Ti plasmid is the tumor-inducing plasmid of Agrobacterium. It is widely used in plant genetic transformation and contains genes involved in opine production.

• Restriction-modification system — bacteria protect their own DNA by methylating specific sequences (modification) and cleave foreign unmethylated DNA at the same sites (restriction); restriction enzymes (Type II) cut at defined palindromic sequences; the basis of recombinant DNA technology.

• Transformation (genetics)
  • electroporation — Electroporation is a method for increasing cell membrane permeability with an electric field so bacteria can take up foreign DNA. It is used to promote transformation and is measured by factors such as the time constant in bacterial protocols.

Molecular tools

• Restriction enzymes (restriction endonucleases) — bacterial enzymes that cut double-stranded DNA at specific short palindromic sequences (restriction sites); produce blunt or sticky ends; used to cut DNA for cloning and mapping; named for the bacterial strain of origin (e.g., EcoRI from E. coli).

• PCR (polymerase chain reaction) — exponential amplification of a specific DNA segment; cycles of denaturation (heat ~95 °C), primer annealing (~55 °C), and extension (72 °C, Taq polymerase); invented by Kary Mullis (~1983; 1993 Nobel Prize); requires two flanking primers and thermostable DNA polymerase. It was invented by Kary Mullis around 1983, earned him the 1993 Nobel Prize in Chemistry, and transformed genetics, forensics, and medicine.

• Sanger sequencing (chain-termination method) — DNA synthesis reactions containing dideoxynucleotide terminators (ddNTPs); synthesis stops when a ddNTP is incorporated; fragments separated by size reveal the sequence; developed by Frederick Sanger (1977); dominant method until next-generation sequencing.

• Gel electrophoresis — DNA, RNA, or protein fragments are separated by size through an agarose (nucleic acids) or polyacrylamide (SDS-PAGE for proteins) gel under an electric field; smaller fragments migrate farther; bands visualized with ethidium bromide or SYBR dyes. Common protein gels use acrylamide/polyacrylamide and can be used in SDS-PAGE and western blot workflows; proteins may also be visualized with Coomassie blue in addition to ethidium bromide or SYBR dyes.
  • DNA gel electrophoresis — DNA gel electrophoresis separates DNA fragments by size in an electric field, usually through an agarose gel. The technique is often used before Southern blotting and can visualize bands with ethidium bromide under UV light.

• Human Genome Project — international effort to sequence all ~3 billion base pairs of the human genome; draft announced 2000 (Clinton/Blair), completed 2003; revealed ~20,000–25,000 protein-coding genes; ~98.5% of genome is non-coding; run by NIH (Francis Collins led U.S. effort) with parallel private effort by Celera Genomics (Craig Venter).

• RNA interference (RNAi) — gene silencing by small double-stranded RNA; siRNAs and miRNAs guide the RISC complex to cleave or block complementary mRNAs; discovered by Fire and Mello in C. elegans (1998; 2006 Nobel Prize); endogenous miRNAs regulate development and gene expression; used experimentally to knock down gene expression.

• CRISPR-Cas9 — a bacterial adaptive immune system adapted for genome editing; a guide RNA (gRNA) directs the Cas9 nuclease to a complementary genomic target, where it makes a double-strand break; the break is repaired by NHEJ (introducing indels) or HDR (precise edits); developed as a gene-editing tool by Doudna and Charpentier (2020 Nobel Prize in Chemistry). It uses PAM sequences and Cas9 domains such as HNH and RuvC to create double-strand breaks.

• Southern blot — detects specific DNA sequences: DNA digested with restriction enzymes → gel electrophoresis → transferred to membrane → hybridized with labeled probe; developed by Edwin Southern (1975). DNA is typically transferred to a nylon or nitrocellulose membrane, and large fragments may be depurinated before transfer.

• Northern / Western blot — Northern blot detects specific RNAs (analogous to Southern but for RNA); Western blot detects specific proteins using antibodies after SDS-PAGE.

• Forensic genetics
  • DNA evidence — DNA evidence refers to biological material analyzed through methods such as PCR, sequencing, and DNA fingerprinting. It became famous in criminal cases involving Gary Dotson, Colin Pitchfork, and laboratory standards maintained by the FBI.
• Molecular biology techniques
  • Blotting — Blotting is a molecular biology technique used to detect a specific DNA or RNA sequence within a larger mixture. It works by transferring material to a membrane and probing it, as in Southern or Northern blotting.

    • Northern blot — A molecular biology technique used to detect specific RNA molecules after gel electrophoresis and transfer to a membrane. It hybridizes a labeled probe to the separated RNA on the membrane to reveal gene-expression levels in a sample. The name mirrors Southern blotting, the better-known DNA method that inspired the pun.

    • Western blotting — Western blotting is a laboratory technique used to detect specific proteins after separation by SDS-PAGE and transfer to a membrane. It is probed with antibodies, often using HRP or alkaline phosphatase labels, and bands may be visualized with chemiluminescence or Ponceau S. Common membranes include nitrocellulose and PVDF, and blocking is often done with milk or BSA. The technique is widely used in medical testing, including confirmation of HIV and Creutzfeldt-Jakob disease, and its name is a pun on Southern blotting.

  • Nucleic acid extraction
    • phenol-chloroform extraction — Phenol-chloroform extraction is a nucleic-acid purification method that partitions proteins and lipids into an organic phase. It is often followed by alcohol precipitation and is a classic method for isolating DNA or RNA.
  • PCR
    • Polymerase chain reaction — Polymerase chain reaction is a laboratory method for amplifying a specific DNA segment outside a cell through repeated cycles of denaturation, primer annealing, and extension. It uses primers that flank the target region, heat cycling, and a heat-stable DNA polymerase such as Taq polymerase from Thermus aquaticus. It was developed by Kary Mullis and has major applications in sequencing, forensic analysis, and real-time quantification, with variants including qPCR, RT-PCR, touchdown PCR, helicase-dependent PCR, and site-directed mutagenic PCR.
      • master mix — A master mix is the precombined PCR solution that usually contains polymerase, buffer, magnesium chloride, and dNTPs. It is aliquoted into reactions so the same mixture can be used repeatedly with only the template changed.

  • site-directed mutagenesis — Site-directed mutagenesis is a lab technique used to introduce chosen DNA changes into a specific gene. It is associated with Kunkel’s method in uracil-deficient E. coli and with applications such as alanine scanning and mutation library construction.

  • yeast two-hybrid assay — A method for detecting protein-protein interactions in Saccharomyces cerevisiae by fusing a bait and prey protein to separate parts of a transcription factor, typically a DNA-binding domain and an activation domain. When the proteins interact, the domains are reunited and a reporter gene such as lacZ is activated. It is commonly built around the Gal4 system. Variants include split-ubiquitin methods for membrane proteins and mammalian adaptations such as KISS.
• PCR

  • annealing step — The annealing step in PCR is the stage when primers bind to the template DNA. It usually requires careful temperature optimization, often around 50 to 60 degrees Celsius.

  • primer dimers — Primer dimers are unwanted PCR by-products formed when primers anneal to each other instead of the target DNA. They often appear as very small bands on a gel and can be reduced by hot-start PCR or by careful master-mix design.

  • quantitative polymerase chain reaction — Quantitative polymerase chain reaction measures DNA amplification during the reaction by detecting fluorescence cycle by cycle. It is widely used to quantify nucleic acids and gene expression, often analyzed with threshold-cycle methods such as the Livak delta-delta-Ct calculation. It can be combined with reverse transcription to measure RNA-derived cDNA, and with other assays such as ChIP-based workflows.
    • Ct value — The Ct value, or cycle threshold, is the qPCR point at which fluorescence rises above background. It is used in relative gene-expression analysis, including delta-delta calculations.
  • Real-time polymerase chain reaction
    • real-time (or quantitative) polymerase chain reaction — Real-time polymerase chain reaction, or qPCR, is a DNA amplification technique that measures product as it accumulates during thermal cycling. It commonly uses fluorescent dyes such as SYBR Green and can detect primer dimers or melting-temperature differences.
  • Steps
    • elongation — In PCR, elongation is the step in which DNA polymerase synthesizes the new strand after denaturation and annealing. It typically occurs near 72 degrees Celsius and is also called extension.
• PCR methods
  • quantitative real time PCR — Quantitative real-time PCR, or qRT-PCR, is a fluorescence-based PCR method used to measure DNA or cDNA amplification as it happens. SYBR green and TaqMan probes are common detection methods, and the technique is widely used for precise quantitation of samples.
• PCR variants
  • touchdown PCR — Touchdown PCR is a PCR variant that begins with a high annealing temperature and then lowers it over successive cycles. The gradual decrease increases specificity by favoring the correct primer-template match early in the reaction.
• DNA sequencing
  • Next-generation sequencing
    • next-generation DNA sequencing — Next-generation DNA sequencing is a high-throughput method for determining the order of bases in nucleic acids from short reads. Common platforms include Illumina, and analysis tools such as Bowtie, Cufflinks, and TopHat are often associated with its output.

  • Sanger method — The Sanger method is the dideoxy DNA sequencing technique that stops strand synthesis with chain-terminating nucleotides. It is named for Frederick Sanger, the only person to win the Nobel Prize in Chemistry twice.

  • shotgun sequencing — Shotgun sequencing is a DNA sequencing method in which long DNA is broken into many fragments, sequenced separately, and assembled by overlap. Associated with Craig Venter’s genome projects, it is useful for large genomes but can struggle with repeated regions.
• DNA sequencing methods
  • Maxam-Gilbert method — The Maxam-Gilbert method is an early DNA sequencing technique that chemically cleaves DNA at specific bases before gel electrophoresis. It is often contrasted with the Sanger method because it relies on base-specific chemical cleavage rather than chain termination.
• DNA extraction
  • purification of DNA — Purification of DNA is the process of isolating DNA, RNA, or related nucleic acids from a sample. Common methods include phenol-chloroform extraction, ethanol precipitation, spin columns, and anion-exchange resins. Successful purification is often checked by a high 260-to-280-nanometer absorbance ratio and stored products may be kept in Tris-EDTA buffer.
• Electrophoresis

  • agarose gels — Agarose gels are porous matrices used in electrophoresis to separate DNA fragments by size and charge. They are commonly soaked in buffer such as TAE and are standard in PCR genotyping and comet assays.

  • Isoelectric focusing — Isoelectric focusing separates proteins by the pH at which they reach their isoelectric point. It uses a pH gradient and electric field, and it is often the second dimension in two-dimensional gel electrophoresis.

• Protein electrophoresis
  • SDS-PAGE — SDS-PAGE is a protein-separation technique that uses detergent and a polyacrylamide gel to sort proteins largely by molecular weight. It is often paired with isoelectric focusing in protein analysis.
• Biotechnology companies

  • Illumina — Illumina is the San Diego-based biotechnology company known for high-throughput DNA sequencing platforms such as HiSeq and MiSeq. It acquired Solexa in 2007 and popularized sequencing-by-synthesis with reversible dye terminators.

  • Illumina, Inc — Illumina, Inc. is the major sequencing company behind platforms such as HiSeq, MiSeq, and NovaSeq. Its systems use sequencing-by-synthesis for short-read DNA sequencing, and the company bought Solexa in 2007.

• Genetics projects
  • International HapMap Project — The International HapMap Project cataloged common human genetic variation by mapping SNPs and haplotypes across multiple populations. It followed the Human Genome Project and became a key resource for association studies.
• Genomics

  • C-value — The C-value is the amount of DNA contained in the haploid genome of a species. The C-value paradox refers to the observation that genome size can include excess DNA not obviously required for development or evolutionary divergence.

  • CpG islands — CpG islands are DNA regions rich in cytosine-phosphate-guanine sequences, and they are often found in promoters. Methylation at these sites can silence gene expression.

  • DNA sequencing
    • Nanopore sequencing — Nanopore sequencing is a long-read DNA sequencing method that identifies bases by measuring changes in electrical current as a strand passes through a tiny pore. It is associated with Oxford Nanopore and instruments such as the MinION.

  • ENCODE — ENCODE is the Encyclopedia of DNA Elements project, a major genomics effort led by the National Human Genome Research Institute. Its published data argued that a large share of the human genome has biochemical activity and regulatory function.

  • Genome projects
    • Celera Genomics — Celera Genomics was the private company that raced the public Human Genome Project to sequence the human genome. It was tied to Craig Venter and was also known for shotgun sequencing work on Drosophila.

  • genome size — Genome size is the total amount of DNA in an organism’s genome, often measured in base pairs or mass. It is the quantity behind the C-value enigma, which notes that genome size does not track organismal complexity well.

  • genomes — Genomes are the complete genetic complements of organisms, and they can be compared by alignment, synteny, and assembly tools such as BLAST and Bowtie. Large sequencing projects, including the Human Genome Project and Celera, focused on reconstructing genomes from short reads.
    • human genome — The human genome is the complete hereditary material of humans. Its sequencing became a major international project and was found to contain far fewer genes than originally expected.
  • Researchers
    • John Craig Venter — American genomics researcher and entrepreneur who founded The Institute for Genomic Research and led Celera Genomics. Celera used shotgun sequencing in competition with the Human Genome Project, and Venter became known for his role in the race to sequence the human genome. He later pursued synthetic biology projects, including work toward the first man-made lifeform, and expeditions exploring marine genetic diversity.
  • J. Craig Venter — J. Craig Venter is the biologist who helped lead Celera Genomics and became a central figure in the race to sequence the human genome. He is also associated with shotgun sequencing and controversial advocacy of gene patenting.
• CRISPR-Cas systems
  • PAM — PAM, short for protospacer adjacent motif, is the short DNA sequence Cas9 must recognize in CRISPR editing. For SpCas9, the PAM is typically NGG, and it lies adjacent to the target sequence rather than inside it.
• gene editing — Gene editing is the technique used to insert, delete, or replace genetic material in living organisms. It is often linked to CRISPR/Cas9, and earlier approaches included TALEN-style systems and other engineered nucleases.

  • gene drives — Gene drives are synthetic genetic systems designed to spread a trait through a population faster than normal inheritance would allow. They have been proposed for controlling insects and other pests, including screwworms.

  • Gene knockout — Gene knockout is a reverse-genetics technique that inactivates a gene to study its function, often through homologous recombination. It is strongly associated with the creation of knockout mice and with the Nobel-winning work of Capecchi, Evans, and Smithies.

  • He Jiankui — He Jiankui is the Chinese biophysicist who announced the creation of Lulu and Nana, the first genetically edited babies, using CRISPR-Cas9. His work sparked major bioethics controversy and led to his firing from SUSTech and detention in China.

  • terminator genes — Terminator genes are a proposed form of genetic use restriction technology designed to make engineered crops sterile in later generations. The technology is controversial and was developed in commercial contexts by companies such as Delta and Pine Land, though it was never widely used.

• Site-specific recombinases
  • Cre recombinase — Cre recombinase is a site-specific recombinase used with loxP sites to conditionally delete, invert, or rearrange DNA. It is widely used in lineage tracing and in systems such as Brainbow and floxed knockouts.
• Protein-protein interaction assays
  • two-hybrid screen — A two-hybrid screen is a molecular biology assay used to detect protein-protein interactions by linking interacting proteins to a reporter system. Yeast versions often use Gal4 fragments and reporters such as HIS3 or lacZ.
• Biophysical techniques
  • FRET — FRET is a distance-dependent nonradiative energy transfer between a donor and acceptor chromophore. It is used as a molecular ruler in biochemistry and cell biology, especially in single-molecule and time-resolved fluorescence assays.
• nucleases — Nucleases are enzymes that cut nucleic acids, including restriction enzymes such as EcoRI that recognize specific DNA sequences. They include endonucleases and exonucleases, and are central to techniques such as DNA digestion and RFLP analysis.

  • Cas9 — Cas9, or CRISPR-associated protein 9, is the DNA-cutting enzyme used in CRISPR genome editing. It binds guide RNA and a PAM-adjacent target to make a double-strand break, and variants such as dead Cas9 and Cas9 nickase are common in gene-regulation and editing techniques.

  • DNase — DNase is a family of enzymes that degrades DNA into reusable nucleotides. The name is used generically for enzymes that break down DNA rather than RNA.

  • Exonuclease — An exonuclease is an enzyme that removes nucleotides one at a time from the ends of a nucleic acid strand. DNA polymerases can have exonuclease activity for proofreading or for removing RNA primers during replication.

  • nuclease — A nuclease is an enzyme that cuts DNA or RNA by cleaving phosphodiester bonds. Common subclasses include endonucleases, exonucleases, and S1 nucleases that act on single-stranded nucleic acids.

• Protein Engineering

  • phage display — Phage display is a protein-engineering technique in which bacteriophages present peptides or proteins on their surfaces while carrying the encoding gene inside. It is used for biopanning, affinity selection, and directed evolution, and it played a role in antibody design and in the development of Humira.

  • recombinant proteins — Recombinant proteins are proteins produced from DNA assembled from different sources and cloned in a laboratory. Examples include therapeutic proteins such as EPO and IGF-1.

• Structural Biology Databases
  • Protein Data Bank — The Protein Data Bank is the major database of experimentally determined macromolecular structures, especially proteins, and is used widely in structural biology. Its entries are commonly derived from X-ray crystallography and are distributed in standardized coordinate formats such as mmCIF.
• Protein assays
  • Bradford assay
    • Bradford protein assay — The Bradford protein assay measures protein concentration by the absorbance shift of Coomassie Brilliant Blue G-250. It is commonly standardized with bovine serum albumin and read spectrophotometrically at about 595 nm.
• Ligand binding assays
  • Scatchard plot — A Scatchard plot is a linearization used in ligand-binding studies that graphs bound over free ligand against bound ligand. Its slope and intercept reveal binding affinity and maximum binding capacity, making it useful in receptor and enzyme kinetics.
• Biochemistry techniques
  • immunoprecipitation — Immunoprecipitation is a lab technique that uses an antibody to pull a specific antigen or protein out of solution. Its chromatin version, ChIP, is used to study DNA binding sites and is often done with beads such as agarose or superparamagnetic microbeads. The process is often shortened to IP, and related forms include ChIP when DNA is involved. Co-immunoprecipitation is a common use of the same method for testing protein-protein interactions.
• bioinformatics — Bioinformatics is the interdisciplinary field that uses algorithms, databases, and statistical methods to analyze biological data, especially DNA and protein sequences. BLAST, Needleman–Wunsch, and related alignment methods are standard clues, along with work at centers such as EMBL-EBI and NCBI.

  • genome assembly — Genome assembly is the process of reconstructing a genome from sequencing reads. It produces contigs and scaffolds, and its quality is often measured with Nx statistics after de Bruijn graph or overlap-layout-consensus methods are applied.

  • sequence alignment — Sequence alignment is the bioinformatics task of arranging DNA, RNA, or protein sequences to highlight regions of similarity. Common tools include BLAST, Clustal, Needleman-Wunsch, and Smith-Waterman, and the results can support phylogenetic analysis.

  • Substitution matrices

    • BLOSUM — BLOSUM is a family of protein substitution matrices used by BLAST to score amino acid alignments. Its name expands to Blocks of amino acid Substitution Matrix, and common versions such as BLOSUM62 are built from conserved blocks of related sequences.
• Transcriptomics
  • RNA sequencing — RNA sequencing is the transcriptomic technique used to measure RNA abundance across a sample, from bulk preparations to single-cell droplet-based assays. Its outputs are commonly normalized with TPM, FPKM, or related measures, and are often analyzed with tools such as Seurat, Cell Ranger, and UMAP or t-SNE for clustering.
• Synthetic biology — Synthetic biology is the field devoted to designing and constructing novel genetic systems for useful purposes. It includes work on genetic circuits, BioBrick standards, and engineered organisms such as E. coli chassis used in iGEM projects.

Model Organisms

• model organisms — Model organisms are species or systems used to study broader biological processes, with classic examples including E. coli, Drosophila, C. elegans, zebrafish, and Neurospora. They are chosen because they are convenient, well-studied stand-ins for answering questions about other organisms.
  • model organism — A model organism is a species used in research because it is convenient for experiments and genetic study. Examples include fruit flies, C. elegans, mice, Arabidopsis, and tobacco mosaic virus in some research contexts.
• E. coli — gram-negative bacterium; workhorse of molecular biology; used to discover the lac and trp operons, DNA replication mechanisms, and as a host for recombinant protein expression; standard laboratory strain K-12. It is a rod-shaped enteric bacterium normally found in the intestines; some strains, including O157:H7, cause severe food poisoning, and its presence in drinking water signals fecal contamination. It has also been widely used in recombinant DNA work and genetic engineering, including classic genetics experiments and work on restriction enzymes.
  • purple-centered colonies with a green metallic sheen — This is the appearance of lactose-fermenting colonies on Eosin-Methylene Blue agar, where strong fermenters can produce a characteristic green metallic sheen. It is a classic identification clue for Escherichia coli on selective culture media.

• Saccharomyces cerevisiae (budding yeast) — unicellular fungus; key eukaryotic model for cell cycle (cyclins/CDKs; Hartwell Nobel Prize 2001), aging, DNA repair, and gene function; amenable to genetic manipulation; ~6,000 genes.

• Drosophila melanogaster — fruit fly; Morgan’s chromosome theory, sex linkage, and genetic mapping; ~14,000 genes; polytene chromosomes; balancer chromosomes; Hox genes (Antennapedia and Bithorax complexes); 4 chromosome pairs; ~4-week life cycle.

• Caenorhabditis elegans (C. elegans) — nematode worm; first multicellular organism with a fully sequenced genome (1998); invariant 959 somatic cells; complete cell lineage mapped; used to discover RNAi (Fire and Mello, 2006 Nobel Prize) and apoptosis pathways (Sulston, Horvitz, Brenner, 2002 Nobel Prize).

• Arabidopsis thaliana — small flowering plant; model for plant genetics and development; small genome (~135 Mb); first plant genome sequenced (2000); used to study flowering-time genes, hormone signaling, and disease resistance. It is also notable for the ABC model of flower development, transformation by Agrobacterium tumefaciens, and studies of self-pollination and immunity. Its full genome was sequenced first, and the chromosome assembly was completed by a genomic initiative in 2001.

• Zebrafish (Danio rerio) — vertebrate model for developmental biology; transparent embryos; forward genetic screens identified key developmental genes; used in regeneration biology and disease modeling; easy to do in vivo imaging.

• Mus musculus (mouse) — primary mammalian model organism; gene targeting/knockout technology (Capecchi, Evans, Smithies; 2007 Nobel Prize); ~85% of genes shared with humans; essential for studying mammalian development, immunology, and cancer.

• Drosophila genes
  • bicoid — Bicoid is a maternal-effect gene and transcription factor in Drosophila that sets up the embryo’s anterior-posterior axis. Its protein gradient is highest at the anterior pole, where it helps specify head and thorax structures and activates genes such as hunchback.
• Fruit flies
  • The Mediterranean fruit fly — The Mediterranean fruit fly, or medfly, is a destructive agricultural pest that has repeatedly plagued California. It is notorious for outbreaks that prompt chemical spraying campaigns such as the Ventura County malathion treatment.

Landmark Experiments

• Griffith’s transformation experiment (1928) — Frederick Griffith injected mice with live rough (non-virulent) Streptococcus pneumoniae + heat-killed smooth (virulent) cells; mice died and live smooth bacteria were recovered, demonstrating that a “transforming principle” could convert non-virulent bacteria; did not identify DNA as that principle.

• Avery-MacLeod-McCarty experiment (1944) — Oswald Avery, Colin MacLeod, and Maclyn McCarty purified the Griffith transforming principle and showed it was DNA (not protein or RNA) by using DNase to destroy transforming activity; first strong evidence that DNA is the genetic material. They worked with strains of Streptococcus pneumoniae and purified the transforming principle from bacterial extracts, building directly on Griffith’s transformation experiment.

• Hershey-Chase experiment (1952) — Alfred Hershey and Martha Chase labeled T4 bacteriophage DNA with ³²P and protein coat with ³⁵S; after infection, ³²P entered bacteria while ³⁵S stayed outside; confirmed DNA (not protein) is the genetic material; used a blender and centrifuge to separate phage coats from infected cells.

  • Alfred Hershey and Martha Chase — Alfred Hershey and Martha Chase performed the 1952 bacteriophage experiment that showed DNA, not protein, is the genetic material. Their work confirmed the conclusions of the Avery-MacLeod-McCarty experiment.

  • Hershey-Chase experiments — The Hershey-Chase experiments used radioactive sulfur and phosphorus to track phage protein and DNA during infection of E. coli. Their results showed that DNA, not protein, entered the bacteria and carried the genetic information.

• Meselson-Stahl experiment (1958) — Matthew Meselson and Franklin Stahl grew E. coli in heavy ¹⁵N medium, then shifted to light ¹⁴N; CsCl density-gradient centrifugation after each generation revealed hybrid-density DNA at first replication, then a mix of hybrid and light DNA; definitively confirmed semi-conservative replication; “the most beautiful experiment in biology.”

• Beadle-Tatum one gene–one enzyme (1941) — George Beadle and Edward Tatum irradiated Neurospora crassa to create auxotrophic mutants blocked at single steps in biosynthetic pathways; concluded each gene encodes a single enzyme; 1958 Nobel Prize; later refined to “one gene, one polypeptide.”

• Miller-Urey experiment (1953) — Stanley Miller and Harold Urey simulated early Earth conditions (water, methane, ammonia, hydrogen; electrical sparks for lightning) in a closed apparatus; produced amino acids and other organic molecules; provided experimental support for the abiotic synthesis of life’s building blocks (Oparin-Haldane hypothesis). The experiment was first reported in 1952 and is commonly dated to 1953.
  • Stanley Miller — Stanley Miller was a graduate student at the University of Chicago who helped demonstrate a possible path to organic synthesis under primordial Earth conditions. He is best known for the Miller-Urey experiment, in which methane, ammonia, hydrogen, and water vapor were exposed to electrical sparks to produce amino acids.

• Luria-Delbrück experiment (1943) — Salvador Luria and Max Delbrück used fluctuation analysis to show that bacterial resistance to phage arises by random mutation before phage exposure, not as a directed response; demonstrated Darwinian evolution in bacteria; won the 1969 Nobel Prize; established that mutation is random with respect to need. The Luria-Delbrück experiment used bacteriophage resistance in bacterial cultures to show that mutations arise spontaneously rather than being induced by selection. Its fluctuating results were later analyzed with fluctuation methods that helped confirm mutation as a random event.

• Watson-Crick double helix / Photo 51 (1953) — James Watson and Francis Crick built a model of DNA as an antiparallel double helix with complementary base pairing; Rosalind Franklin’s X-ray diffraction image “Photo 51” (also insights from Franklin’s measurements shared via Maurice Wilkins without her knowledge) was critical; published in Nature April 25, 1953; Watson, Crick, and Wilkins shared the 1962 Nobel Prize; Franklin had died of cancer in 1958.

• Lac operon (Jacob-Monod, 1961) — François Jacob and Jacques Monod described the lac operon in E. coli as the first model of gene regulation; identified the repressor protein, operator, and inducer (allolactose); demonstrated that gene expression can be controlled at the level of transcription; won the 1965 Nobel Prize in Physiology or Medicine.

• Clonal selection theory (Burnet, 1957) — Frank Macfarlane Burnet proposed that each lymphocyte bears a single antigen receptor; antigen selects and triggers proliferation of matching clones; explains antibody diversity and immunological memory; won the 1960 Nobel Prize.

• Abiogenesis — Abiogenesis is the origin of life from nonliving matter, the process proposed for the first appearance of living cells on Earth. It is associated with the Miller-Urey experiment, primordial soup models, and theories involving ribozymes and early Earth chemistry.

  • Biogenesis — Biogenesis is the principle that living organisms arise only from other living organisms. It is the opposite of spontaneous generation and is associated with Pasteur’s swan-neck flask experiments.

  • Last Universal Common Ancestor — The last universal common ancestor, or LUCA, is the hypothetical cell population from which all current life descends. It is often placed in the early Earth’s hydrothermal vents and is invoked to explain why core molecular machinery such as DNA polymerase is shared by all modern organisms.

  • protobionts — Protobionts are pre-cellular aggregates that may have preceded the first living cells. They could maintain an internal chemical environment and are often discussed through examples such as coacervates, microspheres, and liposomes.

  • spontaneous generation — The discredited theory that living things arise directly from nonliving matter. It was challenged by experiments associated with Redi, Needham, Spallanzani, Schwann, and Pasteur, including demonstrations involving maggots, boiled broth, sealed containers, and Pasteur’s swan-neck flasks.

    • John Needham — John Needham was an English clergyman remembered for arguing that microorganisms could arise by spontaneous generation. He is associated with an experiment on boiled, sealed broth that was taken as evidence for that view.

Studying Cells: Culture, Imaging, and Separation

• Cell biology techniques
  • counting cells — Counting cells is the task of determining how many cells are in a sample, often by using a hemocytometer. It can also be done with Coulter counters, which detect cells by current changes as they pass through a tiny opening.
• Cell culture

  • cell culturing — Cell culturing is the process of growing cells under controlled conditions, usually on nutrient media. Its practical clues include contamination control, media changes, and passaging as cells approach confluence.

  • organoids — Organoids are three-dimensional cell cultures grown from stem cells to mimic the structure and development of organs such as the brain, gut, thyroid, or stomach.

• Cell culture supplements
  • fetal bovine serum — Fetal bovine serum is a common supplement for mammalian cell culture, usually added to media such as DMEM and often starved out before experiments. Its shortened form is FBS, and it is associated with serum proteins such as bovine serum albumin in laboratory work.
• Cell lines

  • HeLa cells — HeLa cells are an immortal human cell line derived from the cervical cancer of Henrietta Lacks. They became a foundational laboratory line and were widely used in biomedical research without her consent.

  • immortal — Immortal describes cells or cell lines that can keep dividing indefinitely rather than undergoing senescence. The term is a standard clue for HeLa, immortalized hybridomas, and other cells made to proliferate beyond the Hayflick limit.

• Centrifugation
  • Svedberg unit — The Svedberg unit, abbreviated S, measures sedimentation rate during centrifugation. It is used especially for ribosomal subunits, as in the familiar 60S and 40S designations.
• Density gradient centrifugation
  • cesium chloride — Cesium chloride is a salt used to make density gradients for centrifugation, especially in DNA experiments. Meselson and Stahl used it in their classic replication study to separate labeled DNA.
• Laboratory equipment
  • Culture dishes
    • Petri dishes — Petri dishes are the shallow circular plates used in laboratories to culture bacteria and other microscopic cells on agar or other growth media. They are standard tools for isolating colonies and testing antibiotic resistance. The dish takes its name from the German biologist Julius Petri and is often just called a plate. It is a basic piece of microbiology equipment.
• Biological stains

  • Giemsa stain — Giemsa stain is a classical biological stain used on blood films, bone marrow, parasites, and chromosomes. It is used in G-banding because it strongly colors AT-rich regions of DNA.

  • trypan blue — Trypan blue is a dye used to stain dead cells and to test cell viability in the lab. Its name refers to trypanosomes, the parasites that cause sleeping sickness, and it was used in work on the blood-brain barrier.

• Stains
  • methylene blue — Methylene blue is a synthetic thiazine dye used as a stain and redox indicator in biology and chemistry. It serves as a counterstain in some bacterial staining methods and can also be used to assess cell viability.
• Microscopy
  • electron microscopy — Electron microscopy uses beams of electrons rather than light to visualize structures too small for ordinary microscopes. Its main forms are scanning and transmission electron microscopy, and cryo-electron microscopy preserves specimens by freezing them in vitreous ice. Common clues include gold sputter-coating, uranyl acetate staining, and the Everhart-Thornley detector.

    • scanning electron microscopy — Scanning electron microscopy uses a focused electron beam and backscattered electrons to produce detailed, three-dimensional surface images. It is contrasted with transmission electron microscopy and is used in environmental variants to observe topography and even living insects.

    • transmission electron microscopy — Transmission electron microscopy is a method in which electrons pass through a very thin sample to form an image. It is analogous to light microscopy, but uses electrons rather than visible light and therefore requires thin specimens.

  • fluorescence microscopy — Fluorescence microscopy uses dyes or proteins that absorb one wavelength and emit a longer one. It is used to localize molecules in cells, with examples including FITC, DAPI, and GFP.

    • STED microscopy — STED microscopy is a super-resolution fluorescence technique developed by Stephen Hell that beats the diffraction limit. It uses a donut-shaped depletion beam to switch off fluorophores around the focal spot and sharpen resolution to nanometer scales.

    • two-photon excitation microscopy — Two-photon excitation microscopy is a fluorescence imaging technique that uses near-simultaneous low-energy photons to excite fluorophores. Its deeper tissue penetration makes it useful for imaging living specimens such as transparent zebrafish brains.

  • FRAP — FRAP, or fluorescence recovery after photobleaching, measures how quickly fluorescence returns to a bleached spot after a targeted laser pulse. It is used to study diffusion, especially lateral movement of molecules in membranes, and is commonly performed with a confocal microscope.

  • Light microscopy

    • bright-field microscopy — Bright-field microscopy is a form of light microscopy in which light passes directly through the specimen. It usually requires staining because the method produces relatively little contrast on its own.

    • confocal microscopy — Confocal microscopy uses point illumination and a pinhole to reject out-of-focus light and improve resolution. It can collect optical slices for z-stacks that are reconstructed into three-dimensional images.

  • microscopes — Microscopes are instruments used to magnify objects too small to see with the naked eye, from simple optical models to electron microscopes. They are associated with Antonie van Leeuwenhoek and with techniques such as oil immersion, dark-field viewing, and fluorescence or scanning varieties. Standard parts include the stage, arm, eyepiece, and objective lenses.

  • photobleaching — Photobleaching is the irreversible loss of fluorescence caused by light damage to fluorophores. It is the phenomenon exploited in FRAP and is a major limitation in fluorescence microscopy and related super-resolution methods.

Bacterial Structure, Growth, and Communities

• Bacterial cell structures
  • capsule — A bacterial capsule is a protective outer covering, usually polysaccharide, that surrounds the cell wall of some bacteria. It helps pathogens evade immune detection, can be seen with the quellung reaction, and in Bacillus anthracis is made of D-glutamic acid.
• Bacterial communication

  • autoinducers — Autoinducers are signaling molecules used by bacteria for quorum sensing, letting cells gauge population density by tracking chemical concentration. They are especially associated with Vibrio species and Aliivibrio fischeri, which uses acyl homoserine lactones.

  • quorum sensing — Quorum sensing is the density-dependent bacterial communication system in which autoinducers trigger coordinated behavior. It is linked to the Lux operon in Vibrio fischeri and to biofilm formation and bioluminescence.

• Bacterial outer membrane components
  • LPS — LPS, or lipopolysaccharide, is the endotoxin found in the outer membrane of gram-negative bacteria. It is built from lipid A, a core polysaccharide or core oligosaccharide, and an O antigen, and is recognized by innate immune receptors such as TLR4. It is often used as an immune stimulant in biology labs, can contribute to septic shock, and is a target of polymyxin antibiotics such as colistin, which bind its outer surface.
• Bacterial secretion systems
  • type III secretion system — A needle-like apparatus used by many gram-negative bacteria to inject effector proteins directly into host cells. It is also called an injectisome and is important in virulence, including in plant pathogens such as Xanthomonas.
• Bacterial spores
  • endospore — An endospore is a dormant, highly resistant structure formed by some bacteria under unfavorable conditions. It allows the organism to survive heat, drying, or nutrient scarcity until conditions improve.
• Bacterial structures
  • Type IV pili — Type IV pili are bacterial surface structures that enable motility through extension, adhesion, and retraction. Their energy source is still not fully understood.
• Chlamydia
  • Chlamydia trachomatis — Chlamydia trachomatis is the bacterium responsible for chlamydia, the most common bacterial sexually transmitted disease. It can also cause trachoma, neonatal conjunctivitis, pelvic inflammatory disease, and lymphogranuloma venereum.
• Enterobacteriaceae

  • Shigella — Shigella is a genus of Gram-negative rod-shaped bacteria best known for causing dysentery. It is a major public-health concern and is often discussed in connection with foodborne and wastewater epidemiology studies.

  • Salmonella — A genus of rod-shaped gram-negative bacteria associated with food poisoning, especially from contaminated meat, raw poultry, and eggs, and with typhoid and paratyphoid fever. It is identified in the Ames test, commonly tracked by O and H antigens, and can be detected with tests such as the TSI slant.

    • Salmonella typhimurium — Salmonella typhimurium is the bacterium commonly used in the Ames test to screen for mutagenicity. It is a Gram-negative rod in the genus Salmonella and is also associated with foodborne illness.
• Microbial communities
  • Biofilm — A community of microbes attached to a surface and embedded in a self-produced matrix of extracellular polymers. Biofilms can form on metal oxides or other solid surfaces, and organisms such as Pseudomonas aeruginosa can produce them. Dental plaque and many persistent infections are classic examples of biofilms.
• Microbial immune systems
  • CRISPR — CRISPR is a bacterial DNA repeat system that provides adaptive immunity by preserving fragments of invading genetic material. It was adapted into a genome-editing system, often paired with the Cas9 nuclease, in which a guide RNA directs a cut near a PAM sequence.
• Microbiology
  • anaerobic — Anaerobic organisms live without oxygen, and obligate forms must avoid it completely. Facultative anaerobes can switch between fermentation and other pathways when oxygen is absent. Methane-producing microbes, sulfate reducers, and organisms in mud or glovebox cultures are classic examples.
    • obligate anaerobes — Obligate anaerobes are organisms that can grow only in the absence of oxygen. Because they cannot handle reactive oxygen species, they settle to the bottom of thioglycolate broth and include methanogens that were harmed during the Great Oxygenation event.
  • Bacterial cell envelope
    • endotoxins — Endotoxins are lipopolysaccharides in the outer membrane of Gram-negative bacteria. Their structure includes lipid A, a core polysaccharide, and an O antigen, and they are distinct from exotoxins.
  • biofilms — Biofilms are surface-attached microbial communities embedded in a protective matrix of extracellular polymeric substances. They are responsible for problems such as dental plaque and persistent infections because they resist antibiotics and are difficult for host defenses to clear.
    • dental plaque — Dental plaque is the sticky biofilm that builds up on teeth between brushings and dental visits. If it accumulates, it hardens into tartar and can lead to gingivitis.
  • Microbial growth models
    • Monod equation — The Monod equation models microbial growth as a function of limiting substrate concentration in a bioreactor. It uses a maximum specific growth rate and a half-saturation constant, and it approaches saturation at high substrate levels.
  • microbiome — A microbiome is the community of microorganisms, or their collective genetic material, in a particular environment. It is studied with tools such as 16S rRNA sequencing, UniFrac, QIIME, and mothur, and it includes gut, skin, and vaginal communities.
    • Gut microbiome — The gut microbiome, or gut flora/gut microbiota, is the community of microorganisms living in the digestive tract, especially the colon. These microbes help break down fiber and other nutrients, including cellulose, produce metabolites such as short-chain fatty acids, synthesize useful compounds such as biotin and vitamin K, and help shape the immune response. The community is often altered or disrupted by diet, antibiotics, probiotics, broad-spectrum antibiotics, or fecal transplant.
• Microbiology techniques
  • streaking — Streaking is the act of running naked or otherwise appearing in public nude, a college prank that briefly became a fad in the early 1970s. It is also the microbiology technique of spreading bacteria across a plate in successive passes to isolate single colonies.

Evolution and Diversity

Connect evolutionary processes to the diversity and relationships of organisms.

Evolution and Natural Selection

• Charles Darwin — On the Origin of Species (1859); proposed evolution by natural selection based on observations from the Beagle voyage (1831–1836), Galápagos finches and tortoises among key evidence. He also wrote The Voyage of the Beagle, The Descent of Man, and later studies on coral reefs, insectivorous plants, barnacles, atolls, earthworms, and expression in humans and animals.

  • On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life — This is Darwin’s 1859 On the Origin of Species by Means of Natural Selection, the foundational text of evolutionary theory. It introduced natural selection and argued for the preservation of favored races in the struggle for life.

  • On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races — Charles Darwin’s 1859 landmark book argues that living things change over time through evolution by natural selection and the preservation of favored variations. Written after Darwin’s Beagle voyage and informed by Alfred Russel Wallace’s similar conclusions, it became especially associated with finches, domestication, and Darwin’s description of it as “one long argument.” The work is organized through chapters on topics such as instinct, hybridism, the geological record, and laws of variation, and it ends by declaring the grandeur of life’s evolutionary history.

  • On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life — Darwin’s On the Origin of Species laid out natural selection as the mechanism of evolution and drew on evidence from Galápagos finches, pigeons, and other observations from the Beagle voyage. It challenged older creationist explanations and became one of the foundational texts of modern biology.

  • The Descent of Man — The Descent of Man is Charles Darwin’s major later work on human evolution and sexual selection. It extends ideas from On the Origin of Species and argues that humans descended from earlier primate ancestors.

  • The Descent of Man and Selection in Relation to Sex — Darwin’s The Descent of Man and Selection in Relation to Sex extends evolutionary theory to humans and develops the idea of sexual selection. It argues that traits such as plumage, odor glands, and musical ability can arise through mate choice and competition.

• Alfred Russel Wallace — independently proposed natural selection simultaneously; co-presented with Darwin at the Linnean Society, 1858. He was a British naturalist and explorer, often called a founder or father of biogeography. He wrote The Malay Archipelago, and the Wallace Line is named for him as the faunal boundary separating Asiatic from Australian species.

• Conditions for natural selection — variation in a population; heritable variation; differential survival/reproduction based on that variation; population responds over time.

• Types of selection — directional (shifts mean phenotype toward one extreme); stabilizing (favors intermediate phenotypes, reduces variance); disruptive (favors both extremes over the intermediate).

• Adaptation — heritable trait that increases fitness in a given environment. The term also titles a Spike Jonze film about Charlie Kaufman’s attempt to adapt The Orchid Thief.
  • Mimicry — Mimicry is an adaptive evolutionary resemblance in which one organism imitates another, often to avoid predators or to attract pollinators. Common subtypes include Batesian and Müllerian mimicry; aggressive mimicry is used to lure prey. Examples include coral and king snakes, hoverflies and wasps, viceroy and monarch butterflies, and orchid pseudocopulation.

    • aggressive mimicry — Aggressive mimicry is a predatory strategy in which an organism resembles something harmless or attractive in order to lure prey. It is famously associated with female Photuris fireflies and other parasites or predators that exploit deception for feeding or attack.

    • Batesian — Batesian mimicry is the kind of mimicry in which a harmless organism resembles a noxious or dangerous one. Predators are fooled into avoiding the mimic because they mistake it for the species with the warning system.

    • Batesian mimicry — Batesian mimicry is a defensive form of mimicry in which a harmless or palatable species imitates a harmful, unpalatable, or otherwise dangerous one to avoid predation. It is named for Henry Walter Bates and is contrasted with warning-color strategies such as aposematism and with Müllerian mimicry. Common examples include hover flies resembling bees and king snakes resembling coral snakes.

    • Henry Bates — Henry Bates was a British naturalist whose work on Amazonian butterflies led to the description of Batesian mimicry. His observations of harmless species resembling toxic ones helped support Darwin’s theory of natural selection.

    • Mertensian mimicry — Mertensian mimicry is a form of mimicry in which a deadly species imitates a less toxic model that predators can survive after encountering. It is often discussed alongside Batesian and Müllerian mimicry in examples such as coral snakes.

    • Muellerian mimicry — Müllerian mimicry is a form of mimicry in which two or more harmful species converge on a similar warning appearance. It differs from Batesian mimicry because all of the species involved are genuinely defended, so predators learn one shared signal more efficiently.

    • Müllerian mimicry — Müllerian mimicry is a form of biological resemblance in which two or more unpalatable or dangerous species evolve similar warning signals. It is an evolutionary pattern in which harmful species converge on the same warning coloration or other signals to deter predators, and unlike Batesian mimicry it benefits all participating species because predators learn to avoid the shared pattern more quickly. It is often discussed alongside aposematism and is sometimes said to blur the line between mimicry and shared defense coloration.

  • spandrels — Spandrels are structures that arise as byproducts of other adaptations rather than as direct products of selection. Gould and Lewontin borrowed the architectural term to criticize strict adaptationist explanations in evolutionary biology.
• Sexual selection — selection based on mate choice or competition; can drive traits that reduce survival (e.g., peacock tail); intersexual selection (mate choice) vs intrasexual selection (male-male competition). It is associated with traits like peacock plumage, Fisherian runaway selection, and Zahavi’s handicap principle.

  • Fisherian runaway — Fisherian runaway, or runaway selection, is a positive feedback process in sexual selection where preference for a trait and the trait itself intensify together. It is commonly illustrated by peacock tails and other exaggerated ornaments that persist even when they reduce survival.

  • handicap principle — The handicap principle is Amotz Zahavi’s signaling theory idea that honest signals must be costly to the signaler. Its classic example is the peacock’s tail or other burdens that demonstrate superior fitness because only strong individuals can afford them.

• Species concepts — biological species concept (Mayr): groups that interbreed and are reproductively isolated from others; morphological species concept (based on physical traits); phylogenetic species concept (smallest clade with a shared derived character); each has trade-offs for asexual organisms and fossil taxa.

• Speciation — allopatric (geographic isolation leads to divergence); sympatric (speciation within a population, often via polyploidy in plants). Speciation is the evolutionary process by which populations diverge until they can no longer interbreed and become separate species. It can also occur parapatrically, and is promoted by reproductive isolation, hybrid incompatibility, and adaptive radiation.

  • allopatric — Allopatric speciation occurs when populations are geographically isolated and diverge until they can no longer interbreed. It is often associated with founder effects, genetic drift, and adaptive radiation in separated populations.

  • Allopatric speciation and Sympatric speciation — Allopatric speciation occurs when populations diverge because they are geographically separated. Sympatric speciation happens without geographic separation, often through chromosomal change or nonrandom mating.

  • Bateson–Dobzhansky–Muller model — The Bateson–Dobzhansky–Muller model explains hybrid incompatibility by showing how separate populations can accumulate mutations that become harmful when combined. It is a classic explanation for reproductive isolation and speciation.

  • cladogenesis — Cladogenesis is the splitting of one evolutionary lineage into two or more distinct lineages. It is associated with punctuated equilibrium and is contrasted with anagenesis, which describes gradual change within a single lineage.

  • Geographic speciation
    • allopatric speciation — Allopatric speciation is the formation of new species through geographic isolation of populations, after they are separated by a geographic barrier. It is contrasted with sympatric speciation, where divergence happens without physical isolation, and is commonly illustrated by barriers like mountains or lakes, or by dispersal after founder events. Biologists such as Ernst Mayr developed classic models of this process, including vicariance and peripatric speciation.
      • peripatric — Peripatric speciation is a form of allopatric speciation in which a small population becomes isolated from a larger one. It is strongly associated with genetic drift and geographic separation, often after migration across a barrier.

  • Haldane’s rule — Haldane’s rule states that in hybrid offspring from two different species or races, the heterogametic sex is more likely to be absent, rare, or sterile. It is a classic principle in speciation and hybrid viability.

  • parapatric speciation — Parapatric speciation is a mode of speciation in which neighboring populations diverge along a gradient without a complete geographic barrier. It is often illustrated by ring species and by cases where adjacent populations experience limited gene flow and partial reproductive separation.

  • peripatric speciation — Peripatric speciation is a form of allopatric speciation in which a small population becomes isolated near the edge of a larger one. It is associated with founder effects and genetic drift, especially when a population spreads into a new environment. The clue set emphasizes that this process limits gene flow between subpopulations separated by distance.

  • ring species — A ring species is a chain of neighboring populations that can interbreed with adjacent groups but whose terminal populations cannot interbreed. This pattern is often cited with examples such as Larus gulls and other geographically wrapped species complexes.

  • stasipatric speciation — Stasipatric speciation is a form of sympatric speciation in which a genetic event such as polyploidy or a heterozygote disadvantage produces reproductive isolation. It is often contrasted with geographic isolation because the split begins within a shared area.

  • sympatric — Sympatric speciation occurs when new species arise in the same geographic area as their ancestor rather than by separation. It can result from reproductive isolation within a shared habitat, including cases involving polyploidy.

  • sympatric speciation — Sympatric speciation is the formation of new species without geographic isolation, often discussed with disruptive selection and hybridization in overlapping ranges. The finch example on Santa Cruz Island is a standard clue for this mode of speciation, which is contrasted with allopatric speciation.
    • sympatry — Sympatric speciation is the formation of new species in the same geographic area as the parent population. It is often associated with polyploidy, especially in plants such as wheat, where reproductive barriers arise without geographic separation.
  • Theory of Speciation — The theory of speciation explains how new species arise, often through geographic isolation and subsequent genetic divergence. Common forms include allopatric, parapatric, and sympatric speciation, and the theory is closely associated with Ernst Mayr.
• Reproductive isolation — pre-zygotic barriers (habitat, temporal, behavioral, mechanical, gametic) and post-zygotic barriers (hybrid inviability, hybrid sterility). Reproductive isolation is the set of prezygotic and postzygotic barriers that prevent populations from interbreeding. It can involve hybrid sterility, hybrid inviability, or other mechanisms that block gene flow and contribute to speciation.

  • Postzygotic isolation — Postzygotic isolation is a reproductive barrier that acts after fertilization, reducing the viability or fertility of hybrids. Its forms include hybrid inviability, hybrid sterility, and hybrid breakdown.

  • Prezygotic isolation
    • Mechanical isolation — Mechanical isolation is a reproductive barrier in which structural differences prevent mating or successful fertilization. It can occur when flowers or other sexual structures no longer match the pollinators or partners of their species.

  • Prezygotic isolation mechanisms — Prezygotic reproductive isolation includes barriers that prevent fertilization before a zygote forms. Common examples are habitat, temporal, behavioral, mechanical, and gametic isolation.

  • Temporal isolation — Temporal isolation is a prezygotic reproductive barrier in which populations breed at different times and therefore cannot exchange gametes. It is often illustrated with species that mate in different seasons or flower on different days.

  • Wallace effect — The Wallace effect is the evolutionary process in which natural selection strengthens reproductive isolation between diverging species. It is associated with A. R. Wallace and with reduced hybridization when differences between populations become more pronounced.

• Fossil record — provides direct evidence of extinct organisms; transitional fossils (e.g., Tiktaalik for fish-tetrapod transition) document evolutionary change.

• Comparative anatomy — homologous structures (same origin, different function; evidence of common ancestry) vs analogous structures (different origin, similar function; convergent evolution).

• Molecular evidence — DNA, protein sequence similarity, shared pseudogenes, and endogenous retroviruses support evolutionary relationships.

• Phylogenetics and cladistics — cladistics classifies organisms by shared derived characters (synapomorphies); a clade (monophyletic group) includes an ancestor and all its descendants; parsimony and maximum likelihood are standard tree-building approaches.

• Modern synthesis — the fusion of Darwinian natural selection with Mendelian genetics in the 1930s–1940s (key figures: Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson, Ronald Fisher, J.B.S. Haldane); unified evolution, genetics, and paleontology; established that natural selection acting on genetic variation is the primary mechanism of evolution. It unified population genetics with natural selection and is associated with figures such as G. Ledyard Stebbins.

• Punctuated equilibrium — the hypothesis (Eldredge and Gould, 1972) that evolutionary change occurs in rapid bursts separated by long periods of stasis in the fossil record, contrasting with strict phyletic gradualism. It was proposed by Niles Eldredge and Stephen Jay Gould in 1972 as an alternative to strict phyletic gradualism.
  • Stasis — In punctuated equilibrium, stasis is the period when a species remains largely unchanged for long stretches of time. The term refers to relative evolutionary stability before a rapid shift follows a new selective pressure.
• Adaptive radiation — rapid diversification of a single ancestral lineage into many ecologically distinct species following access to new resources or environments (e.g., Darwin’s finches in the Galápagos, mammalian diversification after the Cretaceous-Paleogene extinction). Classic examples include Darwin’s finches and the cichlids of East African lakes, often after new habitats open up or after mass extinctions.

  • adaptive radiations — Adaptive radiations are episodes of rapid diversification in which one lineage splits into many related species. Darwin’s finches are a classic example of this evolutionary pattern.

  • Divergent evolution — Divergent evolution, often called adaptive radiation, is the process by which related organisms evolve into different forms to fill different ecological niches.

• Convergent evolution — independent evolution of similar traits in unrelated lineages due to similar selective pressures; produces analogous structures (e.g., wings of birds and bats are homologous, but wings of birds and insects are analogous); contrasts with divergent evolution from a common ancestor producing homologous structures. It also yields homoplastic characters; repeated crab-like body plans are a classic example.
  • carcinization — Carcinization is the repeated evolutionary trend in which unrelated crustaceans evolve crab-like bodies. The term was coined by L. A. Borradaile and is often used as a classic example of convergent evolution.
• Coevolution — reciprocal evolutionary influence between species (e.g., flowers and pollinators, host and pathogen).

  • co-evolution — Co-evolution is the reciprocal evolutionary change of interacting species, each exerting selective pressure on the other. It is often illustrated by predator-prey arms races, plant-pollinator relationships, and the Red Queen hypothesis.

  • Red Queen hypothesis — The hypothesis says organisms must continually evolve to maintain their relative fitness as other species, parasites, and environments change around them. It is used to explain coevolution, especially host-parasite arms races, and the evolution of sex. It was developed by Leigh Van Valen and is named for the character in Through the Looking Glass / Alice in Wonderland.

  • The Red Queen Hypothesis — The Red Queen hypothesis argues that organisms must keep evolving just to maintain their relative fitness in an ongoing coevolutionary race. Its name comes from Lewis Carroll’s Red Queen, and it is often used to explain sexual reproduction and predator-prey arms races.

• Endosymbiotic theory — Lynn Margulis proposed (1967) that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by a host cell; supported by: double membranes, own circular DNA, 70S ribosomes, binary fission, and sequence similarity to α-proteobacteria (mitochondria) and cyanobacteria (chloroplasts). It explains these endosymbionts as becoming organelles after long-term residence in the host cell and is often linked to transferred genes in organelle genomes.

• Three-domain system — Carl Woese (1977) used 16S rRNA sequences to show that life comprises three domains: Bacteria, Archaea, and Eukarya; Archaea are more closely related to Eukarya than to Bacteria; overturned the traditional five-kingdom system.

• Evo-devo — evolutionary developmental biology; Hox genes control body-plan development; small regulatory changes can cause large morphological differences.

• RNA world hypothesis — proposes that early life relied on RNA as both genetic material and catalyst (ribozyme); supported by the discovery that the peptidyl transferase center of the ribosome is RNA-based; rRNA is the catalytic core of the ribosome, making it a ribozyme. Support for the idea also includes ribozymes, self-splicing introns, and laboratory work on autocatalytic RNA systems.

• Evolutionary hypotheses
  • Court Jester hypothesis — The Court Jester hypothesis argues that abiotic factors, especially climate change, are the main drivers of evolution. It is often contrasted with the Red Queen hypothesis.
• Evolutionary theory

  • Cope’s law — Cope’s law is the idea that many animal lineages tend to evolve toward larger body size over time. In biology-oriented questions it is often contrasted with predator-prey escalation and size advantages in defense or attack.

  • Gradualism — Gradualism is the idea that change happens by slow, steady increments rather than sudden leaps. In geology it was associated with Lyell, and in evolution it is often contrasted with punctuated equilibrium.

  • ontogeny recapitulates phylogeny — This discredited theory says embryonic development mirrors evolutionary history, or that an organism’s ontogeny repeats its phylogeny. It is associated with Ernst Haeckel and is often called the biogenetic law; Stephen Jay Gould discussed it in evolutionary theory.

  • Price equation — The Price equation is a mathematical description of evolutionary change that separates selection effects from transmission effects. In reduced form, it relates change in a trait to covariance between trait value and fitness, divided by mean fitness. It is widely used in evolutionary biology and has been adapted to questions in altruism, epidemiology, and economics.

  • punctuated equilibrium — The idea associated with Stephen Jay Gould that evolution proceeds in rapid bursts or brief episodes separated by long periods of relative stasis, rather than as a uniformly gradual process. It is presented as a contrast to models expecting slow, continuous evolutionary change.

  • The Theory of Acquired Characteristics — The theory of acquired characteristics, or Lamarckianism, holds that traits gained during an organism’s lifetime can be inherited by its offspring.

  • Theory of Evolution — The theory of evolution explains the diversification of life by natural processes such as natural selection. It is opposed by creationism and intelligent design, and it was central to debates from the Scopes Trial to later philosophical critiques.

  • Wilberforce-Huxley debate — The Wilberforce-Huxley debate was the famous 1860 Oxford exchange over Darwin’s theory of evolution. It is remembered for Samuel Wilberforce’s remarks about descent from apes and Thomas Huxley’s defense of evolution.

• evolution — Evolution is the biological change of species over generations, often explained by natural selection and diversification from common ancestors. Quizbowl clues may mention Darwin, Wallace, Lamarck, speciation, convergent evolution, or Hardy-Weinberg conditions as contrasts to evolutionary change.

  • adaptations — Adaptations are traits that improve an organism’s fitness in its environment and are favored by natural selection. Examples include the beaks of Galápagos finches, opposable thumbs, and bipedalism.

  • adaptive — Adaptive describes traits or processes shaped by natural selection, including adaptive radiation and the evolution of adaptive optics. In the biology clueing context, it points to evolutionary diversification, while in astronomy it names mirror systems that correct atmospheric distortion.

  • Anagenesis — Anagenesis is evolutionary change within a single lineage, where one taxon is gradually replaced by another without branching. It refers to directional change over time rather than the splitting of a lineage into multiple descendant lines.

  • Baldwin effect — The Baldwin effect in evolutionary theory is the idea that learned behavior can shape evolution by making a phenotype more likely to be genetically assimilated. The same name is also used for an astrophysical relationship involving emission-line equivalent width and luminosity in active galactic nuclei.

  • Clines
    • cline — A cline is a geographic gradient in the frequency of an allele or trait across a species’ range. It can appear in dialect continua and ring species, and the related term ecocline refers to a similar transition zone between ecological regions.
  • extinction — Extinction is the disappearance of a species, including mass extinctions such as the end-Cretaceous event at Chicxulub and the end-Permian die-off. Quizbowl clues often mention background rates, the Allee effect, and anthropogenic causes like habitat loss and climate change.
    • extinct — Extinct describes a species or population with no living members left. The Wake Island Rail is given as an example of a species reaching that status after the last individuals died.
  • Homology and analogy
    • ANALOGOUS — Analogous structures are body parts that perform similar functions but evolved independently. Biologists use the term for similarities caused by similar environmental pressures rather than shared ancestry. The classic example is the wings of different animals or insects that serve the same function without being derived from the same evolutionary origin.

  • homoplasy — Homoplasy is the presence of shared traits that did not come from a common ancestor. It includes convergent evolution and parallel evolution, and it is a common complication in phylogenetic analysis.

  • island gigantism — Island gigantism is the tendency for isolated species to evolve larger body size when predation and competition are reduced. It is used to explain the dodo and giant tortoises of the Galápagos.

  • mass extinction — A mass extinction is a rapid, global die-off in biodiversity in which a large fraction of Earth’s species disappears over a relatively short geologic interval. Examples include the Big Five, the end-Permian crisis or Great Dying, and the end-Cretaceous K–Pg/Cretaceous-Paleogene extinction linked to Chicxulub and the Alvarez hypothesis; the end-Cretaceous iridium anomaly is a classic associated marker. These events mark major geologic boundaries such as the Permian-Triassic and Cretaceous-Paleogene transitions and are commonly associated with asteroid impacts, volcanism, and climate change; other proposed triggers include gamma-ray bursts and geomagnetic reversals. The Anthropocene extinction is often discussed as a possible ongoing sixth extinction, and the topic was studied by Raup and Sepkoski.
    • Holocene extinction — The Holocene extinction is the ongoing loss of species associated with human activity and climate change, often treated as a modern mass extinction event.
  • selection — Selection is the process by which certain phenotypes become more common because they confer an advantage in survival or reproduction. Its forms include natural selection, artificial selection, directional selection, disruptive selection, and sexual selection.

    • artificial selection — Artificial selection is the human-directed breeding of plants and animals for desired traits. Darwin discussed it using fancy pigeons, and later examples include the domestication of the silver fox and the development of crops such as broccoli, cabbage, and kale.

    • genetic hitchhiking — Genetic hitchhiking is the rise in frequency of an allele because it is linked to another allele under positive selection. Also called genetic draft, it is closely associated with selective sweeps and can drive linked variants toward fixation or loss.

  • Social evolution
    • inclusive fitness — Inclusive fitness is the evolutionary idea that an organism’s genetic success includes the reproduction of relatives it helps support. It is used to explain cooperative behavior such as sibling helpers in hyenas and sterile worker castes in insects.

  • speciation AND extinction — Speciation and extinction are the paired processes that create and remove species over evolutionary time. They are the two processes in birth-death models of diversification, and their balance helps explain biodiversity patterns such as the latitudinal diversity gradient.

  • Transitional fossils
    • missing link — The missing link is the hypothetical transitional form once imagined to connect apes and modern humans. The term has also been applied misleadingly to fossils such as Piltdown Man, reflecting outdated linear views of evolution.
• Evolutionary fitness

  • absolute fitness — Absolute fitness is the expected ratio of individuals with a trait after a selection event to the number before it. It measures reproductive success and appears in discussions of the Price equation, Fisher’s fundamental theorem, and Muller’s ratchet.

  • Darwinian fitness — Darwinian fitness is an organism’s reproductive success, often denoted w in evolutionary biology. It underlies concepts such as adaptive landscapes, genetic load, and Hamilton’s rule, and is the standard measure of how well genes are passed to the next generation.

• Evolutionary game theory
  • evolutionarily stable strategy — An evolutionarily stable strategy is a strategy that, once common in a population, cannot be invaded by a rare alternative by natural selection. It is a refined form of Nash equilibrium used in biological game theory, with classic examples drawn from the hawk-dove game, ideal free distribution, and honest signaling.
• Mating systems
  • lekking — Lekking is a mating system in which males display competitively at a common site while females choose among them. It is a classic example of sexual selection and is seen in birds such as sage-grouse and ruffs.
• Animal adaptations
  • camouflage — Camouflage is the use of color, pattern, or disguise to make objects or organisms harder to see. The term covers both military dazzle patterns and biological forms such as disruptive coloration and counterillumination.
    • countershading — Countershading is camouflage in which an animal is lighter underneath and darker on top. It is especially common in sharks and was associated with Abbott Thayer.
• Animal camouflage
  • crypsis — Crypsis is a form of concealment that makes an organism difficult to detect, often by blending into its surroundings. It includes camouflage and is used by animals such as decorator crabs to avoid predators.
• Anti-predator adaptations
  • Aposematism — Aposematism is an antipredator strategy in which toxic or otherwise defended animals advertise danger or bad taste through conspicuous warning signals, often bright colors, odors, or sounds. It is central to Batesian and Müllerian mimicry, where other species copy those signals to avoid predators. Classic examples include poison dart frogs, skunks, and tiger moths.
• Ecogeographical rules
  • Allen’s rule — Allen’s rule states that endotherms in colder climates tend to have shorter extremities and more compact bodies. In its ecological form, populations of the same species living nearer the equator tend to have longer limbs than those toward the poles.
• Evolutionary biology
  • Allometry
    • body size — Body size, or body mass, is a key ecological and evolutionary variable that shapes metabolism, population density, and life-history patterns. It appears in Bergmann’s rule, Kleiber’s law, Cope’s rule, and allometric scaling, and it often shifts toward gigantism or dwarfism on islands.

  • analogous structures — Analogous structures are features that serve similar functions in different species but do not come from common ancestry. Bird wings and insect wings are a standard example of such convergent-evolution traits.

  • antagonistic pleiotropy — Antagonistic pleiotropy is the idea that a single allele can have beneficial effects early in life but harmful effects later on. It is often invoked in explanations of senescence and other tradeoffs in evolution.

  • August Weismann — August Weismann was a German biologist who argued against inheritance of acquired characteristics and promoted the germ-plasm theory of heredity. His work is also associated with the Weismann barrier, which separates germ cells from somatic cells.

  • Evolutionary tempo
    • phyletic gradualism — Phyletic gradualism is the evolutionary view that species change slowly, steadily, and continuously over long periods of time. It stands opposed to punctuated equilibrium, which argues for long stasis interrupted by rapid change.

  • fitness — In evolutionary biology, fitness is an organism’s ability to survive and leave genes in the next generation. It appears in ideas such as Fisher’s fundamental theorem, Wright’s fitness landscape, inclusive fitness, and genetic load.

  • fitness landscapes — Fitness landscapes, also called adaptive landscapes, represent how a genotype or phenotype maps onto reproductive success. They are used to picture peaks and valleys in evolutionary change, including rugged NK-model landscapes.

  • homology — Homology is similarity due to shared ancestry, especially in biology when traits or sequences derive from a common ancestor. It is contrasted with analogy or homoplasy, and shows up in topics like vertebrate forelimbs, orthologs, and paralogs.
    • homologous — Homologous traits or structures are shared because they descend from a common ancestral feature. The term appears in biology for anatomical comparison, chromosome separation in meiosis, and sequence similarity tests such as BLAST.
  • Lamarckism
    • Jean-Baptiste de Monet Lamarck — Jean-Baptiste de Monet, called Lamarck, was a French naturalist and early evolutionary thinker. He is associated with inheritance of acquired characteristics and with early biological classification work.
  • Life-history strategies
    • bet-hedging — Bet-hedging is an evolutionary strategy that lowers average payoff or fitness in exchange for reducing variance across unpredictable environments. It is often illustrated by stochastic phenotype switching, diversified reproduction, or the idea of not putting all one’s eggs in one basket.

  • living fossils — Living fossils are extant organisms that resemble ancient lineages and are often treated as evolutionary relicts or holdovers. Examples include the hoatzin, Amborella, and cycads.

  • Molecular evolution
    • neutral theory — Neutral theory in evolution, associated with Motoo Kimura, argues that genetic drift rather than selection is the main force shaping molecular change. The term also appears in Hubbell’s neutral theory of biodiversity, which treats ecologically similar species as equivalent.

  • phenotypic plasticity — Phenotypic plasticity is the ability of one genotype to produce different phenotypes in different environments. It includes phenomena such as acclimation and polyphenism, and the term is also used by analogy for changing neural traits like synaptic plasticity.

  • Price’s equation — Price’s equation is an evolutionary biology identity that decomposes change in a trait into a selection term and a transmission term. It is often written using the covariance between character value and fitness, and Hamilton’s rule can be derived from it.

  • Social evolution
    • kin selection — Kin selection is the evolutionary process in which organisms help relatives, increasing inclusive fitness through Hamilton’s rule. It explains eusocial castes and altruistic behavior when relatedness makes the benefit outweigh the cost.

      • coefficient of relatedness — The coefficient of relatedness, also written r, measures how closely related two individuals are in genetic terms. It appears in Hamilton’s rule for kin selection, where r times benefit must exceed cost, and is often invoked in discussions of eusociality in haplodiploid species.

      • Green beard effect — The green-beard effect is a form of kin selection in which a recognizable trait is used to direct altruism toward others carrying the same allele. It is a classic model of selective helping and non-helping among organisms with or without the trait.

      • Hamilton’s rule — Hamilton’s rule is the kin-selection inequality that favors altruism when the cost is less than the benefit weighted by relatedness. It is often summarized as C < rB and is used to explain warning calls, self-sacrifice, and inclusive fitness in evolutionary biology.

  • sociobiology — Sociobiology is the study of social behavior through evolutionary biology, especially the role of natural selection and kin selection in animals and humans. It is associated with E.O. Wilson and his 1975 book Sociobiology: The New Synthesis, and later provoked debates over human nature and genetic determinism.

  • Transitional fossils
    • Tiktaalik — Tiktaalik is a famous Devonian transitional fossil often described as a fishapod or missing link between fish and land vertebrates. Neil Shubin helped discover it in 2006 after a long search, and it is especially remembered for features that point toward the evolution of tetrapods.
• Evolutionary ecology
  • r/K selection theory — r/K selection theory classifies species by contrasting reproductive strategies, from rapid reproduction to maintaining populations near carrying capacity. It was developed by MacArthur and E. O. Wilson and uses the two key variables in the Verhulst equation.
• Hominins
  • Ardipithecus — Ardipithecus is an early hominin genus known from species such as A. kadabba and A. ramidus, with ramidus remains dating to about 4.4 million years ago. It is associated with a thin-toothed enamel and evidence for facultative, possibly obligate, bipedalism, including an abductable hallux.
    • Ardipithecus ramidus — Ardipithecus ramidus is an early hominid species discovered in Ethiopia and dated to about 4.4 million years ago. Its fossils suggested bipedalism in a forested environment, challenging the idea that upright walking evolved on the savannah.
  • Australopithecus — Australopithecus is the genus of extinct hominins that includes Lucy and other early human relatives such as afarensis and africanus. Fossils from Taung, Laetoli, Hadar, and Malapa are key clues to this group, often described as the “southern apes.”
    • Australopithecus ethiopicus — Australopithecus ethiopicus is an early hominin known from the so-called black skull. It shows features related to both Australopithecus afarensis and later robust australopithecines.
  • Java Man — The famous Indonesian Homo erectus fossil found by Eugene Dubois on Java, along the Solo River. It was represented by a skullcap and thighbone, was originally called Pithecanthropus erectus, and became a landmark find in the study of early human evolution.

    • Turkana Boy — Turkana Boy is the famous nearly complete Homo erectus skeleton found in Kenya in 1984. The fossil, also called Nariokotome Boy, is one of the most important remains for studying early human anatomy and growth.

    • Eugène Dubois — Eugene Dubois was the Dutch paleontologist who discovered Java Man in 1891 and identified the fossils as Pithecanthropus erectus. His work became important in the search for an evolutionary missing link.

  • Kenyanthropus
    • Kenyanthropus platyops — Kenyanthropus platyops is a fossil hominin species discovered by Meave Leakey’s team at Lake Turkana in Kenya. Its name means “flat-faced man of Kenya,” and it was described from a skull dating to about 3.5 million years ago.

  • Orrorin tugenensis — Orrorin tugenensis is an early hominin from Kenya, nicknamed the “Millennium Man,” and dated to about 6 million years ago. Its fossils include a femur suggesting bipedalism and a humerus indicating that it still climbed trees.

  • Paranthropus — Paranthropus is a genus of early hominins that split from Australopithecus and evolved in eastern and southern Africa. Compared with Homo, its members had smaller brains but larger jaws, and Paranthropus boisei is the best-known example.
• Human evolution
  • hominin fossils — Hominin fossils are the remains of early human relatives, including finds from Olduvai Gorge studied by Mary and Louis Leakey. Common examples include australopithecines and species such as Homo habilis and Homo erectus.
• paleontology
  • fossils — Fossils are preserved remains or traces of once-living organisms, including shells, bones, footprints, coprolites, and index fossils used for relative dating. They are central to paleontology and to the study of sedimentary strata, including famous finds from the Burgess Shale and the work of Mary Anning.
    • dinosaur fossils — The preserved remains of dinosaurs, especially bones, skulls, eggs, and skeletons collected by paleontologists. Notable finds include material from the Flaming Cliffs, Hell Creek, and Sussex, as well as the famous specimen nicknamed Sue. They were central to the Bone Wars rivalry between Edward Drinker Cope and Othniel Charles Marsh; Mary Anning and Othniel Charles Marsh are also associated with their discovery and study.

  • petrified forests — Petrified forests are areas where dead trees have been turned to stone by mineral replacement, preserving trunks and other plant material. Famous examples include Petrified Forest National Park in Arizona and other fossil forest sites around the world.

  • petrified wood — Petrified wood is fossil wood whose original plant tissue has been mineralized and preserved as stone. Its formation is often linked to volcanic ash or other burial that blocks decay, producing tree fossils such as those in Petrified Forest National Park.
• biogeography — Biogeography studies the distribution of living organisms across space and time, including patterns explained by vicariance. A famous subfield is island biogeography, associated with MacArthur and Wilson and with Darwin’s work on the Galápagos finches.

  • Bergmann’s rule — Bergmann’s rule is the ecological pattern that animals in colder climates tend to be larger than related forms in warmer climates. It is often explained by surface-area-to-volume ratio and thermoregulation, and it is contrasted with Allen’s rule, which concerns the size of body parts.

  • Wallace’s Line — Wallace’s Line is a biogeographic boundary in the Indonesian archipelago separating Asian and Australasian flora and fauna. Named for Alfred Russel Wallace, it runs through the Malay Archipelago, often linked to the Sunda and Sahul shelves and the Makassar Strait, and is commonly described as running between Borneo and Sulawesi and between Bali and Lombok.

• Biogeographic boundaries
  • Wallace Line — The Wallace Line is a biogeographic boundary in Indonesia that separates Asian fauna to the west from Australasian fauna to the east. It runs through the region around Bali, Lombok, Borneo, and Sulawesi, and is named for Alfred Russel Wallace.
• Animal body plans
  • cephalization — Cephalization is the concentration of sensory organs and neural control at the front end of an animal’s body, especially in bilaterally symmetrical forms. It is described in planarians, hydras, and some echinoderms as an evolutionary pattern linked to a head region.
• Origin of life hypotheses
  • panspermia — Panspermia is the hypothesis that life can spread through space on comets, asteroids, dust, or other material from one world to another. It was first proposed by Svante Arrhenius and later invoked by scientists such as Fred Hoyle in discussions of life’s cosmic distribution.

Evolutionary Mechanisms in Detail

• Molecular phylogenetics — building evolutionary trees from DNA, RNA, or protein sequence data; maximum likelihood and Bayesian inference are standard methods; 16S rRNA is the standard marker for bacterial phylogeny (Woese); COI gene is standard for animal barcoding.

• Evo-devo and Hox genes — Hox genes encode transcription factors that specify body-axis identity in animals; conserved across bilaterians; mutations produce homeotic transformations (e.g., Antennapedia: leg where antenna should be); the Hox gene cluster is collinear with the body axis it controls; discovery that flies and mice share Hox genes (same homeobox sequence) showed deep conservation.

• Neutral theory of molecular evolution — Motoo Kimura (1968) proposed that most molecular variation is selectively neutral; genetic drift, not natural selection, drives most substitution at the molecular level; explains why synonymous substitutions are far more common than nonsynonymous. It is often linked to the molecular clock, with the substitution rate approximately matching the mutation rate, and Tomoko Ohta’s nearly neutral theory is a later refinement.
  • neutral — Neutral means having no net charge, and it also names a major theory in evolutionary biology in which most mutations do not affect fitness. The word also appears in phrases like neutral current, neutral pion, and neutral theory of molecular evolution.

• Hamilton’s rule and kin selection — W.D. Hamilton formalized inclusive fitness: altruism evolves when rB > C (r = relatedness, B = benefit to recipient, C = cost to actor); explains eusociality in Hymenoptera (haplodiploid relatedness); popularized by E.O. Wilson and Dawkins.

• Reciprocal altruism — Trivers (1971): cooperation between unrelated individuals can evolve if favors are reciprocated; requires repeated interactions and ability to recognize and punish cheaters; basis for game-theoretic models of cooperation (prisoner’s dilemma, tit-for-tat).

• Molecular systematics
  • DNA barcoding — DNA barcoding identifies species by comparing short genetic markers from a specimen to reference sequences. It often uses mitochondrial COI in animals and rbcL or matK in plants, and it has been used to expose food fraud.
• Homeobox genes
  • Hox genes — Developmental transcription factors that control body patterning and segment identity. They encode a conserved DNA-binding homeodomain/homeobox domain, are arranged in clusters, and are expressed in collinear order to pattern the body plan along the anterior-posterior axis. Classic examples include Antennapedia and Ultrabithorax, whose mutations can place body parts in the wrong locations.
    • Antennapedia — Antennapedia is a Drosophila Hox gene whose mutation causes legs to grow in place of antennae. It is associated with the broader Antennapedia complex, which includes other developmental genes such as bicoid and Deformed.
• Limb development
  • Zone of Polarizing Activity — The zone of polarizing activity is a region of the limb bud that helps establish anterior-posterior patterning. It signals with the apical ectodermal ridge and secretes sonic hedgehog to guide limb development.
• Molecular evolution models
  • Jukes-Cantor model — The Jukes-Cantor model is a simple nucleotide substitution model in molecular evolution. It assumes equal base frequencies and equal mutation rates among bases.
• Richard Dawkins

  • The Blind Watchmaker — The Blind Watchmaker is a Richard Dawkins book arguing that natural selection, not chance alone, explains biological complexity. It is a major popular defense of evolution that also discusses the Weasel Program and biomorphs.

  • The Selfish Gene — Richard Dawkins’s influential book argues for a gene-centered view of evolution, in which natural selection acts primarily at the level of genes. It introduced the idea of the meme, challenged group-selection explanations, and became one of Dawkins’s best-known works.

• Richard Dawkins — Richard Dawkins is the evolutionary biologist who popularized the gene-centered view of evolution in The Selfish Gene and later wrote The Extended Phenotype and The God Delusion. He is also credited with coining the term meme as a unit of cultural replication.

• Animal social behavior
  • eusociality — This is the extreme social organization seen in ants, bees, and naked mole rats, marked by reproductive castes and cooperative brood care. It includes forms such as fortress defense and life insurance, and is often discussed in terms of kin selection and haplodiploidy.
• Ants and termites
  • Defense mechanisms
    • autothysis — Autothysis is a self-destructive hive-defense mechanism in which ants or termites rupture themselves to deter attackers.
• Sex allocation
  • Trivers-Willard hypothesis — The Trivers-Willard hypothesis predicts that parents in better condition will tend to invest more in sons, while those in poorer condition will tend to invest more in daughters. It is often discussed in evolutionary biology as a rule about sex-ratio bias and parental investment.
• Sexes
  • males — Males are the sex class associated in many species with XY chromosomes in mammals and ZZ chromosomes in birds. Biology clues about males often mention Bateman’s principle, sexual selection, and sequential hermaphroditism.

Taxonomy and Diversity of Life

Classification system

• Linnaean hierarchy — Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Mnemonic: “Dear King Philip Came Over For Good Soup.”

• Binomial nomenclature — genus + species (italicized); devised by Carl Linnaeus (18th century); Systema Naturae (1735) established the system; Linnaeus is the “father of modern taxonomy.”
  • specific — Specific is the adjective used in binomial nomenclature for the second word after the genus name. In chemistry and biology it also appears in terms like specific rotation and stereospecific reaction. It can also modify quantities measured relative to a standard, such as specific heat and specific gravity.

• Carl Linnaeus — Swedish botanist (1707–1778); developed binomial nomenclature and the hierarchical classification system in Systema Naturae; also developed the scale for temperature (though the Celsius scale is named for Anders Celsius, not Linnaeus). He is also known as Carl von Linné or Carolus Linnaeus, and his work shaped plant classification in Species Plantarum and related botanical systems.

• Phylogenetics — classification based on evolutionary relationships; modern cladistics groups organisms by shared derived characteristics (synapomorphies). It studies evolutionary relationships, usually represented with trees showing common ancestry and divergence. It overlaps with cladistics and taxonomy, and its computational methods are often associated with Joseph Felsenstein and Bremer support.

  • clade — A clade is a monophyletic group consisting of a common ancestor and all of its descendants on a phylogenetic tree. It is the taxonomic equivalent of a monophyletic group and is used to distinguish true evolutionary groups from assemblages defined by superficial similarities or homoplasy.

  • clades — A clade is a monophyletic group consisting of a common ancestor and all of its descendants. Clades are the basic units used in phylogenetic trees and in modern evolutionary classification.

  • long branch attraction — Long branch attraction is a phylogenetic error in which rapidly evolving lineages are mistakenly grouped together. It is a classic problem for parsimony methods and is often discussed through the Felsenstein zone.

  • maximum parsimony — Maximum parsimony is a phylogenetic criterion that chooses the tree requiring the fewest evolutionary changes. It aims to reduce homoplasy, though it can suffer from long-branch attraction in the Felsenstein zone.

  • molecular clocks — Molecular clocks use genetic change to estimate the timing of evolutionary divergence. They are calibrated with fossil evidence and are often applied through methods such as BEAST to place a timescale on phylogenetic trees.

  • monophyly — Monophyly is the property of a taxonomic group that includes an ancestor and all of its descendants. Clades are monophyletic groups, identified by shared derived characters called synapomorphies.

  • outgroups — Outgroups are taxa used to root a phylogenetic tree by providing a comparison point outside the group of interest. Because they are more distantly related than the ingroup members are to one another, they help identify shared derived traits.

  • phylogenetic — Phylogenetic is the adjective for methods and trees that infer evolutionary relationships from common ancestry. It also appears in phylogenetic signal and comparative methods such as independent contrasts and generalized least squares.

  • phylogenetic tree — A phylogenetic tree is a branching diagram used to show evolutionary relationships among taxa. It may be rooted with an outgroup or molecular clock, and it is also called an evolutionary tree, phylogeny, or dendrogram.

  • phylogenetic trees — Phylogenetic trees are branching diagrams that represent evolutionary relationships among organisms. They are built with methods such as maximum parsimony, bootstrapping, neighbor joining, and Bayesian approaches, and they may be rooted with an outgroup.

  • phylogenies — Phylogenies are diagrams that show hypothesized evolutionary relationships among organisms or genes. They are often drawn as trees or cladograms with branching ancestry and may include an outgroup.

  • phylogeny — Phylogeny is the evolutionary history of a species or group, often represented with branching trees. The term is also used more generally for phylogenetic relationships shown in cladograms, dendrograms, and other tree diagrams.

• Taxonomic ranks
  • Domain — This is the taxonomic level added in part because of study of the Archaea. It refers to the domain, the rank above kingdom in the modern biological hierarchy.

The three domains

• Bacteria — prokaryotes; peptidoglycan cell walls; diverse metabolisms; include pathogens (E. coli, Streptococcus) and beneficial organisms (gut microbiome, nitrogen fixers). It is also called Eubacteria, the older name for the same domain. Examples include Salmonella and E. coli; bacteria are often identified by Gram staining and by sensitivity to antibiotics such as penicillin.
  • Actinobacteria

    • Corynebacterium — Corynebacterium is a genus of Gram-positive, club-shaped bacteria. It includes the diphtheria pathogen and industrial species used in amino acid production, especially Corynebacterium glutamicum.

    • Mycobacterium — Mycobacterium is a genus of acid-fast bacteria identified by Ziehl-Neelsen staining and often cultured on Löwenstein-Jensen medium. It includes the agents of tuberculosis and leprosy, and its members are distinguished by a waxy cell wall that helps some species survive inside macrophages.

  • Actinomycetes — Actinomycetes are branching, filamentous bacteria that were once mistaken for fungi. Common in soil, they include Streptomyces and are a major source of antibiotics.

  • Agrobacterium — Agrobacterium is a plant-associated bacterium best known for transferring Ti plasmids that trigger crown gall disease and are widely used to transform plants such as Arabidopsis. Its vir genes are induced by plant signals like acetosyringone, and the bacterium’s genetic material can be integrated into host genomes.

  • Alphaproteobacteria
    • Agrobacterium tumefaciens — Agrobacterium tumefaciens is the plant-transforming bacterium that transfers Ti plasmids into plant cells through a type IV secretion system. It is famous for causing crown gall tumors and for its use in making transgenic plants such as Arabidopsis.
  • bacilli — Bacilli are rod-shaped bacteria. Examples include the bacteria associated with tetanus, botulism, and salmonella poisoning.
    • anthrax bacterium — Anthrax is a disease-causing bacterium historically associated with cattle.
  • Bacillus — Bacillus is a genus of Gram-positive rod-shaped bacteria that includes Bacillus subtilis, Bacillus thuringiensis, and Bacillus anthracis. Members of the genus are known for endospore formation, biocontrol toxins, and other widely studied laboratory species.

    • Bacillus subtilis — Bacillus subtilis is a common model bacterium for studying endospore formation and is found in the GI tract of ruminants. A protease named subtilisin is associated with this species and helps generate the Klenow fragment in lab work.

    • Bacillus thuringiensis — Bacillus thuringiensis is a soil bacterium known for making Cry and Cyt toxins that kill many insect larvae by punching holes in the gut lining. Its initials, Bt, are widely used for the toxin genes inserted into genetically modified crops.

  • bacteria or bacterium — Bacteria are prokaryotic microorganisms such as E. coli that lack a nucleus and can exchange genetic material by conjugation and transformation. They include Gram-positive and Gram-negative forms, nitrogen-fixing symbionts, and cyanobacteria, and some cause diseases like cholera.

  • Bacterium — A bacterium is a single-celled prokaryotic microorganism, distinct from viruses and eukaryotic pathogens. The clues contrast it with diseases such as Rocky Mountain spotted fever, German measles, diphtheria, and rabbit fever.

  • Cell envelope
    • bacterial cell wall — The bacterial cell wall is the peptidoglycan-rich outer structure that protects bacteria from osmotic lysis. It is targeted by lysozyme and beta-lactam antibiotics, and its thickness distinguishes Gram-positive from Gram-negative cells.

      • peptidoglycan — Peptidoglycan, also called murein, is the mesh-like polymer that forms bacterial cell walls. It contains N-acetylglucosamine and N-acetylmuramic acid cross-linked by short peptides, making it a target of beta-lactam antibiotics and lysozyme. Gram-positive bacteria have especially thick layers of it, while innate immunity can recognize its fragments through receptors like NOD2.

      • peptidoglycan or murein — Peptidoglycan, also called murein, is the structural polymer that makes up most bacterial cell walls. Penicillin inhibits its synthesis by binding PBPs and blocking cell wall formation.

    • gram-negative — Gram-negative bacteria have an outer membrane containing lipopolysaccharides and do not retain crystal violet in Gram staining. Their cell envelope includes a periplasm between the outer membrane and the plasma membrane.
  • Cell shape
    • rod-shaped — Rod-shaped describes bacteria with elongated cylindrical bodies. This morphology is called a bacillus, in contrast to spherical cocci.
  • Clostridia
    • Clostridium difficile — Clostridium difficile is a spore-forming bacterium that causes antibiotic-associated diarrhea and pseudomembranous colitis. Its toxins damage the colon, and infections are often treated with vancomycin, fidaxomicin, or fecal microbiota transplant after antibiotic disruption of gut flora.
  • Clostridium
    • Clostridium botulinum — Clostridium botulinum is the anaerobic bacterium that produces botulinum toxin, one of the most potent known neurotoxins. It can grow in improperly preserved food and cause bulging cans, then produce the paralysis associated with botulism. The toxin is also used cosmetically as Botox.

  • cocci — Roughly spherical bacteria, as opposed to rod-shaped bacilli or spiral forms. They often appear in clusters, chains, or pairs depending on the species, and the term includes many Gram-positive bacteria; streptococci are a familiar example.

  • cyanobacteria — Cyanobacteria are photosynthetic bacteria once called blue-green algae. They are important for nitrogen fixation, stromatolites, the Great Oxygenation Event, and the endosymbiotic origin of chloroplasts.

    • akinetes — Akinetes are dormant cells formed by cyanobacteria, the photosynthetic bacteria once called blue-green algae. These organisms include heterocyst-forming nitrogen fixers and are central to the Great Oxygenation Event and chloroplast endosymbiosis clues.

    • cyanobacteria or blue-green algae — Cyanobacteria are photosynthetic prokaryotes that were among the first organisms to release oxygen into Earth’s atmosphere. They are often called blue-green algae and are thought to be the ancestors of chloroplasts through endosymbiosis.

    • Spirulina — Spirulina is a blue-green algae marketed as a nutrient-rich supplement. It is often promoted for its vitamins and minerals and is sometimes claimed to help lower cholesterol.

  • Gammaproteobacteria
    • Vibrio — Vibrio is the genus of curved Gram-negative bacteria that includes the cholera pathogen Vibrio cholerae. Other members include V. fischeri, famous for quorum sensing and bioluminescence, and V. vulnificus and V. parahaemolyticus, which can cause serious infections.
      • Vibrio fischeri — Vibrio fischeri is the luminous marine bacterium whose lux operon became a classic model for quorum sensing. It colonizes the bobtail squid and is typically grown in salty media.
  • Gram-negative bacteria — Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide, so they do not retain crystal violet in Gram staining. They stain red with safranin and include organisms such as E. coli.

    • periplasm — The periplasm is the gel-like space between the inner and outer membranes of Gram-negative bacteria. It is the compartment that many secretion systems and transport proteins must cross or exploit before cargo reaches the outside of the cell.

    • Pseudomonas — Pseudomonas is a gram-negative bacterial genus that includes species associated with burn infections, cystic fibrosis, and biofilms. Quizbowl clues often single out P. aeruginosa for exotoxin A and hot tub folliculitis, and P. putida for oil-spill bioremediation and the first GMO patent dispute.

      • Pseudomonas aeruginosa — Pseudomonas aeruginosa is a gram-negative opportunistic bacterium and a major cause of hospital-acquired infection, especially in patients with cystic fibrosis. It can cause hot tub folliculitis and ecthyma gangrenosum, and it is known for pigments and toxins such as pyocyanin and exotoxin A.

  • Gram-positive bacteria — Gram-positive bacteria retain crystal violet in Gram staining because of a thick peptidoglycan cell wall, so they appear purple under the microscope. They often have teichoic acids, a reduced periplasm, and include many endospore-forming Firmicutes. Common examples include Staphylococcus.

  • Lactobacillus — Lactobacillus is a genus of rod-shaped bacteria widely used in lactic acid fermentation. Its species are common in yogurt, sourdough, kimchi, the vaginal microbiome, and dental plaque.

  • Listeria — Listeria is a genus of bacteria that includes Listeria monocytogenes, a cause of serious food-borne illness. It is associated with unpasteurized dairy products and can lead to meningitis, miscarriages, and stillbirths.

  • Mycoplasmas
    • Mycoplasma — Mycoplasma is a genus of tiny bacteria noted for lacking cell walls and for its pleomorphic, cholesterol-dependent growth in culture. Members of the genus include important causes of atypical pneumonia and species such as M. genitalium, which is studied for its very small genome.

  • pilus — A pilus is a bacterial appendage used in conjugation to transfer plasmids between cells. Different pili also mediate attachment, twitching motility, and biofilm formation.

  • proteobacteria — This large bacterial phylum is often called the purple bacteria and includes five major classes: alpha, beta, gamma, delta, and epsilon. It is the group most closely related to mitochondria.

  • Rickettsia — Rickettsia is a genus of obligate intracellular gram-negative bacteria that includes the agents of Rocky Mountain spotted fever and epidemic typhus. Members are associated with arthropod vectors and are notable for their close evolutionary relation to the bacterial ancestor of mitochondria. These bacteria include species such as prowazekii, conorii, akari, and felis.

  • Rickettsiales
    • Rickettsiae — Rickettsiae are small, nonmotile intracellular parasites transmitted by arthropods such as ticks and mites. They are associated with diseases such as typhus and trench fever, and the name honors Howard T. Ricketts.
  • Spirilla
    • Spiral / Spirillum — Spiral, or spirillum, is one of the basic bacterial shapes. It refers to bacteria with a curved or helical form rather than the spherical or rod-shaped forms.
  • spirochetes — Spirochetes are corkscrew-shaped Gram-negative bacteria with periplasmic flagella. They include Treponema pallidum, which causes syphilis, and Borrelia burgdorferi, which causes Lyme disease.
    • spirochaete — A spirochaete is a gram-negative, corkscrew-shaped bacterium that is usually flexible, helical, and motile. It includes Leptospira, Borrelia, and Treponema pallidum, the cause of syphilis and yaws.
  • Staphylococci
    • Staphylococcus — A genus of Gram-positive cocci that grow in clusters rather than chains and are distinguished in part by the coagulase test. Medically important species include Staphylococcus aureus, which can cause MRSA, toxic shock syndrome, and other serious hospital-acquired infections and may produce Panton-Valentine leukocidin, along with coagulase-negative species such as S. epidermidis and S. saprophyticus.
  • Streptococci
    • Streptococcus — Streptococcus is a genus of spherical, chain-forming bacteria that includes major pathogens such as S. pyogenes, S. pneumoniae, and viridans streptococci. It is associated with strep throat, scarlet fever, and pneumonia, and is classically grouped by Lancefield grouping; S. pyogenes is known for the M protein and for Cas9 used in CRISPR.
  • Structures
    • endospores — Endospores are durable, dormant bacterial structures produced by organisms such as Bacillus and Clostridium. They are highly resistant to heat, UV radiation, desiccation, and other harsh conditions.
      • bacterial endospores — Bacterial endospores are dormant survival structures made by organisms such as Bacillus and Clostridium. They resist heat, desiccation, and chemicals because of their tough spore coats and dipicolinic acid-rich cores, and they are targeted by special staining and sterilization methods.
  • Thermus
    • Thermus aquaticus — Thermus aquaticus is a heat-loving bacterium from hot springs and hydrothermal vents. It is famous for yielding Taq polymerase, a thermostable enzyme that made PCR practical.
  • Wolbachia — Wolbachia is a genus of intracellular alpha-proteobacteria best known for infecting many insects and nematodes. It can cause reproductive effects such as feminization in some woodlice and is used to reduce dengue transmission by infecting Aedes aegypti mosquitoes.
• Archaea — prokaryotes; no peptidoglycan; often extremophiles (thermophiles, halophiles, methanogens); more closely related to Eukaryota than to Bacteria. They have ether-linked membrane lipids and unusual cell-wall chemistry, and their ribosomes resemble eukaryotic ones more than bacterial ones. They include methanogens, halophiles, thermophiles, and acidophiles/thermoacidophiles. The group was first delineated as a separate lineage by Carl Woese, with George Fox associated with its discovery.

  • Asgard archaea — Asgard archaea are a proposed archaeal superphylum or clade thought to be closely related to eukaryotes. Their named groups include Lokiarchaeota, Thorarchaeota, Odinarchaeota, and Heimdallarchaeota, and they are often discussed in relation to LECA and FECA.

  • Hyperthermophiles
    • Pyrococcus furiosus — Pyrococcus furiosus is a hyperthermophilic archaeon whose DNA polymerase is used in PCR for its accuracy and heat tolerance. It grows optimally at about 100 degrees Celsius and is a source of the enzyme Pfu polymerase.
  • methanogens — Methanogens are anaerobic archaea that produce methane, often in marshes, sediments, and animal intestines. They are important decomposers and are also used in sewage treatment and in biogas production.

• Eukarya — membrane-bound nucleus; includes Protista, Fungi, Plantae, Animalia. It is one of the three domains in Carl Woese’s system, alongside Bacteria and Archaea. Eukaryotic cells are distinguished by having nuclei, and this domain includes humans, amoebas, dolphins, and yeast.

• Actinobacteria
  • Streptomyces — Streptomyces is a large genus of soil-dwelling actinobacteria that produces the earthy odor compound geosmin. It includes hundreds of species and is a major source of natural and clinically used antibiotics, including daptomycin.
• Magnetotactic bacteria
  • magnetosomes — Magnetosomes are intracellular structures in certain bacteria that contain magnetite crystals. They help magnetotactic bacteria orient themselves in magnetic fields and position themselves near low-oxygen environments.
• Staining techniques

  • acid-fast staining — Acid-fast staining is a differential stain used to identify organisms such as Mycobacterium that resist decolorization because of waxy, mycolic-acid-rich cell walls. It commonly uses carbol fuchsin and a counterstain such as methylene blue, with positive cells retaining the red stain.

  • Gram stain — The Gram stain is a classic microbiology test developed by Hans Christian Gram in 1884 that separates bacteria into gram-positive and gram-negative groups based on cell wall structure. It uses crystal violet, iodine, and a decolorizer, followed by safranin or another red counterstain; bacteria with thick peptidoglycan walls retain the crystal violet and appear purple, while others are counterstained pink/red.

• Eukaryotes
  • Protists or Protista — Protists, or Protista, is the eukaryotic kingdom used for many organisms that do not fit neatly into the plant, animal, or fungal kingdoms. It includes amoebas, paramecia, euglena, slime molds, kelp, and other mostly unicellular microorganisms.
• Life domains
  • Eukaryota — Eukaryota are organisms whose cells contain a nucleus and membrane-bound organelles. This domain includes animals, plants, fungi, and protists, and it is often discussed in connection with endosymbiosis and the origin of mitochondria. Their ribosomes have 60S and 40S subunits.
• Microorganisms
  • extremophiles — Extremophiles are organisms that thrive in conditions too hot, salty, acidic, or pressurized for most life. They include many archaea and other microbes used in discussions of black smokers, acid lakes, and PCR enzymes.

    • Alkaliphiles — Alkaliphiles are organisms, including archaea, that thrive in high-pH environments. They survive by keeping excess alkali out of their cells and by using enzymes adapted to those conditions.

    • extremophile — An extremophile is an organism that can survive or even thrive in harsh conditions that would kill most life, such as extreme heat, acidity, or salinity. Thermophiles and halophiles are common examples.

    • Halophiles
      • Halophile — Halophiles are organisms, including some archaea, that thrive in very salty environments. They are associated with places such as the Dead Sea, the Great Salt Lake, and African soda lakes.
    • thermophiles — Thermophiles are heat-loving extremophiles that thrive at temperatures far above ordinary biological norms. They are found in places such as hot springs and deep-sea vents, and Thermus aquaticus is a classic example.

Eukaryotic kingdoms

• Protista — paraphyletic grouping; mostly unicellular; includes algae, amoebas, paramecia, slime molds; many are photosynthetic. The term was coined by Ernst Haeckel for organisms that are generally unicellular or simple multicellular forms lacking true tissues. It is a traditional catch-all for eukaryotes that are not plants, animals, or fungi, and includes protozoa, algae, diatoms, dinoflagellates, amoebas, paramecia, and slime molds.

  • amoeba — Amoebas are unicellular eukaryotic protists that move and feed with pseudopodia, constantly changing shape. They engulf food by phagocytosis and include species such as Amoeba proteus, Entamoeba histolytica, and Naegleria fowleri. Free-living forms may inhabit mud or sludge; some marine forms such as foraminiferans and radiolarians secrete shells.

  • contractile vacuole — The contractile vacuole is an organelle in many freshwater protists that expels excess water. By pumping fluid out of the cell, it helps maintain osmotic balance and salt concentration.

  • dinoflagellates — Dinoflagellates are marine protists with two flagella set in grooves called the cingulum and sulcus, and many have armored thecae. Their blooms can cause red tides, and some species form symbiotic zooxanthellae or produce toxins such as saxitoxin.
    • Pfiesteria piscicida — Pfiesteria piscicida is a fish-killing dinoflagellate associated with outbreaks in Chesapeake Bay and several North Carolina river systems. It has been linked to lesions, respiratory problems, nausea, and memory loss in exposed people.
  • Euglena — Euglena is a genus of flagellated protists with an elongated cell, a nucleus, pigment-containing chloroplasts, and a pellicle rather than a rigid cell wall. They have a red eyespot for light detection and are often studied as a classic example of organisms with mixed plantlike and animal-like traits: they can photosynthesize in light but become heterotrophic in darkness, and are often described as mixotrophs because they can photosynthesize and also feed by ingestion.
    • Euglenas — Euglenas are freshwater flagellates that can both photosynthesize and feed heterotrophically. They are known for their flagellum and for storing food as paramylon rather than starch.

  • foraminiferans — Foraminiferans are shelled protists whose calcium-carbonate tests often become fossils and sedimentary deposits. They are important climate proxies and help form materials such as the chalk cliffs of Dover.

  • Protozoans
    • protozoan — Protozoans are single-celled eukaryotic heterotrophs, often treated as animal-like protists, especially forms that ingest food. Common groups include amoebas, ciliates, flagellates, foraminiferans, and apicomplexans; examples include the malaria parasite Plasmodium. They are sometimes discussed in connection with coral diseases caused by ciliate species. Historically, they were grouped under the name Acrita and divided into Sarcodina, Mastigophora, Ciliata, and Apicomplexa.

      • Cryptosporidium parvum — Cryptosporidium parvum is a coccidian protozoan parasite that causes watery diarrhea and abdominal cramps. It is especially dangerous in AIDS patients, where it can produce severe, chronic diarrhea and dehydration.

      • Foraminifera — Foraminifera are microscopic marine protozoans that build calcium-carbonate shells. Their accumulated shells can lithify into chalk.

      • Kinetoplastids
        • Trypanosoma — Trypanosoma is a genus of parasitic protozoans that includes the species causing African sleeping sickness and Chagas disease. They are known for kinetoplast DNA, variant surface glycoproteins, and a distinctive RNA-editing system.
          • variant surface glycoproteins — Variant surface glycoproteins are the Trypanosome cell-surface proteins responsible for antigenic variation in African sleeping sickness. They are transcribed from bloodstream expression sites and are periodically replaced during differentiation by procyclins.

      • Plasmodium — Plasmodium is the protozoan genus that causes malaria and is spread by Anopheles mosquitoes. Its life cycle includes sporozoites, merozoites, and schizogony, and PfEMP1 is a notable associated protein. Important species include P. falciparum, P. vivax, and P. ovale.

      • Sporozoa — Sporozoa is the old protozoan group for parasitic forms that produce spores during part of their life cycle. The term appears in older biology classifications of unicellular organisms rather than modern taxonomic schemes.
• Fungi — heterotrophic; cell walls of chitin; digest food externally (absorptive nutrition); include molds, yeasts, mushrooms; hyphae form mycelium. Important decomposers. They reproduce with spores and structures such as asci and basidia, and they form lichens with algae and symbiotic mycorrhizae with plant roots. Important compounds associated with fungi include aflatoxin, ergot, and penicillin; antifungal drugs include azoles and echinocandins.
  • Ascomycetes — Ascomycetes are the sac fungi, a major class of Eumycota that includes baker’s yeast. They are defined by producing spores in saclike asci.

    • Neurospora — Neurospora is an ascomycete genus best known through Neurospora crassa, a bread mold used by Beadle and Tatum to establish the one gene-one enzyme hypothesis. It became a classic model for studying meiosis and recombination.

    • Neurospora crassa — Neurospora crassa is a red bread mold and classic model organism used in genetics and circadian-rhythm research. It is associated with Beadle and Tatum’s one gene-one enzyme work, ordered tetrads, and mechanisms such as repeat-induced point mutation.

  • Ascomycota — Ascomycota are a major fungal phylum, named for the saclike asci in which spores are formed. They reproduce sexually through asci and are one of the two large phyla in Dikarya. They include yeasts such as Saccharomyces, as well as Candida, Cordyceps, Neurospora, and Aspergillus.
    • ergot — Ergot is the fungus that grows on rye and other grasses and produces toxic alkaloids that can cause ergotism. It is also the source of compounds tied to LSD and other hallucinogens, and its medicinal derivatives can cause vasoconstriction and even gangrene.

  • Aspergillus — Aspergillus is a genus of filamentous fungi noted for 45-degree branching hyphae and invasive disease in immunocompromised patients. Clues for it include aflatoxin production, the koji mold used to ferment soy and sake, galactomannan testing, and fungus balls in lung cavities. Related clues include A. fumigatus in research on gene deletion libraries and the aflatoxin-producing species A. flavus and A. parasiticus. Another common species is A. oryzae, used in food fermentation.

  • Basidiomycota — Basidiomycota is a fungal phylum that includes mushrooms, toadstools, shelf fungi, rusts, smuts, puffballs, boletes, and shiitake. Its members produce sexual spores externally on club-shaped basidia, often after clamp connections form in the mycelium.

  • Cordyceps — Cordyceps is a genus of parasitic fungi known for infecting and manipulating insect hosts. Some species are used in traditional medicine, while others are famous for the way they eventually sprout stalks from the bodies of their hosts. The genus became widely recognized in popular culture through The Last of Us.

  • Deuteromycota — Deuteromycota is the fungal group of imperfect fungi that are classified without a known sexual stage. These mitosporic fungi reproduce asexually and include organisms such as Penicillium and Rhizoctonia.

  • Eumycota — Eumycota is the fungal phylum name sometimes given as the lone phylum of the kingdom Fungi. It is a traditional classification term in older biology material.

  • Fission yeasts
    • Schizosaccharomyces pombe — Schizosaccharomyces pombe is the fission yeast model organism used in cell-cycle research, famous for wee1 mutants that make tiny cells. Paul Nurse used this unicellular eukaryote to study cdc2 and the regulation of mitosis.
  • Fungal cells
    • heterokaryon — A heterokaryon is a fungal cell containing two genetically distinct nuclei in the same cytoplasm. These structures can arise after cell fusion and are important in fungal reproduction and compatibility studies.
  • Fungal reproduction
    • plasmogamy — Plasmogamy is the fusion of cytoplasm between fungal cells without immediate fusion of nuclei. It produces a dikaryotic or heterokaryotic stage in organisms such as basidiomycetes and zygomycetes before karyogamy.

  • Fusarium — Fusarium is a genus of ascomycete fungi that includes important plant pathogens. Species in the group cause Panama disease in bananas and head blight in cereals such as wheat and barley.

  • haustoria — Haustoria are specialized structures used by parasitic fungi to penetrate host tissues. They have sharp tips that increase contact with host cell membranes and help the fungus absorb nutrients.

  • Hyphae — Hyphae are the filamentous structures that make up the bodies of multicellular fungi. They are threadlike fungal threads that collectively form a mycelium, which spreads through the substrate and supports fungal growth. Hyphae grow at their tips, are often divided by septa, and can specialize into structures such as haustoria, appressoria, and arbuscules in symbiosis with plants.

    • Septa — Septa are cross-walls that divide fungal hyphae into separate cells or compartments. Fungi lacking these internal partitions are called coenocytic; septa are part of vegetative growth in more advanced fungi.

    • Septae — Septae are perforated walls that divide fungal hyphae into sections. They create partitions within the filamentous body of many fungi.

  • Mildew — Mildew is a fungal growth that appears in powdery and downy forms and attacks plants such as grapevines, lilacs, and willows. The term also refers to the visible growth that appears on damp clothing, food, and bathroom surfaces.

  • molds — Molds are filamentous fungi that commonly grow on spoiled bread, fruit, and cheese. They also appear in biology and industry as a general type of fungus distinct from mildews and other fungal forms.
    • Mold — Mold is a rapidly growing fungus that reproduces asexually and often appears as fuzzy growth on bread or other food. Rhizopus is a common bread mold.
  • Mushrooms — Mushrooms are the familiar fruiting bodies of fungi, recognized by their caps, gills, and hyphae. They appear in ethnomycology and in Anna Tsing’s The Mushroom at the End of the World, and include psilocybin-containing species used historically as hallucinogens; matsutake are a famous example. They also include edible varieties such as shiitake and portobello, poisonous species, and appear in video games such as Pokémon.

    • Armillaria bulbosa — Armillaria bulbosa is a mushroom-forming fungus associated with honey mushrooms. It is notable for being a large, potentially monstrous fungus rather than the edible mushroom itself.

    • mushroom — A mushroom is a fungus with a cap and stalk, often described by the terms pileus and stipe. Common edible varieties include button, portobello, shiitake, crimini, and oyster mushrooms, while some species such as the death cap are poisonous.

    • The Mushroom — A mushroom is a fungus whose fruiting body appears in many poisonous and medicinal species, including fly agaric and Amanita phalloides. It is also the object in the Alice in Wonderland riddle that can make one grow taller or shorter.

  • Mycelia
    • Mycelium — Mycelium is the vegetative body of a fungus, made of branching hyphae that spread through a substrate. In fairy rings, it can expand outward as the center exhausts its food supply, and fungal growth at the tips of hyphae is organized by spitzenkörper.
  • Yeasts — Yeasts are single-celled fungi used in baking, breadmaking, and brewing, and in fermentation. Saccharomyces cerevisiae is the standard/classic example and a model organism in genetics; yeasts include Candida and other organisms that reproduce by budding, with some species also used in two-hybrid assays. They may also reproduce by fission.

    • Baker’s yeast — Baker’s yeast is the unicellular fungus Saccharomyces cerevisiae, a common laboratory ascomycete and classic model organism in genetics, cell biology, and molecular biology. It is used in baking, brewing, and fermentation, and in tools such as yeast two-hybrid screens; its GAL4 activator is a standard component of these assays. It was the first eukaryote to have its genome sequenced and has been used to study the cell cycle and mating type switching.

    • Saccharomyces — Saccharomyces is a genus of fungi best known for the yeasts used in brewing and breadmaking. Its species include Saccharomyces cerevisiae, a standard model organism and an important beer yeast.

    • yeast — Yeast is the unicellular fungus used in baking and brewing, with Saccharomyces cerevisiae as the standard species name. Clues about yeast often mention fermentation, model-organism genetics, and species such as Candida and Saccharomyces pastorianus.

• Plantae — multicellular; photosynthetic; cellulose cell walls; alternation of generations (sporophyte/gametophyte). Divisions: non-vascular (mosses, liverworts) → seedless vascular (ferns, horsetails) → seed plants (gymnosperms: conifers; angiosperms: flowering plants).

• Animalia — multicellular; heterotrophic; no cell walls; diverse body plans. Major phyla: Major phyla include Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata; it includes modern humans.

• Ciliophora — Ciliophora are alveolate protozoans characterized by hairlike cilia used for movement and nuclear dimorphism, with one macronucleus and one or more micronuclei performing distinct genetic roles. Familiar examples include Paramecium, Tetrahymena, Stentor, and Stylonichia.

  • Paramecium — Paramecium is a genus of ciliate protozoa known for cilia, conjugation, and a macronucleus-micronucleus split. Its familiar species include P. aurelia and P. bursaria, and it is often used in ecology and microbiology clues.

  • Tetrahymena — Tetrahymena is a genus of ciliate protists widely used as a model organism in biology. It was important in discoveries about telomerase and chromatin regulation, and its species include Tetrahymena thermophila and Tetrahymena pyriformis. Like other ciliates, it has separate macronuclear and micronuclear genomes.

  • Tetrahymena thermophila — Tetrahymena thermophila is a free-living ciliate protozoan used as a model organism. It was important to discoveries involving telomerase, ribozymes, and ciliate conjugation.

• Metamonads
  • Diplomonads
    • Giardia lamblia — Giardia lamblia is an intestinal protozoan parasite that causes giardiasis, often contracted from contaminated water. It produces severe diarrhea, stomach cramps, and flattened intestinal villi, and its trophozoites can show an owl’s-eye appearance.
• Photosynthetic organisms
  • algae — Algae are photosynthetic aquatic organisms that include green, red, and brown forms, as well as diatoms and many phytoplankton. They are central to coral bleaching, red tides, lichens, and other ecological processes.
    • brown algae — Brown algae are multicellular marine algae, also called Phaeophyta, whose brown color comes largely from the carotenoid fucoxanthin. Their reproductive cells have two flagella. They include kelps and other seaweeds such as Sargassum and Macrocystis; large forms use holdfasts, pneumatocysts, and related structures to thrive underwater.
      • Fucales
        • Sargassum — Sargassum is a genus of brown seaweed that forms floating mats at sea. It gives its name to the Sargasso Sea and has become notable for massive beaching events in the Gulf of Mexico.
      • kelp — Kelp refers to large brown algae in the order Laminariales that form underwater forests. These seaweeds are ecologically important and are used in products such as alginate and soda ash.

    • coccolithophores — Coccolithophores are calcifying phytoplankton whose tiny carbonate plates, or coccoliths, contribute heavily to limestone and calcareous ooze. They are also studied by remote sensing because their blooms reveal patterns in ocean chemistry and carbon dioxide.

    • diatoms — Diatoms are silica-shelled algae and among the most abundant phytoplankton. Their frustules are built from two interlocking valves, and their ornate microscopic skeletons can accumulate into large geological deposits. They are often split into centric and pennate forms; preserved remains can be used to study past water conditions. Their biology is often linked to auxospores, size reduction after division, and their importance in marine and freshwater ecosystems.
      • Diatom — Diatoms are silica-shelled unicellular algae whose frustules are built from interlocking valves. They are a major component of plankton and are known for the near-indestructibility of their microscopic cell walls.
    • Green algae — Green algae are a major group of photosynthetic protists, mostly aquatic, distinguished by chlorophylls a and b. They include Chlamydomonas, Chlorella, Volvox, Spirogyra, and sea lettuce, and include lineages related to land plants; they are often treated as plant ancestors.

      • Spirogyra — Spirogyra is a genus of filamentous green algae common in freshwater and often seen as slimy pond scum. Its name refers to the helical arrangement of its chloroplasts.

      • Volvocales

        • Volvox — Volvox is a genus of green algae that forms spherical colonies made of many flagellated cells. It is a model for multicellularity and colony inversion, and its close unicellular relative is Chlamydomonas.

    • Rhodophyta — Rhodophyta, or red algae, are mostly marine photosynthetic algae. Their red color comes largely from phycobilin pigments, especially phycoerythrin, which helps them absorb blue light in deeper water. They are important sources of agar and carrageenan and include edible seaweeds such as nori, as well as forms such as Porphyra, Palmaria, Polysiphonia, and coralline algae. Red algae lack flagellated cells and characteristically have pit connections, unstacked thylakoids, floridean starch, and complex life cycles involving carposporophytes and cystocarps.

    • seaweed — Seaweed is the broad name for marine algae such as kelp, nori, and other edible or commercially useful macroalgae. It forms underwater forests, supplies carrageenan and other products, and includes the large brown algae that dominate many coastal ecosystems.
• Protozoan organelles
  • Trichocysts — Trichocysts are ejectable structures found in some protozoa, especially Paramecia. They may be mucoid, filamentous, or toxic and can be discharged for defense or to immobilize prey.
• Protozoans
  • Stentor — Stentor is the name of a Greek herald said to have a voice as loud as fifty men. It is also a trumpet-shaped genus of protozoans that can regenerate from fragments.
• Radiolarians
  • radiolarian — Radiolarians are marine protozoans known for elaborate siliceous skeletons that can accumulate as deep-sea sediment. They can grow to several millimeters across and are especially notable for their glasslike shells.
• Fungal phyla
  • Chytridiomycota — A fungal phylum distinguished by mobile, flagellated cells; its gametes bear flagella. It includes aquatic fungi such as Allomyces and species such as Batrachochytrium dendrobatidis, the chytrid fungus blamed for major amphibian die-offs. It is often treated as the only motile fungi.
• Fungal structures
  • coenocytic — Coenocytic structures are multinucleate and lack septa, allowing cytoplasm and organelles to move freely. The term is often applied to fungal hyphae that are continuous tubes rather than divided into cells.
• Fungal toxins
  • Phallotoxins
    • phalloidin — Phalloidin is a bicyclic heptapeptide toxin from the death cap mushroom that binds F-actin and blocks actin depolymerization. Fluorescently tagged phalloidin is commonly used to stain and image thin filaments.
• grasses — Grasses are the Poaceae, the plant family that includes cereal grains, bamboo, maize, rice, wheat, and barley. They dominate prairies, savannas, and other grassland ecosystems, and many species have adaptations that let them recover quickly after grazing and fire.

  • bamboo — Bamboo is a fast-growing woody grass with segmented hollow stems, used to make many instruments across Asia and beyond, including shakuhachi, bansuri, panpipes, nohkan, and bamboo versions of gamelan instruments. It is also used in percussion traditions such as tinikling, names the bamboo grove of the Seven Sages, appears in Chinese annals and poems written on bamboo strips, and is the main food source of pandas.

  • Citronella — Citronella is a fragrant tropical grass, especially known for the sharp-smelling oil distilled from it. That oil is commonly used in insect repellents and in perfumes and soaps.

  • maize — A major cereal crop domesticated from teosinte in Mesoamerica and now grown widely in the Americas and beyond. Corn/maize was a staple of Pre-Columbian civilizations and the U.S. Midwest, and is used for food, animal feed, ethanol, high-fructose sweetener, sweetcorn, tortillas, nixtamalization, the Three Sisters, and ritual drinks such as chicha. It is central to many Mesoamerican and Native American origin myths, including stories featuring the Hero Twins, Quetzalcoatl as an ant, and yellow and white meal used in human creation. Barbara McClintock used maize to discover transposons.

  • sweetgrass — Sweetgrass, also known as Hierochloe odorata, is a fragrant grass burned in some Native American smudging rituals and used in basket-making traditions. It is the title plant of Robin Wall Kimmerer’s Braiding Sweetgrass, a book that links Indigenous ecological knowledge to ideas like the Honorable Harvest.

  • wheat — Wheat is the staple cereal grain processed into flour and bread, and it was central to ancient and modern grain economies. Clues point to its role in Roman annona distributions, the Green Revolution, and American bonanza farms and Great Plains agriculture.

• Malvaceae
  • baobab tree — Baobab trees are the swollen-trunked members of the genus Adansonia, especially famous in Madagascar and the African Sahel. They are huge water-storing trees with dramatic “upside-down” silhouettes and large indehiscent fruits. Six of the nine species occur in Madagascar, where the Avenue of the Baobabs near Morondava is a famous landmark.
• Millets
  • Pearl Millet — Pearl millet is a drought-resistant grain important across the Sahel and in parts of India and West Africa. It was domesticated in northern Mali in the third millennium BCE and remains a staple crop in regions such as Rajasthan and Borno State.
• Arachnids

  • book lungs — Book lungs are respiratory organs found in arachnids and some other arthropods. They consist of stacked, plate-like surfaces filled with hemolymph that exchange gases with the air.

  • mites — Mites are tiny arachnids studied in predator-prey ecology, including Carl B. Huffaker’s classic experiments using oranges and rubber balls to show how spatial heterogeneity can stabilize populations. They are also implicated in colony collapse disorder through species such as Varroa and Acarapis, and the group excludes ticks.
    • A mite — A mite is a tiny arachnid related to spiders, including forms such as ticks and chiggers. The clue asks for the four-letter common name of those animals.
  • Mites and ticks
    • Mite — A mite is a very small arachnid, often parasitic, related to ticks and other members of the Acari. The word can also refer to a tiny coin of very small value in older usage.
  • Spiders — Eight-legged arachnids that recur in mythology, religion, and literature. Associated figures include Anansi, Spider Woman, Arachne, Iktomi, and Grandmother Spider, and they appear in works such as Frost’s “Design” and Calvino’s The Path to the Nest of Spiders. Spider deities also appear in Mesoamerican art, and spiders commonly serve as enemies or characters in fantasy and video games.
    • Recluse spiders
      • Brown recluse — A venomous spider of the genus Loxosceles, recognized by a violin-shaped marking on its body. Its bite can cause tissue necrosis or necrotic lesions, and it is among the few spiders in the United States capable of killing a human.

    • spider — A spider is an eight-legged arachnid associated in myth with Anansi and Jorōgumo, and with Arachne, who was transformed into a spider by Athena after a weaving contest. It also appears in Roussel’s ballet-pantomime The Spider’s Feast.

    • Spiderwebs — Spiderwebs are the silk structures spiders use to trap prey, with many forms such as orb webs, sheets, funnels, and tangled webs. They can also appear in pop culture as the title of a No Doubt song and as the object of several Pokémon clues about trapping moves and sticky webs.

    • Venomous spiders
      • black widow and brown recluse — The black widow and brown recluse are the two North American spiders whose bites can be fatally dangerous to humans. Both are notable for being colorful in a subtle way and for avoiding contact with people.

  • spinnerets — Spinnerets are arachnid organs on the abdomen that produce silk through spigots and are used for tasks such as wrapping prey and making egg cases. Most spiders have three pairs, though some families have two or four pairs.

  • Tarantulas — Tarantulas are large, hairy mygalomorph spiders that belong to the arachnids. Their name comes from the Italian dance tarantella, which was once linked to the fear of their bite.
    • Tarantula — A tarantula is a large spider, often associated with dramatic rumors but generally not dangerous to humans. The name also evokes the Italian city of Taranto and the spider’s iconic place in natural history and culture.
  • ticks — Ticks are blood-sucking arachnid parasites that can transmit Lyme disease, Rocky Mountain spotted fever, babesiosis, and other infections. Some species, such as deer ticks and lone star ticks, are also associated with alpha-gal allergy.

    • deer tick — A deer tick is a blood-feeding arachnid that can transmit Lyme disease and other illnesses such as Rocky Mountain spotted fever. It is noted for swelling as it feeds and is closely related to other disease-carrying ticks and mites.

    • tick — A tick is a blood-feeding arachnid in the subclass Acarina, often noted for transmitting Lyme disease. The word also names the sound of a clock and a small unit of stock-price movement.

  • Widow spiders
    • Black widow spider — The black widow spider is a venomous Latrodectus spider recognized by the red hourglass on the female’s abdomen. Its bite is usually not fatal but can cause severe abdominal pain and muscle spasms.
• Arthropod anatomy

  • exoskeleton — An exoskeleton is a hard external supporting structure found in arthropods, crustaceans, and many mollusks. Because it is shed during molting, it limits growth until a new covering is formed.

  • Sensory appendages

    • antennae — Antennae are the sensory appendages on the heads of arthropods, especially insects. The same word also refers to radio devices that convert electrical signals into electromagnetic waves and back again.
• Arthropod vision
  • compound eyes — Compound eyes are made of many ommatidia and are a hallmark of arthropod vision. In Drosophila, genes such as Pax help control their development, and the trait is often discussed through the fly’s classic red-and-white eye-color genetics.

    • compound eye — A compound eye is a visual organ made of many ommatidia, each contributing to a mosaic image. It is the characteristic eye type of arthropods such as Drosophila and other insects.

    • Ommatidium — An ommatidium is one of the repeating units that make up the compound eyes of insects and other arthropods. Each ommatidium contains a cornea and crystal cone that focus light onto receptor cells. A normal ommatidium contains eight photoreceptor cells, and sevenless mutants disrupt development of the seventh cell.

• Arthropoda — Arthropoda is the largest animal phylum, including insects, arachnids, crustaceans, myriapods, trilobites, and eurypterids. Its members are jointed-legged invertebrates with chitinous exoskeletons, segmented bodies or tagmata, jointed appendages, and open circulatory systems with a hemocoel. They grow by molting through ecdysis, and many terrestrial forms have Malpighian tubules; compound eyes are common, and they may have a stomatogastric nervous system and copper-based hemocyanin rather than hemoglobin. The name means “jointed feet.”
  • Arachnids

    • Arachnida — Arachnida is a class of chelicerate arthropods that includes spiders, scorpions, ticks, mites, and harvestmen. Members typically have two main body regions and eight legs as adults, distinguishing them from insects. Many species breathe with book lungs; the class also includes the subclass Acari and the extinct eurypterids.

    • Varroa destructor — Varroa destructor is a parasitic mite of honeybees that feeds on haemolymph. It helps spread Deformed Wing Virus and other pathogens.

  • Body regions
    • abdomen — In arthropods, the abdomen is the rear body region behind the head and thorax. It is also the section that carries many of the structures moved or signaled during the honey bee waggle dance.

  • centipedes — Centipedes are many-legged arthropods, with some species popularly described as having more than 100 legs. They can be confused with millipedes, and they also appear in cultural clues such as George Humphries’s poem and the Pokémon Scolipede and Centiskorch.

  • cephalothorax — A cephalothorax is the fused head-and-thorax body region found in many arthropods. It is especially associated with spiders, whose bodies are divided into the cephalothorax and abdomen.

  • Chelicerates
    • horseshoe crab — The horseshoe crab is a marine arthropod often called a living fossil because it has changed little since the Cambrian. Its copper-based blood is harvested for bacterial testing, and the best-known species is Limulus polyphemus.
  • Crustacea — Crustacea is the arthropod subphylum that includes crabs, lobsters, and many other aquatic forms. Members commonly have two pairs of antennae and often pass through a nauplius larval stage.
    • Barnacles — Barnacles are sessile marine crustaceans in the infraclass Cirripedia, with a free-swimming larval stage before they attach to rocks, shells, driftwood, boats, whales, and other hard surfaces. Their settlement and attachment are driven by cement glands and protein cues in the larval stage. Barnacles were studied extensively by Darwin, and Connell’s work on Chthamalus and Balanus helped define realized versus fundamental niches.
      • Barnacle — A barnacle is a marine crustacean that often attaches permanently to rocks, docks, or whales. Its feathery appendages filter food from the water, and Joyce’s wife Nora Barnacle shares the name.
    • Branchiopods
      • Daphnia — Daphnia are transparent freshwater water fleas used widely in biological experiments. Their laterally compressed bodies and model-species status, including D. pulex and D. magna, are the main identifying clues.

    • copepods — Copepods are tiny crustaceans that dominate zooplankton and make up a huge share of animal biomass in the oceans. They are often mentioned with krill and salps, and many species also undergo diel vertical migration.

    • Crabs — Crabs are decapod crustaceans with clawed, oval bodies and many specialized forms, including fiddler crabs, hermit crabs, coconut crabs, blue crabs, Japanese spider crabs, and red crabs. Their evolutionary tendency toward crab-like body forms is called carcinization. They also appear in literature and mythology, including Gabriel García Márquez’s A Very Old Man with Enormous Wings and the Popol Vuh, and in references to Sartre, Chinese mitten crabs, and Christmas Island red crabs.

    • Hermit crabs — Hermit crabs are crustaceans that live in empty gastropod shells and practice a form of commensalism called metabiosis. Some related crablike creatures, including the coconut crab, are mentioned with them in clues because of their size and shell-using habits.
      • hermit crab — A hermit crab is a crustacean that protects its soft abdomen by living in empty snail shells. It can also be linked to the mutualism with sea anemones and to its exchange of shells in chains with other hermit crabs.

    • krill — Krill are small shrimp-like crustaceans, especially abundant in Antarctic waters and crucial in marine food chains. Clues include Euphausia superba, diel vertical migration, calyptopis and furcilia larvae, and their importance as food for whales.

    • mantis shrimp — Mantis shrimp are stomatopod crustaceans famed for their explosive raptorial claws and shell-shattering punches. They have unusually complex vision, including the ability to detect circularly polarized light.

    • Sea Monkeys — Sea Monkeys are a marketed form of brine shrimp, originally sold as an “Instant Life” novelty pet. Harold von Braunhut popularized them through mail-order kits and other promotional gimmicks.

  • hemolymph — Hemolymph is the bloodlike fluid of arthropods in open circulatory systems. It combines functions analogous to blood and interstitial fluid and may carry oxygen with hemocyanin.

  • Insects — Insects are the class of arthropods that includes bees, ants, termites, flies, beetles, butterflies, aphids, cicadas, Drosophila, and the cochineal insect, a source of red dye. They are six-legged, usually with three pairs of legs and one pair of antennae, and have a body divided into head, thorax, and abdomen. Their development can include pupa and imago stages, and some species such as periodical cicadas have unusually long underground life cycles. Insects are the most species-rich class of animals, outnumbering all other life forms combined in known species diversity. The field of entomology studies them. They also appear in literature and allegory, including Karel Čapek’s insect play and works that use insects as symbols for human life.
    • Ants — Ants are eusocial insects in the family Formicidae, with elbowed antennae, compound eyes, and formic acid in their poison glands. Familiar examples include fire ants, leafcutter ants, army ants, carpenter ants, and Argentine ants. They are noted for superorganismal social behavior studied by E. O. Wilson, and for the Martialis heureka oddity; they also appear in mythological and literary traditions, including Myrmidon ancestry and natural history writing.

      • fire ants — Fire ants are stinging invasive ants native to South America, best known in the United States as a major introduced pest. They can form living rafts on water, and Solenopsis invicta is a common species name for the red imported fire ant.

      • Leafcutter ants — Leafcutter ants are social ants in the genera Atta and Acromyrmex that cultivate fungus by feeding it fresh plant material. They are famous for carrying loads many times their body weight and for highly organized caste-based labor.

      • queen ants — Queen ants are the reproductive females of ant colonies, and similar queens exist in other eusocial insects such as bees. They are central to studies of caste structure, haplodiploidy, and colony reproduction in eusocial societies.

    • aphids — Aphids are small sap-feeding insects that attack plants and are often protected and milked by ants. They are notorious agricultural pests and common prey for ladybugs. Many species reproduce by parthenogenesis; some can give birth to already-pregnant daughters. They also show polymorphism between winged and wingless forms and communicate alarm with trans-beta-farnesene. Notable examples and associations include Acyrthosiphon pisum and Buchnera endosymbionts, and they are known for cornicle secretions.

    • Cicada — Cicadas are xylem-feeding insects known for loud tymbal sounds and, in some species, synchronized 13-year or 17-year emergence cycles. North American periodical cicadas belong to Magicicada and include broods such as Brood X and Brood XIX. They are loud, short-lived adults and are sometimes called locusts in North American dialects, though they are not true locusts.

    • cockroach — A cockroach is the many-legged insect associated with Gregor Samsa in The Metamorphosis and with other literary and political metaphors. It can also be used figuratively for vermin-like pests or dehumanizing insults.

    • Cockroaches
      • Blattodea — Blattodea is the insect order that includes cockroaches, with many species adapted to tropical habitats and some living in houses. Members of the order have flattened bodies and are known for rapid running legs.
    • Coleoptera — Coleoptera is the insect order of beetles, an enormously diverse and species-rich group and the largest order in the animal kingdom. Its members have hardened forewings called elytra that protect the membranous hind wings, and they undergo complete metamorphosis. The order includes weevils, fireflies, ladybugs, bombardier beetles, dung beetles, scarabs, mealworms, and flower chafers, and is commonly divided into major suborders such as Adephaga and Polyphaga.

      • coleopteran — Coleopterans are beetles, the insect order whose name refers to hardened forewings that form a sheath. It is one of the largest animal orders by species count.

      • elytra — Elytra are the hardened forewings of beetles that protect the flying wings beneath them. In games like Minecraft, the Elytra are wearable and let the player glide from great heights.

      • Glow-worms — Glow-worms are luminous insects or larvae whose light appears in poems by Cowper, Wordsworth, Marvell, and Shelley. They are especially associated with the night and with the imagery of small living lamps or country comets.

      • Glowworms
        • railroad worm — The railroad worm is a bioluminescent South American beetle larva and female that shows rows of glowing green spots and a red head light. Its nickname, ferrocarril, alludes to its resemblance to a lit train.
      • Jewel beetles
        • Emerald Ash Borer — The emerald ash borer is a metallic green beetle, Agrilus planipennis, that attacks ash trees in the genus Fraxinus. It has caused massive North American tree loss and is monitored with traps and surveillance for associated wasps.

      • ladybird beetle — Ladybird beetles are small red or yellow beetles often marked with black spots and widely used in biological pest control. They were famously imported to California from Australia to control the cottony cushion scale that threatened citrus trees in the 1890s.

      • Weevils — Weevils are beetles in the superfamily Curculionoidea, known for their long snouts and largely plant-eating habits. Some species are agricultural pests, including cotton weevils and other damaging crop-eaters.

        • boll weevil — A cotton pest native to Mexico that spread through the southern United States and devastated Southern cotton crops, leading many farmers to diversify their crops. It is associated with the Enterprise, Alabama monument.

        • Boll Weevils — Boll weevils are insects that damage cotton, and the term also became a nickname for conservative Southern Democrats in Congress. The phrase ties together the insect and the political label.

    • crickets — Crickets are nocturnal orthopteran insects related to grasshoppers and katydids. They are recognized by their chirping, which comes from rubbing their front wings together, and by the use of their leg tympana to hear it.

    • Dragonflies
      • dragonfly — Dragonflies are predatory insects with two pairs of wings, long bodies, and a life around lakes and wetlands where they hunt smaller insects. Their group includes species such as the Yellow Darter, Keeled Skimmer, and Common Hawker, and giant ancient forms once had enormous wingspans.
    • Dragonflies and damselflies
      • Odonata — Odonata is the insect order containing dragonflies and damselflies. These paleopteran insects cannot fold their wings back over the abdomen, and their lineage includes forms that grew large during the Carboniferous.

    • earwigs — Earwigs are nocturnal insects in the order Dermaptera, with the characteristic pincers at the end of the abdomen. Their name comes from an old belief that they crawl into sleeping people’s ears.

    • Fireflies — Fireflies are bioluminescent insects whose light is produced by luciferase acting on luciferin. They appear in Japanese literature and film titles, and their flashes are famously used by Photuris females to lure Photinus males.
      • A firefly — A firefly produces light through a chemical reaction involving luciferin and luciferase. The clues also allow the related names lightning bug and glow worm.

    • fleas — Fleas are the wingless blood-sucking insects in the order Siphonaptera. They are known for powerful jumping, for transmitting plague and typhus, and for appearing in literary works from Donne to absurdist drama.

    • flies — Flies are a common literary and artistic symbol, appearing in myths, paintings, and poems alike. They also show up in references to Beelzebub, Io’s torment, and the insect model organism Drosophila melanogaster.

      • alkali fly — The alkali fly is a salt-tolerant insect from the Ephydridae family found at Mono Lake. It can trap an air bubble in its hairs and feed underwater, and the Kucadikadi were known for eating its pupae.

      • Diptera — Diptera is the insect order of flies and mosquitoes, named for having two wings. Quizbowl clues often point to Drosophila melanogaster for sex linkage and to Rhagoletis pomonella as a classic example of sympatric speciation.

      • Fruit flies
        • Drosophila — Drosophila is the model genus of fruit flies used throughout genetics and developmental biology. Its species, especially Drosophila melanogaster, are famous for white-eyed mutants, polytene chromosomes, and classic work by Morgan, Dobzhansky, and others.
      • mosquitoes — Mosquitoes are blood-feeding flies in the family Culicidae and among the deadliest animals to humans. Their aquatic pupae are called tumblers, and notable genera include Anopheles, Aedes, and Culex, including Aedes aegypti. They are notorious disease vectors, especially for malaria, yellow fever, West Nile virus, dengue, Zika, and encephalitis; females transmit many of these illnesses. They are also a major target of Wolbachia, sterile-male, and gene-drive control programs.

        • Aedes — Aedes is a genus of mosquitoes that includes species such as A. aegypti and A. albopictus. These mosquitoes are important vectors for dengue, Zika, yellow fever, and chikungunya, and their spread has been targeted with Wolbachia-based control efforts.

        • Aedes aegypti — Aedes aegypti is the mosquito species most often associated with transmitting several viral diseases, especially yellow fever and dengue. Its genus name is Aedes, and it is the classic disease-vector mosquito in many biology questions.

        • Anopheles — Anopheles is a mosquito genus best known for transmitting malaria, including Plasmodium vivax. In quizbowl it is often clued as the mosquito prompt for malaria vectors.

          • Anopheles mosquito — Anopheles mosquitoes are the primary vectors for Plasmodium, the protozoan genus that causes malaria. These mosquitoes transmit the parasite between human hosts, making them one of the most important disease carriers in biology.

      • screw worm — The screw worm is a parasitic fly whose females lay eggs in open wounds, and the larvae feed on living flesh. It is especially notorious for causing deadly myiasis in livestock, such as cattle.

      • the housefly — The housefly is the common scavenging fly Musca domestica. It can spread disease mechanically after feeding on refuse and manure, where its larvae develop.

      • the tsetse fly — The tsetse fly, genus Glossina, is the insect vector associated with African sleeping sickness. It transmits trypanosomes that can cause the disease and is also known for harboring the endosymbiont Wigglesworthia.

      • tsetse fly — The tsetse fly is an African insect of the genus Glossina that transmits sleeping sickness to humans. It also spreads nagana in livestock and is named for its role in destroying cattle.

    • Hymenoptera — Hymenoptera is the insect order that includes ants, bees, wasps, sawflies, and horntails. Its adults typically have membranous wings joined by hamuli, and many members show haplodiploidy and eusocial behavior.

    • Lepidoptera — Lepidoptera is the insect order that includes butterflies and moths, with scaly wings and caterpillar larvae. Common clues include the monarch and viceroy, industrial melanism in the peppered moth, and the chrysalis stage of metamorphosis.
      • butterfly — Butterflies are lepidopteran insects that emerge from chrysalises and are studied by lepidopterists. They are known for bright wings, nectar feeding, and behaviors such as puddling, and are often contrasted with moths; examples associated with mimicry and migration include monarchs, viceroys, and Heliconius. The term also appears in Zhuangzi’s dream and in works such as Julia Alvarez’s In the Time of the Butterflies, David Henry Hwang’s M. Butterfly, The Insect Play, and Bradbury’s “A Sound of Thunder,” as well as in science in the butterfly effect and as a transition state in chemistry.
        • Brush-footed butterflies
          • viceroy butterfly — The viceroy butterfly is the North American species famous for resembling the monarch in a classic mimicry example. Its resemblance to the monarch helped make it a standard example in discussions of mimicry and warning coloration.

        • butterfly wings — Butterfly wings are the insect structures referred to in these clues. They were used in a painting technique that involved pressing natural objects into wet paint, and they are associated with pterins, which are pigments.

        • Milkweed butterflies
          • Monarch butterflies — Monarch butterflies are the orange-and-black milkweed butterflies famous for migrating to Mexico and for the poisonous defense that makes the viceroy’s mimicry famous. They also title Julia Alvarez’s novel In the Time of the Butterflies, and Barbara Kingsolver’s Flight Behavior uses their appearance in Appalachia as a climate-change clue.
        • monarch butterfly — The monarch butterfly is the orange-and-black milkweed butterfly famous for its multigenerational migration from North America to overwintering sites in Mexico. Its larvae feed on milkweed, giving the adult a bitter taste and a classic example of chemical defense in mimicry.

      • Caterpillar — A caterpillar is the larval stage of a butterfly or moth, and Eric Carle’s The Very Hungry Caterpillar is the classic children’s book about one eating through many foods before metamorphosis. The same answer can also appear in industrial history clues, such as references to the Caterpillar company in labor disputes.

      • Caterpillars — Caterpillars are the larval stage of butterflies and moths. They are often identified by their role in metamorphosis and, in some literature, by their silk-producing pupal association.

      • moth — A moth is an insect often contrasted with butterflies and used in literary and pop-culture prompts. It also appears in the title of the Japanese monster film Gojira vs. Mosura, where it refers to Mothra.

      • Silkmoths
        • silkworm — The silkworm is the larva raised for sericulture, especially on mulberry leaves, to produce silk. It is the larval form of the silk moth Bombyx mori and has long been cultivated in China. Chinese tradition links it to Leizu, and later legend and history connect its eggs and silk to the spread of silk production beyond China, including to Byzantium. It also appears in Chinese idioms.

    • lice — Lice are small parasitic insects that infest hair and clothing and include sucking forms such as those in Anoplura. Their spread has been used in studies of human clothing history, and ancient medical texts already describe remedies for driving them away.

    • mantises — Mantises are predatory insects known for their folded raptorial forelegs and triangular heads. They appear in fiction and games as agile, territorial bugs, and Pokémon such as Scyther and Lurantis are based on them.
      • praying mantis — The praying mantis is an insect named for its upright forelegs, which give it the appearance of praying. It is famous for predation and for sexual cannibalism in some species.
    • moths — Moths are feathery-antennaed lepidopterans, distinct from butterflies but in the same insect order. Virginia Woolf wrote about the death of a moth in an essay that appears in her posthumous collection The Death of the Moth.

      • Biston betularia — Biston betularia is the peppered moth, a classic example of natural selection studied by H.B.D. Kettlewell. Its melanic form became common in polluted industrial areas and later declined as air quality improved.

      • Geometrid moths
        • peppered moths — Peppered moths, Biston betularia, are the classic example of industrial melanism and directional selection. Kettlewell’s studies linked their color shift to predation pressure in polluted versus clean environments.
          • Bernard Kettlewell — Bernard Kettlewell studied industrial melanism in peppered moths through field experiments comparing survival on light and dark backgrounds. His work became a famous example of natural selection in action, though later critics questioned his methods and results.
      • Gypsy moth — The gypsy moth is a destructive forest moth, Lymantria dispar, whose larvae defoliated oak trees after escaping from an experiment in Massachusetts. It is also used as a symbol for the Roma in Maus.
    • Orthoptera — Orthoptera is the insect order that includes grasshoppers and crickets. These insects are commonly grouped by their jumping hind legs and, in many questions, appear as a straightforward taxonomy identification clue.
      • Grasshoppers — Grasshoppers, or locusts, are jumping insects often cited in biblical and agricultural contexts. They can swarm destructively in large numbers, a behavior tied to major crop damage and historical plagues.
    • Orthopterans
      • locusts — Locusts are swarming grasshoppers that appear in the Bible as a plague and in the Book of Revelation as tormented, scorpion-tailed beings. They are associated with John the Baptist’s diet of wild honey and locusts, and with the eighth plague of Egypt in Exodus and Joel’s prophetic imagery. Quiz clues may also mention the year of the locust in American history.
        • locust swarm — A locust swarm is a mass outbreak of migratory locusts or grasshoppers in which huge numbers gather and devour crops and vegetation in their path, often devastating large regions. Historical examples include the Exodus narrative, destructive infestations in the American West, and major modern outbreaks in East Africa, the Middle East, and other parts of Africa and Asia.

    • termites — Termites are eusocial wood-eating insects that break down cellulose and live in colonies with a queen. They are classified as the insect order Isoptera and are often discussed as one of the social orders of insects. They also appear in ecological and anthropological contexts such as chimp tool use, Azande oracles, and granary-collapse divination.

    • True bugs
      • Hemiptera — Hemiptera is the insect order containing the true bugs, including stink bugs, cicadas, aphids, water striders, and bedbugs. Its name refers to the half-hardened forewings characteristic of the group.
        • Homoptera and Hemiptera — The traditional designation of the true bugs paired Homoptera with Hemiptera as the two orders making up that group. The clue asks for both names together rather than one single insect order.
    • Wasps — Wasps are narrow-waisted hymenopteran insects in the suborder Apocrita, including hornets and yellow jackets. They are often eusocial or parasitoid, and some species are used in biological control. Certain parasitoid wasps force spiders to spin cocoons for their young. The wasp is also linked to the Sphex concept of seemingly purposeful but mechanical behavior.

      • Wasp — A wasp is an insect that appears in literature as a symbol or plot device, including in works by George Herbert and Iain Banks. The word also titles Aristophanes’ comedy The Wasps.

      • Yellow jacket — A yellow jacket is a bright-colored paper wasp in the family Vespidae. Georgia Tech uses the Yellow Jacket as its mascot.

  • millipedes — Millipedes are diplopod arthropods with two pairs of legs per body segment, setting them apart from centipedes. They are generally detritivores and include species that defend themselves with chemical secretions.

  • Myriapoda — Myriapoda is the arthropod subphylum that includes centipedes and millipedes. Its name means “many-legged.”

  • trilobites — Extinct marine arthropods of the class Trilobita, named for their three-lobed bodies. They were once-dominant in Paleozoic seas, lived in shallow waters, often rolled into defensive balls, and disappeared at the end of the Paleozoic. They had compound eyes and jointed legs ending in claws, and are classic abundant index fossils from the Cambrian and later Paleozoic strata.

    • The trilobite — The trilobite was an ancient marine arthropod that became extinct in the Permian period. It is famous as an abundant fossil and as one of the first animals known to have developed eyesight.

    • trilobite — Extinct Paleozoic marine arthropods that first appeared in the Cambrian. They were abundant and are common fossils because they molted hard exoskeletons, and their fossils are often used to mark early geologic strata. Their three-lobed bodies left many specimens split along the head and thorax, and some species had remarkably early compound eyes.

• Auks
  • puffin — Puffins are seabirds in the auk family, noted for their colorful bills and black-and-white bodies. They nest in burrows on remote coasts and are related to the extinct great auk rather than penguins.
• Bears
  • Giant pandas
    • pandas — Pandas are the black-and-white bears native to China that became symbols of conservation and diplomatic gifting. They also show up in pop culture through Kung Fu Panda, famous zoo stories, and references to their distinctive thumb-like sesamoid bone.
  • polar bear — The polar bear is the Arctic’s largest land carnivore, a cold-climate marine mammal adapted to sea ice. It hunts seals and other marine prey, appears in Greenland’s symbolism and Inuit myth as Nanook, and is also depicted as a speaking, armored creature in His Dark Materials and other literary and pop-culture references to the species.

• Beluga — Beluga can refer to the white whale, closely related to the narwhal, or the beluga sturgeon, the largest freshwater fish. The sturgeon gives its name to beluga caviar.

• Big cats
  • panther — A panther is not a separate species but a black color variant of a leopard or jaguar. The word is often used for large cats with melanism.
• Cervidae
  • antlers — Antlers are the branched, bony head structures grown and shed by deer and related cervids. Quizbowl clues may tie them to jackalopes, the Shope papillomavirus, and mythic creatures like the wendigo, whose authentic myths do not include them.
• Chelicerates
  • horseshoe crabs — Horseshoe crabs are marine arthropods often called living fossils because they have changed little over hundreds of millions of years. Their blood is prized for limulus amebocyte lysate, which is used to detect bacterial endotoxins in medicine.
• Civets
  • masked palm civet cats — Masked palm civets are small mammals implicated in the transmission of SARS to humans. They were also reported as a traditional delicacy in Guangdong and were ordered destroyed in large numbers by Chinese authorities.
• Cyprinidae
  • carp — Carp are freshwater fish in the cyprinid family, including koi and several common Asian varieties. They also recur in East Asian folklore and art, where they can symbolize persistence, transformation, or good fortune.
    • Asian carp — Asian carp is the name for several invasive carp species in North America, especially bighead and silver carp. They are notorious for leaping from the water and threatening to spread through the Mississippi watershed into the Great Lakes.
• Decapod crustaceans
  • Lobsters — Lobsters are decapod crustaceans and a common subject in banquet still lifes, especially in Dutch painting. They appear in surreal and pop-cultural art contexts, including works associated with Dalí and Alexander Calder, and in pop-culture cooking scenes such as the dish-cooking flashback in 22 Jump Street. They are also a common culinary subject, especially in imagery about seafood, boiling, and seafloor scavengers.
• Deer

  • rut — Rut is the seasonal period when male deer and other antlered ungulates are especially active and compete for mates. It is often triggered by shortening days, and bucks may use their antlers in dominance fights before the young are born in spring.

  • White-tailed deer — The white-tailed deer is a widespread North American deer species known for its white underside to the tail and its status as a common roadkill animal. It is also the state mammal of Ohio and appears in historical references to deer hunts, frontier settlements, and Native hunting practices.

• Delphinidae
  • orca — An orca, or killer whale, is a large delphinid predator that lives in pods and hunts fish, seals, penguins, sharks, and other marine mammals. It appears in creation stories and folk traditions, and the name is often associated with Shamu and the film Free Willy. Captive orcas such as Tilikum and J35 have drawn attention in discussions of animal welfare and conservation.
• Dromaeosaurs
  • Utahraptor — Utahraptor is a large dromaeosaur named for Utah and known for the enlarged sickle claw on its foot. It is also the dinosaur character in Dinosaur Comics that appears as a more level-headed companion to T-Rex.
• Echinoderm water vascular system
  • stone canal — The stone canal is part of the echinoderm water vascular system, carrying water from the madreporite to the ring canal. It is a calcified duct named for the ringed calcium carbonate around it.
• Elephants

  • Asian elephants — Asian elephants were an important symbol of kingship and warfare in Southeast Asia, often depicted with howdahs and used in battle. They are associated with rulers such as Naresuan of Ayutthaya, whose duel with Mingyi Swa became famous.

  • Jumbo the Elephant — Jumbo was P. T. Barnum’s famous elephant, later remembered for dying in a railroad collision in Ontario. The animal’s posthumous fame has made it a recurring mascot and trivia subject.

  • Topsy — Topsy was a circus elephant whose killing became notorious in accounts of early electrocution demonstrations, especially at Coney Island and in Edison-linked film footage. She is also remembered from retellings that confuse her with other elephants and from questions about her being led across the Brooklyn Bridge as a publicity stunt. The name Topsy also appears as the enslaved child in Uncle Tom’s Cabin, a different and unrelated character who is bought as a companion to Eva and declares ignorance of God and her mother.

• Giraffidae

  • giraffe — The giraffe is the tall African mammal with seven cervical vertebrae that Lamarck once imagined evolving by stretching. It appears in Renaissance art backgrounds and is paired with the okapi as a classic heterochrony example.

  • Okapi — The okapi is a forest-dwelling ungulate from the Ituri rainforest of central Africa and the only living relative of the giraffe. Its zebra-like striped legs and later scientific description by Harry Johnston are standard clues, along with its reputation as a rare and elusive animal.

• Great apes

  • common chimpanzee — The common chimpanzee is a great ape species studied in early primate fieldwork by researchers supported by Louis Leakey. It is one of the best-known nonhuman primates in African ecology and anthropology.

  • gorilla — A gorilla is the largest of the great apes and the animal behind clues about silverbacks, chest-beating, and Koko’s sign-language use. It is also the subject of selective-attention examples like the invisible gorilla experiment.

  • orangutan — A great ape in the genus Pongo, including the Bornean and Sumatran species; the only living members of their genus. Orangutans are native to Southeast Asia, especially Borneo and Sumatra, and are known for their reddish-brown hair. The name comes from Malay words meaning “man of the forest.” Edgar Allan Poe’s “The Murders in the Rue Morgue” features an orangutan that escapes from a sailor and is blamed for murders, and “Hop-Frog” also invokes an orangutan in a violent mistaken-identity plot.

  • orangutans — Orangutans are great apes in the genus Pongo, with Bornean, Sumatran, and Tapanuli species. They also appear in literary clues such as Poe’s “Hop-Frog” and “The Murders in the Rue Morgue,” where one is implicated in murder.

• Hemichordates
  • acorn worm — Acorn worms are hemichordates that live in burrows in shallow marine sediment. They are named for the acorn-shaped proboscis they use when digging.
• mammals — Mammals are synapsid vertebrates and the class Mammalia. They are distinguished by hair, milk-producing glands, and a jaw-to-ear evolutionary transition that resulted in three middle ear bones. Living mammals include monotremes, marsupials, and placentals; most species have seven cervical vertebrae, and most give live birth. The group underwent a major radiation after the end-Cretaceous extinction event.
  • Afrotherians
    • Aardvark — The aardvark is a nocturnal African mammal, especially of southern Africa, and the sole living member of the order Tubulidentata, in the family Orycteropodidae. It has a long snout and enamel-free teeth, feeds on ants and termites, can reach large size, and its name means “earth pig”.
  • Agoutis
    • agouti — Agouti can refer to the mammalian pigment gene whose dominant mutations cause yellow coat color and obesity, as well as the agouti-related peptide involved in appetite regulation. It is also the name of a burrowing rodent of the family Dasyproctidae found from southern Mexico to southern Brazil, with a slender body, coarse glossy fur, and powerful hindquarters.
  • Ailurids
    • red panda — The red panda, Ailurus fulgens, is a small Himalayan mammal that is not closely related to the giant panda. It appears in modern pop culture as the animal form taken by Mei Lee in Turning Red and as the species associated with Aggretsuko.

  • anteater — An anteater is a toothless mammal that feeds mainly on ants and termites. The group includes the giant anteater and is known for a long snout, a tubular tongue, and heavy foreclaws used for digging.

  • antelopes — Antelopes are a large group of hoofed ruminants that includes goat-antelopes like the chamois, as well as African species such as the nyala and impala. They also appear in African sculpture through ciwara carvings, and Samson Kambalu’s Antelope is a Fourth Plinth work featuring John Chilembwe.

    • antelope — An antelope is a hooved mammal in the bovine family, including forms such as gazelles, kudus, oryxes, and gemsboks. Quizbowl clues often point to their speed, grazing habits, and animal names attached to trains, parks, or geographic features.

    • gazelles — Gazelles are slender antelopes found in Africa and Asia, often praised for their grace and speed. Common named species include Thompson’s, Grant’s, and Speke’s gazelles.

    • Oryx — An oryx is a desert antelope with long, straight horns, sometimes called a saber antelope. Species such as the Arabian and scimitar oryx are commonly referenced in wildlife clues.

    • Springbok — A springbok is a small gazelle-like antelope from southern Africa and the national mascot of South Africa. Its scientific name is Antidorcas marsupialis. The name is also used for South Africa’s national rugby and soccer teams, making it a frequent source of controversy in sports and post-apartheid identity.

    • Wildebeest — A large African antelope in the genus Connochaetes, also called a gnu. Wildebeests have a heavy head, horns, and a long tail, and are often described as ox-like in appearance. The black and blue species live in large herds on the savannas of central and southern Africa. They are famous for huge migrations across the Serengeti and the Mara River, where many are killed by crocodiles during river crossings.

  • Badger — Badgers are stocky burrowing mustelids with striped faces, known for underground setts and nocturnal habits. Species include the honey badger and stink badger. They appear in children’s and fantasy literature, including The Wind in the Willows, The Sword in the Stone, and works associated with Beatrix Potter and Tolkien, and they are also famous as animal and sports mascots, especially at the University of Wisconsin.

  • bats — They are the only flying mammals, in the order Chiroptera, and are associated with echolocation. They are important disease reservoirs and are linked to rabies, Marburg and Ebola transmission, MERS, SARS, and white-nose syndrome. They also appear in mythic stories, including Camazotz and the decapitation of Hunahpu, and in folklore as creatures turned from humans by gods.

    • Bat — Bats are nocturnal flying mammals in the order Chiroptera, named from Greek roots meaning hand-wing. Many endangered species are named for their regions, such as the Indiana, Hawaiian hoary, and Virginia big-eared bats. The word also appears in pop culture as a Batman prefix and in jokes about guano and cave-dwelling animals.

    • Chiroptera — Chiroptera is the mammalian order that includes all bats, including the vampire bat. Clues for this group often point to animated bat characters or notes that the answer is not Rodentia.

    • Old World fruit bats — Old World fruit bats, or megabats, are a bat family that includes flying foxes and usually does not rely on echolocation. They are often discussed as potential reservoirs for diseases such as Ebola.

    • vampire bats — Vampire bats are the blood-feeding bats of the family Desmodontidae, including the common, white-winged, and hairy-legged species. They use sharp incisors and an anticoagulant in their saliva to feed on birds and livestock, and are notable for walking on all fours on the ground.

  • Bears

    • brown bear — The brown bear is the large animal central to Ainu ritual sacrifice traditions and to the Sankebetsu bear attack, the deadliest such incident in Japanese history. It is also a recurring mascot-like animal in Duolingo lore.

    • brown bears — Brown bears are large omnivorous mammals that appear in mythology and folklore from Russia to the Baltic and the Arctic. Clues may mention the Ainu iomante ceremony, the Russian euphemism medved, or legendary bear-slaying heroes such as Lāčplēsis.

      • grizzly bears — Grizzly bears are the North American brown bears marked by the muscular shoulder hump used for digging. They are the bears associated with Timothy Treadwell and the documentary Grizzly Man.

      • Kodiak bear — A giant subspecies of brown bear native to the Kodiak Archipelago in Alaska. It is also called the Alaskan brown bear, is known scientifically as Ursus middendorffi, and is one of the largest living bear species; the staged record describes it as the largest living land carnivore.

    • Hsing-Hsing — Hsing-Hsing was a giant panda at the National Zoo. He suffered from kidney disease and cancer, and later died in 1999 at age 28.

    • The Grizzly Bear — The grizzly bear is the large North American bear found in the high country and known for its massive size and strength. It is often identified by its yellowish-brown to russet coat and the Latin name Ursus horribilis.
  • Big cats
    • Panthera — Panthera is the genus of the big cats, including lions, tigers, leopards, and jaguars. It is the biological group behind references to tigers, lions, and other large felids, including the species used in the name of Tiger King.
  • Bovids
    • Sheep — Wool-bearing livestock that recur in literature, religion, and myth. In Abrahamic imagery they appear in Jesus’s parables, the lost sheep parable, and as symbols of his role as the Good Shepherd; Agnus Dei is another related image. Sheep also appear in Orwell’s Animal Farm and the famous black sheep nursery-rhyme line. They have practical associations with lamb in sacrificial and culinary traditions, and with New Zealand’s huge sheep population and named breeds or tribal confederations.
      • Domestic sheep — Domestic sheep are the wool-bearing farm animals cloned as Dolly, the first mammal cloned from adult cells. They are historically associated with Merino wool, the Mesta system in Castile, and huge flocks in Australia and New Zealand.
  • Bovines

    • American Buffalo — American buffalo, or bison, are the great plains mammals central to western ecology, Native American myth, and U.S. frontier history. They also appear in David Mamet’s American Buffalo and in the nickname Buffalo Bill.

    • bison — A large bovine animal native to North America and Eurasia. Also called buffalo, it was central to Plains lifeways and later to campaigns of mass slaughter that nearly drove it extinct. It appears in connections to pishkun cliff drives, pemmican, winter counts, the conservation movement led by the American Bison Society, and the Sioux name tatanka.
      • buffalo — Buffalo, more specifically the American bison, is the animal tied to Great Plains history, buffalo jumps, and the buffalo soldiers. Other clues include the Lakota sacred animal, the Great Race, and the city of Buffalo, New York and its chicken wings.

    • ox — An ox is a castrated or working bovine, often used in mythology and art as a symbol of strength or burden. Clues may reference the Cowherd and Weaver Girl legend or Picasso’s Guernica.

    • water buffalo — The water buffalo is a domesticated bovine whose milk is used in mozzarella, especially in its Italian Mediterranean breed. The phrase “water buffalo” also became famous as an insult in a University of Pennsylvania harassment incident involving Eden Jacobowitz.

    • yak — A long-haired bovine native to Tibet and Central Asia, adapted to high-altitude life on the Tibetan plateau by its thick coat, low sweat-gland count, and oxygen-carrying blood cells. Its scientific name is Bos grunniens, and the female is called a dri or nak.
  • Camelids

    • guanaco — The guanaco is a wild South American camelid and one of the llama’s closest relatives. It lives in the Andes and Patagonia and is often cited as an ancestor of the domesticated llama.

    • llamas — Llamas are hardy Andean camelids domesticated as pack animals and used in trade across the highlands. They are closely tied to caravan transport, charki jerky, and South American pastoral life.

      • llama — The llama is a domesticated South American camelid native to the Andes. It was historically used as a pack animal in the mountains of western South America and is often contrasted with the alpaca.

  • Carnivora — Carnivora is a mammalian order that includes dogs, cats, bears, raccoons, seals, and walruses. Despite the name, not every member is strictly flesh-eating; the group contains many classic predators. Giant pandas are also classified within this order.

  • Carnivorans

    • hyena — Hyenas are carnivorous mammals closely related to mongooses rather than dogs, and they include striped, brown, spotted, and aardwolf forms. They are often noted for scavenging behavior, laughing vocalizations, and unusual folklore surrounding their sex and symbolism.

    • hyenas — Hyenas are carnivorous mammals noted for their laughing call, powerful jaws, and scavenging habits. Folklore from Africa and the Mediterranean often treats them as shape-shifting or ominous creatures, especially in stories of werehyenas and speech-imitation.
      • Aardwolf — A small African hyena relative with a striped coat, a coarse mane that bristles when threatened, and five toes on its forefeet. It has a weak jaw and a long neck, feeds mainly on termites and other small insects, and lives in burrows on dry plains of eastern and southern Africa. Its name is Afrikaans for “earth wolf.”

    • Raccoon — A raccoon is a masked North American mammal in the genus Procyon. It is a familiar pest and pop-culture creature, appearing in examples such as Meeko, Tom Nook, Rocket, and the trash-raiding animals in urban legends and cartoons.

    • raccoons — Raccoons are the masked mammals behind clues about Rocket in Guardians of the Galaxy, the trash-panda internet nickname, and the famous Toronto skyscraper climb. Older history clues also point to their pelts, which were used for frontier clothing and became a Whig symbol.
  • cattle — Cattle are domesticated bovines that include breeds such as Brown Swiss, Red Danish, Red Poll, Galloway, and Black Angus. The term can also be answered simply as cows.

    • aurochs — Aurochs were extinct wild cattle of Europe and Asia and are considered ancestors of domestic cattle. They are often identified with animals depicted in Paleolithic cave paintings such as those at Lascaux.

    • Calf — A calf is a young cow, and vellum for medieval manuscripts could be made from its skin. Quizbowl clues may also use the golden calf from Exodus or the fattened calf in the Parable of the Prodigal Son.

    • Charolais — Charolais is a French beef cattle breed, typically white and large, known for a short coat that thickens in winter. It has been used in the United States to develop the Charbray breed.

    • cow — A cow is the bovine animal tied to Hathor in Egyptian religion and to the Milky Way in later mythic imagery. Clues may also point to the golden calf of the Israelites, Nandi in Hindu tradition, or modern works and films titled First Cow.

    • cows — Cows are domesticated bovines raised for milk, meat, and other agricultural uses. They appear in myths and musicals alike, from the theft of the brown cow in the Ulster cycle to Milky White in Into the Woods, and cowpox was used in the development of the first vaccines.

  • Cetaceans — Cetacea is the mammalian order that includes whales, dolphins, and porpoises. It is divided into mysticetes and odontocetes; many toothed cetaceans use echolocation. Cetaceans are adapted to aquatic life with blowholes, and some species have baleen or a melon for sound production and sensing.

    • Baiji — The baiji was a freshwater dolphin and cetacean from China’s Yangtze River. It is now widely regarded as functionally extinct. Its decline was driven by pollution, dams, and ship noise that disrupted sonar navigation. It is also remembered from Last Chance to See and from efforts that documented its rapid decline before it disappeared.

    • baleen whales — Baleen whales are the filter-feeding cetaceans that lack teeth and instead use baleen plates to strain food from the water. This group includes blue whales and humpbacks, and it is also called Mysticeti.

      • Humpback whale — The humpback whale, Megaptera novaeangliae, is a baleen whale known for its long pectoral fins and complex songs. It also appears in clues about Star Trek IV and famous whales such as George and Gracie.

      • humpback whales — Humpback whales are baleen whales known for their long migrations and elaborate breeding-season songs. Their curved backs and small dorsal fins give them their name, and they are the animals the Enterprise crew rescue in The Voyage Home.

    • Beluga whales
      • Beluga whale — Beluga whales are small white whales with a rounded forehead bulge. They are often associated with aquarium rescues and are well known as Arctic cetaceans.

    • blue whale — The blue whale is the largest animal ever known, a baleen whale also called Sibbald’s rorqual or the sulphur-bottom. Clues often mention its immense size, its rare population, and possible hybridization with the fin whale.

    • dolphin — Dolphins are marine mammals famous in myth for rescuing Arion and carrying off Dionysus’s pirates. They are also the animals behind the oracle of Delphi’s emblematic name and a common quizbowl answer for the bottle-nosed dolphin.

    • melon — A melon, also called junk, is the fatty forehead organ of toothed whales that helps focus and direct sound for echolocation. It is especially associated with dolphins and porpoises, where it sits above the nasal passages.

    • river dolphins — River dolphins are freshwater or brackish cetaceans, including the boto and the Amazonian river dolphin. They are associated with the Amazon and Orinoco basins and with Amazonian folklore about encantados.

    • sperm whale — The sperm whale is the largest toothed whale and one of the deepest-diving whale species. It is famous for eating giant squid and for producing ambergris and spermaceti. Sperm whales are the giant deep-diving cetaceans central to Herman Melville’s Moby-Dick, which follows Captain Ahab’s obsessive hunt for the white whale.

    • vaquita — The vaquita is the critically endangered porpoise endemic to the Gulf of California. Its clues usually pair it with the totoaba and the Gulf’s conservation crisis, especially the gillnet fishery that threatens both species.
  • Civets
    • Civet — A civet is a catlike viverrid mammal native to Africa and Asia. Civets are associated with the musk-like secretion used in perfume and with kopi luwak coffee.
  • Common hippopotamus
    • Fiona the Hippo — Fiona the hippo is a Cincinnati Zoo animal who became an online sensation after being born prematurely and surviving intensive care. She was the first of her species to be photographed in utero at that zoo and became a local mascot and viral star.
  • Deer

    • caribou — Caribou are the North American form of reindeer, especially associated with Arctic, woodland, and barren-ground populations. They are northern deer whose males and females both grow antlers, and they are known for an audible clicking sound from a tendon in the foot. They range across Canada, Alaska, and other cold northern regions and are known for large seasonal migrations to calving grounds on the Arctic coast.

    • reindeer — Reindeer, also called caribou in North America, are semi-domesticated hoofed animals herded by peoples such as the Sami and Nenets. They figure in Arctic subsistence and migration patterns, as well as in Santa imagery and documentary films.

    • stag — A stag is the male deer often used in mythological and literary clues. It is the animal into which Actaeon is transformed, and it is also the form of the Ceryneian Hind’s male counterpart that appears in broad nature and heraldry clues.

  • Dermopterans
    • Dermoptera — Dermoptera is the mammalian order that contains the colugos, sometimes called flying lemurs. These animals are the only living members of the order.
  • domestic pigs — Domestic pigs are the domesticated form of suids and are the source of pork, swine, and hog meat. They appear in examples ranging from cured pork fat such as lardo to BLT sandwiches. They are also the animals associated with African swine fever, a serious viral disease that has affected outbreaks in places such as Singapore, Bulgaria, and China.
    • Poland-China — This is a breed of domestic pig named for Poland and China.
  • Equids

    • zebra — A zebra is the striped African equine of the horse family, with species such as Grevy’s, plains, and mountain zebras. Quiz clues often mention its black-and-white stripes, endangered status of the Grevy’s variety, and the extinct quagga.

    • zebras — Zebras are striped equines often used as an obvious identification answer in animal clues. They appear in migration and biology hints, and are a common comparison point for the striped fish danio rerio and the giraffe-relative okapi.

      • quagga — The quagga was an extinct zebra relative from South Africa. It was striped mainly on the head, neck, and forequarters, with the rest of the body brown, and it was recognized as a distinct species only after it had disappeared.
  • Eulipotyphla
    • shrews — Shrews are tiny insectivorous mammals, among the smallest mammals on earth, with extremely high metabolisms and constant feeding needs. They are often described as mouse-like and are related to moles.
      • Shrew — Shrews are tiny, insect-eating mammals in the family Soricidae, closely related to moles. Some shrews can echolocate, and their name appears in Shakespeare’s The Taming of the Shrew.

  • European rabbit — The European rabbit, Oryctolagus cuniculus, is the invasive mammal that spread disastrously through Australia. It is known for myxomatosis control efforts and for having been transported across much of Europe and beyond as a domesticated species.

  • Felids

    • a cat — A cat is the animal in Schrödinger’s famous thought experiment, where it is described as simultaneously alive and dead. Cats also appear in literary clues such as Gray’s poem on the death of a favorite cat and Saki’s talking Tobermory.

    • Smilodon — Smilodon is the genus of saber-toothed cat found in the Americas during the Pleistocene. It is the famous California tar pits fossil associated with oversized canines.

  • giant panda — The giant panda is the black-and-white bear native to China that eats almost entirely bamboo and is notoriously difficult to breed in captivity. It is strongly associated with conservation, the World Wildlife Fund logo, and panda diplomacy, and has a thumb-like wrist bone. Breeding centers in Chengdu, historic gifts of pandas to the United States, and the well-known giant panda birth at the Atlanta zoo are also associated with it. It was once grouped with the raccoon family but is now placed in Ursidae. The animal appears in children’s media, as the mascot for the Chinese fast-food chain Panda Express, and in Desiigner’s song “Panda,” later sampled by Kanye West. It is not the red panda, despite the similar name.

  • Giraffids
    • The okapi — The okapi is an African herbivore related to the giraffe. It is recognized by its striped legs and its reddish-brown body.
  • Great apes
    • orang-utan — The orangutan is the great ape of Borneo and Sumatra whose name means “man of the woods” in Malay. It is the world’s largest tree-dweller and is often mentioned as Pongo pygmaeus.

  • Hamsters — Hamsters are small rodents kept as pets, including the Syrian or golden hamster and Campbell’s dwarf hamster. They are also used in biomedical research, from Chinese hamster ovary cells for antibody production to classic experiments on viral transformation.

  • Herpestids
    • mongooses — Mongooses are small carnivorous mammals known in folklore for fighting snakes. They appear in The Jungle Book’s “Rikki-Tikki-Tavi” and in names like Operation Mongoose and the Mongoose Gang.

      • A mongoose — A mongoose is a small carnivorous mammal known for its snake-fighting reputation. Kipling’s Rikki-Tikki-Tavi features a mongoose as its title character.

      • meerkat — A meerkat, also called a suricate, is a small southern African mongoose relative that lives in colonies and stands watch for predators. The animal is especially familiar from the Lion King character Timon.

      • mongoose — The mongoose is a small carnivorous mammal famous for killing snakes, especially in stories like Rikki-Tikki-Tavi. In literature and folklore it appears in tales about mongoose-hound motifs, and introduced mongooses caused ecological problems in places like Jamaica.

  • Hominids — Hominids are the great apes, the taxonomic family that includes humans. The term is often used to distinguish the human lineage from other primates at the family level.

  • Horse-donkey hybrids
    • Hinny — A hinny is the offspring of a male horse and a female donkey.

  • Insectivora — Insectivora is the traditional mammalian order that once grouped animals such as moles, shrews, and hedgehogs. Its name reflects a diet of insects, but it is an older classification now considered obsolete or heavily revised in modern taxonomy; several groups, including tree shrews and hedgehogs, were split off in later systems, though the name still appears in older biology contexts.

  • Lagomorphs

    • Lagomorpha — Lagomorpha is the mammalian order that includes rabbits and hares, along with pikas. Its clues often center on rabbits such as Bugs Bunny, Roger Rabbit, Thumper, and Peter Rabbit, and on the fact that these animals are not rodents.

    • Rabbits — Rabbits are prolific lagomorph mammals, often contrasted with hares. They appear as tricksters or companions in stories such as Br’er Rabbit, Harvey, and the animals of Watership Down, and recur in myth and folklore through the moon rabbit and Easter traditions. They have also been used in laboratory antibody production and cancer research, and were targeted by Australia’s myxoma-virus control campaign.

      • bunny — A bunny is a rabbit, the animal famously associated with Margaret Wise Brown’s Goodnight Moon and The Runaway Bunny. The clue set also treats it as a generic costume or mythic creature term. It can also appear as a character persona or mascot name.

      • Swamp Rabbit — A swamp rabbit is a large rabbit associated with wetland habitats in the southeastern United States. The clue points to Jimmy Carter’s reported encounter with one during a 1979 fishing trip.

  • Macropods
    • tree kangaroos — Tree kangaroos are arboreal macropods in the genus Dendrolagus, most of which live in New Guinea. They are unusual among kangaroos for adapting to life in the trees, with only one Australian species.
  • Marsupialia — Marsupialia is the infraclass of mammals whose young are born very undeveloped and complete development attached to nipples in a pouch (marsupium). It includes kangaroos, koalas, wallabies, opossums, bandicoots, and the Tasmanian devil, and is especially associated with the fauna of Australia. Females are described as having two external vaginas, and males as having a two-pronged penis.
    • Bandicoots
      • bandicoot — Australian marsupials in the family Peramelidae, distinguished by fused toes on the hind feet and a large number of incisors; the name is sometimes translated as “pig rat.” They are the animal group associated with Crash Bandicoot and related game characters such as Coco and Crunch.
    • Koalas
      • Koala — Koalas are sedentary arboreal marsupials native to Australia with stout bodies, round ears, and large spoon-shaped noses. They feed mainly on eucalyptus leaves and live in eucalyptus trees; their specialized digestive system includes a large caecum, and young must lick specialized maternal feces to acquire bacteria needed to digest their food. Koalas are often incorrectly called bears, and their closest living relative is the wombat. They are also noted for long sleep, chlamydia outbreaks, dehydration, and habitat loss as major threats.
    • Macropods
      • kangaroos — Kangaroos are large Australian marsupials known for hopping on powerful hind legs and balancing with their tails. Their family includes related forms such as wallabies, and the quokka is a smaller relative in the same group.
        • Kangaroo — A kangaroo is the hopping marsupial symbol of Australia, whose young are called joeys and are carried in a pouch. Males are also known for boxing fights, and some species are large enough to be prominent national symbols.

    • Marsupial — Marsupials are mammals that give birth to very undeveloped young and commonly rear them in a pouch, as in kangaroos and opossums. Quizbowl clues often mention the thylacine, wombats, the monito del monte, and the Virginia opossum as a North American member.

    • Numbat — The numbat is a small Australian marsupial that feeds almost entirely on termites. Despite having no pouch, it gives birth to a few young each year, and its gray-brown coat with dark stripes gives rise to the name banded anteater.

    • opossums — Opossums are the common marsupials of the Americas, with the Virginia opossum being the U.S. species most often mentioned. They are noted for a grasping tail and for often pretending to be dead when threatened.
      • opossum — The opossum is the only marsupial native to North America north of Mexico. It is an arboreal mammal with a prehensile tail and unusually low body temperature, which helps explain its resistance to rabies.
    • possums — Possums are marsupials, with the Virginia opossum being the only marsupial native to North America. They often appear in animated films as woodland animals that can play dead, including characters like Crash and Eddie.
      • Possum — A possum is a nocturnal marsupial, especially the Australian and New Guinea animals in the phalanger group. They are represented by species such as the brush-tailed, ring-tailed, and pygmy possums, and are not closely related to the North American opossum.
    • Thylacines
      • Thylacine — The thylacine was a carnivorous, striped, dog-like marsupial native to Australia and Tasmania. It was also called the Tasmanian tiger or Tasmanian wolf, and is generally believed extinct; the last known individual died in 1936.

    • Virginia opossum — The Virginia opossum, Didelphis virginiana, is the only marsupial native to the United States. It is famous for “playing dead” and for scavenging opportunistically for food.

    • wallabies — Wallabies are smaller macropod marsupials related to kangaroos and native to Australia and nearby islands. Examples include the nail-tailed, pademelon, red-necked, and rock wallabies.

    • wombats — Wombats are burrowing marsupials native to Australia, including the common, southern hairy-nosed, and northern hairy-nosed species. They are known for cube-shaped droppings and a rear anatomy that makes them difficult for predators to bite from behind.
      • wombat — A wombat is an Australian marsupial known for its burrowing habits, stocky body, and cube-shaped droppings. It is native to Australia and Tasmania and is often described as a slow, heavy herbivore with powerful digging claws.
  • Monotremes — Monotremes are the egg-laying mammals represented today by the duck-billed platypus and the echidnas, or spiny anteaters. They are found only in Australia and New Guinea and are distinguished by a cloaca, a single opening for the urinary and reproductive tracts; the name comes from Greek for “single opening.” They also lack nipples, secreting milk through skin patches, and males of some species have venomous ankle spurs.
    • echidnas — Echidnas are Australian monotremes related to the platypus and commonly called spiny anteaters. They include short-beaked forms and rarer long-beaked species such as St. David’s echidna, and one species is named for Sir David Attenborough.
      • Echidna — The echidna, or spiny anteater, is one of the egg-laying monotremes found in Australia and New Guinea. It has a long snout and sticky tongue for eating termites and ants, and its young hatch from eggs.
  • Old World monkeys — Old World monkeys are the cercopithecids, a primate group with downward-facing nostrils and a name tied to the Old World. They include the animals associated with Sun Wukong and Hanuman in Journey to the West and related zodiac lore.
    • Baboons
      • baboon — A large ground-dwelling Old World monkey of the genus Papio, known for its hairless buttocks and omnivorous habits. In Egyptian religion, one form of the god Babi took this animal shape, and Jeff Getty received a baboon bone-marrow transplant in hopes of resisting HIV.

    • Barbary apes — Barbary apes are tailless macaques, scientifically Macaca sylvanus, found in North Africa and on the Rock of Gibraltar. They are the only monkeys native to Europe and are often described as semi-wild in Gibraltar.

    • Rhesus monkey — The rhesus monkey, Macaca mulatta, is the primate that gives its name to the Rh blood factor. It is also a common biomedical model and the natural host of herpes B virus.

  • Pangolins — Pangolins are scaly anteaters, mammals in the order Pholidota and family Manidae, named from a Malay word for “roller” because they curl into a ball when threatened. They feed on termites and ants by flicking them into toothless mouths, and have drawn attention in discussions of wildlife trade and zoonotic disease.

  • Perissodactyls

    • Baluchitherium — Baluchitherium is an extinct giant rhinoceros relative known from Oligocene fossils first found in southern Pakistan. It is sometimes described as one of the largest land mammals of all time.

    • rhinoceroses — Rhinoceroses are large odd-toed ungulates famous in art, natural history, and conservation. They appear in famous depictions from Dürer’s woodcut to reports on endangered subspecies such as the northern white rhinoceros.

      • a rhinoceros — A rhinoceros is the animal famously depicted in Albrecht Dürer’s inaccurate woodcut, with armor-like plates and a back horn. The creature is also central to Eugène Ionesco’s play Rhinoceros, where the transformation of people into rhinos becomes an absurdist image of conformity. Both clues point to the same horned mammal.

      • rhinoceros — Rhinoceroses are large horned mammals known for their armored-looking skin and endangered status. They are also famous from Dürer’s 1515 woodcut and from historical encounters involving gifts to European rulers.

  • Pholidota
    • pangolin — Pangolins are toothless mammals of the genus Manis, found in tropical Africa and Asia, that use long tongues to eat insects and curl into a ball for defense. They are also called scaly anteaters and are placed in the order Pholidota, with armor-like scales and powerful claws for breaking open logs.
  • Pigs
    • Warthog — A warthog is a rugged African pig with prominent facial bumps and curved tusks, adapted to open grasslands and lightly wooded areas. The name is also used for the jeep-like vehicle in Halo and for the A-10 Warthog nickname.
  • Primates — Primates are a mammalian order that includes lemurs, tarsiers, monkeys, apes, gorillas, humans, and other related species. They are characterized by grasping hands or opposable thumbs, stereoscopic vision, and relatively advanced brains; in haplorhines, they also lack the ability to synthesize vitamin C. The order includes Australopithecus and other early hominins.
    • apes — Apes are the primates referenced by names like Bored Ape Yacht Club and Planet of the Apes. The clues also point to apes in African mythic journeys to the underworld and in Classical Chinese poetry, where their cries are a traditional literary image.

      • African apes — Gorillas, common chimpanzees, and bonobos are the three species commonly listed as African apes.

      • ape — Ape is the broad animal clue behind several literary works, from Kafka’s “A Report to an Academy” to O’Neill’s The Hairy Ape. The clue also points to stories of humans or apes transformed by imitation, including an Ellison tale in which a scientist is changed into a sex-driven ape.

      • gibbons — Gibbons, or hylobatids, are the lesser apes of Southeast Asia, including the siamang, black gibbon, white-handed gibbon, and hoolock gibbon. They are a family within the superfamily Hominoidea and are known for their arboreal lifestyle.
        • gibbon — Gibbons are small apes of the family Hylobatidae, native to the forests of Southeast Asia. They are known for their long arms, upright swinging movement, and tailless bodies, and unlike the great apes they have small buttock callosities.

      • Gigantopithecus — Gigantopithecus was an extinct giant ape known mainly from teeth and jawbones. It may have been the largest primate ever and is sometimes linked to legends of the Yeti.

      • gorillas — Gorillas are the largest living primates, with silverback males leading troops and mountain gorillas famous from Dian Fossey’s research. Clues may also mention knuckle-walking, western lowland gorillas, or the claimed sign-language experiments on captive individuals.
        • Individual gorillas
          • Koko — Koko was a gorilla studied and trained by Francine Patterson and Ronald Cohn for her use of American Sign Language. Born Hanabi-Ko, she became widely publicized for her communication skills and for media attention surrounding her interactions with humans and other animals, including reports about her kitten All Ball, an AOL chat, and controversies over interactions with visitors.
        • mountain gorillas — Mountain gorillas are endangered highland gorillas of the Albertine Rift, especially in the Virunga region and Bwindi. They are famously associated with primatologist Dian Fossey.

    • aye-aye — The aye-aye is a nocturnal Malagasy primate, Daubentonia madagascariensis, related to lemurs. It is famous for its long middle finger used to extract insect larvae from bark, and its name sounds like a sailor’s assent.

    • bonobo — A distinct species of great ape in genus Pan found only in the Democratic Republic of the Congo, in the Congo Basin south of the Congo River. Bonobos are closely related to chimpanzees and are often described as having a more matriarchal social structure and greater sexual behavior; Kanzi is a famous bonobo trained with lexigrams.

    • bush baby — A bush baby is a small nocturnal primate in the family Galagidae, known for its large eyes and childlike calls. These arboreal animals live in sub-Saharan Africa and are often called galagos.

    • chimpanzees — Chimpanzees are great apes studied by Jane Goodall at Gombe Valley in Tanzania. They are known for vocalizations, tool use, complex social behavior, and language-learning experiments, including work involving Washoe and Nim Chimpsky. Their behavior includes termite fishing, and Goodall’s Gombe research made them famous; the Gombe Chimpanzee War destroyed the Kahama band and involved the Kasakela community. They also appear in several film and television references.

      • common chimpanzees — Common chimpanzees, or chimps, are the ape species studied by Jane Goodall and famous for tool use, territorial border patrols, and social intelligence. They are among humanity’s closest living relatives and include the ASL-trained subject Nim Chimpsky.

      • Nim Chimpsky — Nim Chimpsky was the chimpanzee raised by humans in Herbert Terrace’s sign-language experiment. He is also the subject of a 2011 documentary that follows his rescue and later life after the project ended.

    • Genus Homo
      • Homo — Homo is the Latin genus name for humans and the first word in titles such as Homo economicus and Homo ludens. It also appears in the biological genus Homo, which includes Homo sapiens and extinct species like Homo erectus. Quizbowl clues can also define it as the prefix meaning same, or the highest occupied molecular orbital abbreviated HOMO.
    • Hominins
      • Australopithecus boisei — Australopithecus boisei is a robust australopithecine fossil famous as Mary Leakey’s ‘Nutcracker Man’ from Olduvai Gorge. It was once placed in the genus Zinjanthropus and is now treated as a side branch rather than a direct human ancestor.

    • Howler Monkey — Howler monkeys are large New World monkeys known for their loud vocalizations. Their enlarged voice box produces a trumpet-like call, and some species feed heavily on leaves that require detoxification.

    • lemur — Lemurs are Madagascar’s distinctive primates, including ring-tailed, ruffed, and sifaka forms. They are often cited as endemic Malagasy fauna and as examples of the island’s unusual primate evolution.

    • lemurs — Lemurs are Madagascar’s endemic primates, including ring-tailed lemurs, indris, sifakas, aye-ayes, and mouse lemurs. They are associated with fady taboos and the fossa, and have been used as a flagship conservation group for the island’s unique wildlife; the old speculative continent name Lemuria is also associated with them.

    • Lorisiformes
      • Loris — Lorises are nocturnal primates in the family Lorisidae, related in appearance to lemurs and including the potto and bush baby. The family is named for the Asian loris species, and its members are adapted to nocturnal life.

    • mandrills — Mandrills are large Old World monkeys native to western and central African forests. They are noted for their vivid blue cheeks, red nose, and other brightly colored facial markings.

    • Primate — A primate is a high-ranking bishop in some Christian traditions, and the same word also names a mammalian order. In biology, primates include monkeys, apes, lemurs, and tarsiers.

    • Ramapithecus — Ramapithecus is an extinct primate once proposed as an early human ancestor and sometimes treated as a dead-end line. It was known from fossil remains found across Africa, Europe, and Asia and was long placed near Australopithecus in older classifications.

    • Tarsiers — Tarsiers are small nocturnal primates of Southeast Asia that cling vertically to trees and leap with elongated ankle bones. They are famous for their enormous eyes, large ears, and a head that can rotate far around, and have long been seen as bridging prosimians and monkeys.
  • Rodentia — Rodentia is the largest order of mammals. Rodents are defined by a single pair of ever-growing incisors in each jaw, adapted for gnawing. The group includes mice, rats, squirrels, beavers, guinea pigs, capybaras, prairie dogs, porcupines, nutria, the Gambian pouched rat, and naked mole rats, and it is widely used in laboratory genetics, psychology, and biomedical research. Common laboratory rat strains include Wistar, Sprague Dawley, and Long-Evans; rats have been used in experiments on mazes, overpopulation, conditioning, Little Albert, and the Morris water maze.

    • beaver — Beavers are semi-aquatic rodents whose pelts fueled the North American fur trade and gave their name to conflicts such as the Beaver Wars. Their image appears in place names and historical references across colonial North America, especially where competition over fur commerce drove expansion and rivalry.

    • beavers — Beavers are large semiaquatic rodents famous for dam building, lodge construction, and castoreum. Their pelts were central to the North American fur trade and the Beaver Wars, and beaver fur helped drive the rise of European felt hats.
      • beaver dams — Beaver dams are the stream barriers built by beavers to create ponds, protect lodges, and store food. They are classic examples of ecosystem engineering and have drawn attention for their effects on landscapes from Canada to Tierra del Fuego.
    • Capybaras
      • Capybara — The capybara is the largest living rodent, native to Central and South America and a member of the family Hydrochoeridae. It is a brown, semi-aquatic animal with partially webbed feet and a blunt snout, resembling an oversized guinea pig or small pig. It is sometimes called a water hog, and Brazilian sites such as Serra da Capivara are named for it.

    • Gerbil — A gerbil is a small desert rodent with long hind legs and a long tail, closely associated with pet ownership and laboratory use. Its family classification has been revised, and common trivia about it often references the Mongolian gerbil lineage.

    • Groundhog — The groundhog, or woodchuck, is a North American marmot celebrated on Groundhog Day each February 2nd. Its scientific name is Marmota monax, and it is known for burrowing and hibernation.

    • Guinea pig — A small South American rodent, the guinea pig is a domesticated cavy with the species name Cavia porcellus. It is widely used in laboratories and was historically used by scientists such as Lavoisier and Koch. The animal also gave rise to the phrase for someone used in a risky experiment and appears in cultural references such as mock bullfight traditions and the Guinea Pig Club.

    • house mouse — Mus musculus is the common house mouse and standard mammalian model organism used in genetics, immunology, and biomedical research. It is widely used for knockout and transgenic studies, including Cre/Lox and CRISPR-based gene editing, and for preclinical testing and cancer research. Common inbred strains include BALB/c and C57BL/6. Mice are small rodents, smaller than rats, and also appear in literature, fables, myths, and mock-epic tales such as Aesop and the Batrachomyomachia.

      • knockout mice — Knockout mice are laboratory mice engineered so that a particular gene has been inactivated. They are made using modified stem cells and are commonly used to study gene function.

      • transgenic mouse — A transgenic mouse is a laboratory mouse whose genome has been altered by inserting cloned DNA into an embryo and implanting it into a surrogate. These mice are central tools in genetic research because they can express or inherit engineered genes.

    • mole-rats — Mole-rats are burrowing rodents that include the naked mole-rat and the Damaraland mole-rat. They are famous for eusocial colonies with castes and for unusual pain insensitivity linked to a lack of substance P.

    • muskrat — A muskrat is a North American semiaquatic rodent related to voles and other cricetids. Its name comes from the odor-producing musk glands near the body, and its pelt was once an important fur-trade commodity.

    • Myocastoridae
      • Nutria — The nutria, also called the coypu, is a large semiaquatic rodent native to South America and the only living species in the family Myocastoridae. It is valued for fur but has become an invasive pest outside its native range, including the southeastern United States, where it damages crops and earthen irrigation works. It has a long tail.

    • naked mole-rat — Hairless eusocial rodents native to East Africa that live in underground colonies with a single breeding queen. They are noted for unusual pain resistance and cancer resistance, and for an insect-like queen-based social structure.

    • porcupines — Porcupines are quilled rodents whose defensive spines are used in some Native American textile and beadwork traditions. Despite the common belief, they do not shoot their quills, but their barbed quills can be painful and difficult to remove.

    • rats — Rats are rodents often blamed for plague and used in classic psychology experiments on learning, conditioning, memory, and maze navigation, including work by Tolman, Watson, and Lashley. They are typically larger than mice. Literary and fictional examples include the rats in Camus’s The Plague, Orwell’s 1984, Templeton in Charlotte’s Web, and torment scenes involving rats; they also appear in references to Ratcatcher, Rat Film, Ganesha’s mount, the Little Albert experiment, and invasive species such as Polynesian rats on Easter Island.

    • squirrels — Squirrels are rodents known for hiding nuts and acorns, and some species can leap head-first down trees or glide as flying squirrels. They also appear in myth and folklore, from Ratatosk in Norse tradition to stories about animal pox viruses.

      • Flying squirrel — A flying squirrel is a gliding squirrel that spreads a skin membrane between its limbs to move through the air. Rocky from Rocky and Bullwinkle is a well-known fictional example.

      • Prairie dogs — Prairie dogs are colonial burrowing rodents of the genus Cynomys found in the western United States and northern Mexico. They are named for their barking calls and live in large underground towns or colonies. Their communication has been studied for its referential specificity and colony-specific dialects.

    • The chinchilla — The chinchilla is a South American rodent native to the Andes and prized for its fur. It is associated with fur farms because many animals are needed to make a single coat.

    • Voles and lemmings
      • A lemming — A lemming is a small rodent with a short tail and a reputation for sudden population booms. The animal is famously, and misleadingly, associated with mass migrations and collective suicide myths.
    • woodchucks — Woodchucks are also called groundhogs, and they are the rodents behind the “how much wood” tongue twister. They appear in poems by Robert Frost and others as animals associated with hibernation and burrowing.

  • sea otters — Sea otters are marine mammals known for floating on their backs, using rocks to crack shellfish, and holding hands while sleeping. They are a keystone species in kelp forest ecosystems because their predation on sea urchins helps prevent overgrazing.

  • Sirenians

    • dugongs — Dugongs are shallow-water sirenians related to manatees and often called sea cows. They feed on seagrass beds and are especially associated with northern Australia, the Red Sea, and parts of the Seychelles.

    • manatees — Manatees are large aquatic mammals in the order Sirenia, often called sea cows. They live in warm coastal and river waters, are related to dugongs, and are frequently injured by boat propellers.

    • Sirenia — Sirenia is the order of aquatic herbivorous mammals that includes manatees and dugongs. These animals are hairless except for bristles around the mouth and are often compared to mermaids.

    • Steller’s sea cow — Steller’s sea cow was a large sirenian marine mammal native to the Bering Strait and extinct by the 18th century. It is known from Georg Wilhelm Steller’s descriptions and was hunted out soon after its discovery.

  • sloths — Sloths are slow arboreal mammals in the order Pilosa, famous for their algae-covered fur and unusual adaptations for hanging in trees. Both living tree sloths and extinct ground sloths show up in questions about their reduced mobility, neck vertebrae counts, and their place in popular culture.

    • sloth — Sloths are slow-moving tree-dwelling mammals, and the name also shows up in pop culture as a character in the Pusheen universe and in the DMV gag from Zootopia. The term also covers extinct ground sloths such as Megalonyx and Mylodon, which are noted for helping spread avocado seeds.

    • three-toed sloth — A mostly arboreal mammal in the genus Bradypus, adapted to hanging in tropical trees. It is known for its very slow metabolism and movement, long claws, ability to hang upside down for long periods, and good swimming ability. It has nine cervical vertebrae, allowing its head to rotate far around, and is often described as one of the slowest-moving mammals.

  • Tigers
    • Siberian tiger — The Siberian tiger, also called the Amur tiger, is the northern subspecies of Panthera tigris altaica. It appears on the coat of arms of the Jewish Autonomous Oblast and has been associated with resistance in Korea after Japanese eradication campaigns.
  • Tubulidentates
    • Tubulidentata — Tubulidentata is the mammalian order that contains aardvarks alone. Its only family is Orycteropodidae.
  • Ungulates — Ungulates are hoofed mammals, traditionally divided into the odd-toed perissodactyls and even-toed artiodactyls. Familiar examples include horses, deer, cattle, sheep, goats, camels, rhinos, and cows, and many Eocene mammals were also grouped among them; cetaceans are close relatives in modern classifications.
    • Artiodactyla — Artiodactyla is the mammalian order of even-toed ungulates, including deer, pigs, camels, bovines, antelope, sheep, giraffes, and other familiar hoofed mammals. Its members bear weight on an even number of toes and are defined by having a functional, symmetrical third and fourth digit. It is sometimes referenced through examples such as Bambi or Porky Pig.

      • a goat — A goat is the animal named by these clues, including the rival school mascot theft and Edward Albee’s The Goat, or Who Is Sylvia? Albee’s play centers on Martin falling in love with a goat named Sylvia, which is killed by Stevie.

      • hippopotamus — A hippopotamus is a large African amphibious mammal whose name means “water horse.” It appears in Egyptian mythology through Taweret and other protective deities, and it is also the animal behind the game Hungry Hungry Hippos.

      • hippopotamuses — Hippopotamuses are large semiaquatic African mammals, often shortened to hippos, known for their barrel-shaped bodies and river habitats. Their name means “water horse.” They appear in children’s literature such as George and Martha, in Egyptian religion and iconography as red animals or as Taweret, the goddess depicted in hippopotamus form, and in modern ecological stories such as Pablo Escobar’s escaped herd in Colombia and the feral hippos at Hacienda Nápoles.

        • a hippopotamus — A hippopotamus is one of the animals targeted in a Rubens hunt scene that also includes a crocodile. The painting shows mounted hunters attacking the animals in a dramatic Baroque composition.

        • hippo — The hippopotamus is a large African semi-aquatic mammal and even-toed ungulate, despite its bulky land-animal appearance. It is known for its huge mouth, thick skin, and dangerous territorial behavior. The name comes from Greek for “river horse,” and it is related more closely to whales than to most other large mammals. In Egyptian myth, Taweret was commonly represented as a hippopotamus, and hippos were associated with Set. A signaling pathway named Hippo shares the animal’s name and is involved in organ-size control.

      • pronghorn antelope — The pronghorn antelope is the fastest North American mammal and the sole living member of its family. Its long legs and large respiratory system help it reach roughly 50 miles per hour.

    • moose — The moose is the largest member of the deer family, with broad antlers, a shoulder hump, and a throat bell. Its name comes from an Algonquin term meaning “eater of twigs,” and it often feeds on aquatic plants in summer.

    • Perissodactyla — Perissodactyla is the mammalian order of odd-toed ungulates, including horses, tapirs, and rhinoceroses. These animals bear weight mainly on one or three toes on each foot, unlike the even-toed ungulates in Artiodactyla.
      • Tapir — A tapir is a stocky odd-toed ungulate with a short, trunk-like snout. Living species include the mountain, Baird’s, Brazilian, and Malayan tapirs. Tapirs live in Asia and Latin America and are related to horses and rhinoceroses rather than to pigs or elephants.
    • ruminants — Ruminants are cud-chewing mammals with multi-chambered stomachs, including the reticulum, rumen, omasum, and abomasum. They digest cellulose with microbial help and include animals such as cattle and giraffes.
  • whales — Whales are the huge marine mammals that include toothed forms like sperm whales and filter-feeding baleen whales. They are associated with echolocation, melon organs, whale falls, and the historic association of blue whales as the largest animals on Earth. They were hunted for ambergris and inspired works such as Moby-Dick, Free Willy, and Blackfish; whalers also used their remains in scrimshaw. In Moby-Dick, Ahab is obsessed with a white whale.
    • Delphinidae
      • orcas — Orcas, also called killer whales, are large toothed whales in the dolphin family known for coordinated hunting and their place as top marine predators. They are associated with Blackfish, SeaWorld controversies, and cultural reverence among the Lummi and Ainu.
    • Mysticetes
      • baleen — Baleen is the keratinous filtering structure used by mysticete whales to strain krill and other small prey from seawater. It distinguishes baleen whales from toothed whales, or odontocetes.
    • narwhal — The narwhal is the Arctic whale famous for the long tusk projecting from its head. It is also the animal on the Nunavut coat of arms and is found around Baffin Island.
  • Xenarthrans

    • Anteaters — Anteaters are South and Central American mammals in the order Pilosa, closely related to sloths and specialized for eating ants and termites. Their long tongues and toothless, tube-like snouts are recurring clues, especially for the giant anteater and the tamanduas.

    • Armadillo — A nocturnal xenarthran mammal known for its armored body of bony plates, burrowing habits, and ability to roll into a ball. Notable examples include the nine-banded armadillo and the pink fairy armadillo; some populations in the southern United States can carry leprosy, and humans can contract the disease from them.

    • Edentata — Edentata is the old mammalian order name for sloths, anteaters, and armadillos. The name means “toothless” and refers to animals with reduced or absent teeth.

    • Xenarthra — Xenarthra is the mammalian order that includes armadillos, anteaters, and sloths. The name refers to the unusual vertebral articulations that gave the group its distinctive classification.

• pigeons — Pigeons are the homing birds long used for wartime messaging and the subject of famous behavior experiments. They carried news like Cher Ami and G.I. Joe, were studied by B. F. Skinner in conditioning projects, and inspired both military surveillance schemes and musical or theatrical oddities. The extinct passenger pigeon and domestic rock pigeon are also relevant species in the broader pigeon family.
  • Carrier pigeon — Carrier pigeons are homing pigeons used to deliver messages over long distances. Cher Ami was one famous carrier pigeon, remembered for delivering messages during World War I.
• Pigs
  • Sows
    • sow — A sow is an adult female pig, and the word also means to plant seeds in soil. In mythology clues, it is the kind of animal slain by Theseus in the story of the Crommyonian sow.
• Sirenians

  • Dugong — A dugong is a sirenian marine mammal sometimes called a sea cow, found in the Indian and Pacific Oceans. It has tusks and a tail with two flukes, and it is often contrasted with the manatee. Clues often emphasize its saltwater habitat and its historical hunting for oil.

  • manatee — The manatee is a large aquatic mammal in the genus Trichechus, closely related to the dugong. It lives in warm coastal waters and rivers and is often called a sea cow.

• Snake sensory organs
  • Pit organ — Pit organs are infrared-sensing structures in snakes that detect heat, especially in pythons, boas, and crotalines. In pit vipers they sit in paired facial pits between the eye and nostril, helping species like rattlesnakes and moccasins locate warm prey.
• Symmetry

  • bilateral symmetry — Bilateral symmetry is the body plan in which an organism can be divided into matching left and right halves along a longitudinal axis by a single plane. It is seen in humans and many other animals, including worms, and is characteristic of most animal phyla and groups such as chordates, annelids, and mollusks.

  • radial symmetry — Radial symmetry is body-plan symmetry around a central axis, common in organisms such as jellyfish and adult echinoderms. Its special five-part form is pentaradial symmetry, seen in adult starfish and other echinoderms.

• Tetrapods
  • Amphibia — Amphibia is the tetrapod vertebrate class that includes frogs, toads, salamanders, newts, and caecilians. Its name means “dual life”; the group is divided into the orders Anura, Urodela, and Apoda. Amphibians are typically cold-blooded, have permeable skin and no amniotic eggs, and many undergo metamorphosis from aquatic larvae such as tadpoles to adults. Their study is often associated with Xenopus, the Spemann-Mangold organizer and induction experiments, and chytridiomycosis.

    • Amphibian — An amphibian is a cold-blooded vertebrate such as a frog or salamander. Common examples include the hellbender and cave salamanders.

    • Anura — Anura is the amphibian order that includes frogs and toads. Its name refers to the lack of a tail in adult forms. Its members reproduce by external fertilization and amplexus.

    • Apoda, Caudata, and Salientia — Apoda, Caudata, and Salientia are the three traditional orders of amphibians. Apoda contains caecilians, Caudata contains salamanders and newts, and Salientia contains frogs and toads.

    • Caecilians — Caecilians are limbless, wormlike amphibians in the order Gymnophiona, also called Apoda. They live mostly underground in tropical regions and are the least familiar of the three living amphibian orders.

    • frog — Frogs are amphibians that begin life as tadpoles and undergo metamorphosis into lung-breathing adults. They recur in literature, folklore, mythology, games, and internet memes, including Aristophanes’ The Frogs, Mark Twain’s The Celebrated Jumping Frog of Calaveras County, Hop-Frog, Who Will Run the Frog Hospital?, and Frogger. In world myth they can be liminal creatures, and Native American stories include Frog as a culture figure.
      • Xenopus — Xenopus is a genus of African frogs widely used in developmental biology, especially for studies of gastrulation and embryology. Its large oocytes and classic organizer experiments make species like X. laevis and X. tropicalis common model organisms.
        • Xenopus laevis — Xenopus laevis, the African clawed frog, is a major model organism in developmental biology because of its large eggs and embryos. It was central to classic experiments on embryonic induction, organizer activity, and the pregnancy test work of Lancelot Hogben.

    • newts — Newts are the amphibians central to Karel Čapek’s war satire and to the Fantastic Beasts protagonist Newt Scamander. Čapek’s play imagines intelligent salamanders used for labor and then rebelling against humanity in a global conflict.

    • salamanders — Salamanders are the fire-elemental amphibians associated with burning wood, regeneration, and the order Urodela.

      • axolotl — The axolotl is a neotenic Mexican salamander often used in regeneration and developmental biology, especially for limb and heart studies. Its distinctive gill stalks and ability to survive in a larval form make it a classic model organism. Julio Cortázar’s short story “Axolotl” uses the creature as a surreal symbol of identification and transformation. In the story, the narrator becomes obsessed with the animals at the Jardin des Plantes until he imagines himself inhabiting one.

      • axolotls — Axolotls are salamanders famous for their frilly external gills and neotenic appearance. Julio Cortázar’s story “Axolotl” uses them as a haunting symbol of transformation and identification.

      • newt — Newts are amphibians, with varieties such as the red-spotted and crested newts. They are also the creatures in Karel Čapek’s story of intelligent beings that are enslaved and eventually rebel.

      • salamander — Salamanders are amphibians in the order Caudata, including lungless forms such as Ensatina eschscholtzii. They can also appear as symbolic fire animals, as in Fahrenheit 451 and Gaudí’s Park Güell.

    • Tadpoles
      • Tadpole — A tadpole is the larval, fish-like stage in a frog’s development. It is the juvenile amphibian form that later undergoes metamorphosis and develops air-breathing lungs.
    • Toads
      • cane toad — The cane toad, also called the marine toad or Rhinella marina, is an invasive toxic amphibian introduced to Australia, Florida, and other places to control agricultural pests. It became notorious for spreading rapidly and poisoning native predators and pets, thriving in environments where it was never meant to live.
  • Reptiles and Amphibians — Reptiles and amphibians are the subjects of herpetology. The field covers animals such as snakes, lizards, frogs, and salamanders.
• Toothed whales
  • dolphins — Dolphins are intelligent marine mammals and toothed whales, often treated as a separate marine-mammal group. They are known for bottlenose species, echolocation, and signature whistles, and examples include the pink Amazon river dolphin and the baiji. In Greek myth, Poseidon sends one to win Amphitrite, and Dionysus turns pirates into dolphins; dolphins also appear in music, including David Bowie’s lyrics.
    • Bottlenose dolphins — Bottlenose dolphins are the best-known dolphin species, identified by their bottle-shaped snout and ability to communicate with whistles and clicks. They sleep with one brain hemisphere at a time and are often given the species name Tursiops truncatus.
• tortoises — Tortoises are land-dwelling testudines, often contrasted with turtles, and they appear in biology, folklore, and poetry alike. Quizbowl clues may mention their shells in Chinese legend, Lawrence’s poems about them, or giant species such as those in the Galápagos.
  • Tui Malila — Tui Malila was an extremely long-lived tortoise associated with the Tongan royal family. Queen Elizabeth II visited it in 1953 when it was said to be 176 years old.