Biology

Cell Biology

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.
• Plasma membrane — phospholipid bilayer with embedded proteins; selectively permeable; fluid mosaic model (Singer and Nicolson, 1972).
• Nucleus — contains DNA on chromosomes; surrounded by the double nuclear envelope with nuclear pores; contains the nucleolus (site of rRNA synthesis and ribosome assembly).
• 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.
• Mitochondria — site of aerobic respiration; double membrane (inner membrane folded into cristae); own circular DNA and 70S ribosomes; endosymbiotic origin.
• Chloroplast — site of photosynthesis in plants and algae; double membrane plus internal thylakoid membranes (stacked into grana) and stroma; own circular DNA; endosymbiotic origin.
• Lysosome — contains hydrolytic enzymes (acid pH); digests cellular debris, food particles, and foreign material; absent in plant cells (vacuoles serve analogous functions).
• Vacuole — storage organelle; central vacuole in plant cells maintains turgor pressure.
• Cytoskeleton — three types: microtubules (tubulin, 25 nm; form spindle fibers, cilia, flagella), intermediate filaments (structural support), microfilaments (actin, 7 nm; cell movement, cytokinesis).
• 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.
• Peroxisome — contains catalase; breaks down H₂O₂ and fatty acids via beta-oxidation.

Membrane transport

• Passive transport — no energy required; moves down concentration gradient. Includes simple diffusion, facilitated diffusion (via channel or carrier proteins), and osmosis (water via aquaporins).
• 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.
• Endocytosis / exocytosis — bulk movement of materials into or out of the cell via vesicles; phagocytosis (solid particles), pinocytosis (fluids).
• Osmosis — water moves toward the region of lower water potential (higher solute concentration). Hypotonic solution causes cell swelling; hypertonic causes shrinkage (crenation in animal cells, plasmolysis in plant cells).

Cell division

• Mitosis — division producing two genetically identical daughter cells; phases: PMAT (Prophase, Metaphase, Anaphase, Telophase) plus Cytokinesis. Somatic cell division.
• 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.
• 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 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).
• 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.
• Cancer — results from mutations in proto-oncogenes (become oncogenes) and/or tumor suppressor genes (e.g., TP53, Rb); cells divide uncontrollably.
• 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).
• 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.
• 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.

Biomolecules

• 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).
• Lipids — hydrophobic; includes triglycerides (energy storage), phospholipids (membranes), steroids (cholesterol, hormones). Saturated fats have no double bonds; unsaturated have one or more.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• ATP (adenosine triphosphate) — the universal cellular energy currency; hydrolysis of the terminal phosphate bond releases ~7.3 kcal/mol (standard conditions).
• 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).

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) — occur in the stroma; use ATP and NADPH to fix CO₂ into G3P (glyceraldehyde-3-phosphate) via RuBisCO; 3 CO₂ fixed per turn → net gain of 1 G3P after 3 turns.
• 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).

Cellular respiration

• Overall equation — C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~30–32 ATP (aerobic).
• Glycolysis — cytoplasm; splits glucose (6C) into 2 pyruvate (3C); net yield: 2 ATP, 2 NADH; no oxygen required.
• 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.

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.
• 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.

Transcription and translation (Central Dogma)

• Central dogma — DNA → RNA → Protein (Crick, 1958). Reverse transcriptase (in retroviruses) allows RNA → DNA, an exception.
• 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).
• 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.
• 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).
• 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.
• 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).

Gene regulation

• 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.

Mutations

• Point mutations — substitutions: missense (change amino acid), nonsense (create stop codon), silent (synonymous, no amino acid change).
• Frameshift mutations — insertions or deletions shift the reading frame; generally severe.
• 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).

Virology and Microbial Genetics

Viruses

• 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.
• 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.
• 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).
• 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.
• Viroid — the simplest known pathogen: a small, circular, single-stranded RNA molecule with no capsid; infects plants; mechanism involves RNA interference-like silencing.
• 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.

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.
• 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.
• 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.

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.
• 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.
• 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).
• 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).
• Northern / Western blot — Northern blot detects specific RNAs (analogous to Southern but for RNA); Western blot detects specific proteins using antibodies after SDS-PAGE.

Mendelian and Population Genetics

Mendelian genetics

• Law of Segregation — alleles separate during gamete formation; each gamete carries one allele per gene.
• 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).
• Dominance — complete dominance (one allele fully masks the other); incomplete dominance (heterozygote is intermediate); codominance (both alleles expressed equally, e.g., AB blood type).
• Dihybrid cross — two heterozygous parents yield 9:3:3:1 phenotypic ratio among offspring.
• 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.
• 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.
• Sex-linked traits — genes on X chromosome (X-linked); males (XY) are hemizygous; examples: red-green color blindness, hemophilia A.
• 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.

Genetic disorders

• 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.
• 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).
• 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.
• 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.
• 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.
• 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.

Extensions of Mendelian genetics

• Polygenic inheritance — multiple genes affect a single trait (e.g., skin color, height); produces a continuous distribution.
• Pleiotropy — one gene affects multiple phenotypic traits (e.g., sickle-cell mutation affects RBC shape, oxygen carrying, and malaria resistance).
• 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).
• 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.

Population genetics

• 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.
• 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).
• Gene flow — movement of alleles between populations via migration; tends to homogenize populations; counteracts divergence caused by drift and selection.
• Natural selection — differential reproductive success based on heritable traits; alters allele frequencies directionally.
• Mutation — the ultimate source of new alleles.
• 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.

Evolution and Natural Selection

• 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.
• 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.
• 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).
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• Alfred Russel Wallace — independently proposed natural selection simultaneously; co-presented with Darwin at the Linnean Society, 1858.
• 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).
• 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).
• Adaptation — heritable trait that increases fitness in a given environment.
• Speciation — allopatric (geographic isolation leads to divergence); sympatric (speciation within a population, often via polyploidy in plants).
• Reproductive isolation — pre-zygotic barriers (habitat, temporal, behavioral, mechanical, gametic) and post-zygotic barriers (hybrid inviability, hybrid sterility).
• 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.
• Coevolution — reciprocal evolutionary influence between species (e.g., flowers and pollinators, host and pathogen).
• Evo-devo — evolutionary developmental biology; Hox genes control body-plan development; small regulatory changes can cause large morphological differences.

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.”
• 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).
• Phylogenetics — classification based on evolutionary relationships; modern cladistics groups organisms by shared derived characteristics (synapomorphies).

The three domains

• Bacteria — prokaryotes; peptidoglycan cell walls; diverse metabolisms; include pathogens (E. coli, Streptococcus) and beneficial organisms (gut microbiome, nitrogen fixers).
• Archaea — prokaryotes; no peptidoglycan; often extremophiles (thermophiles, halophiles, methanogens); more closely related to Eukaryota than to Bacteria.
• Eukarya — membrane-bound nucleus; includes Protista, Fungi, Plantae, Animalia.

Eukaryotic kingdoms

• Protista — paraphyletic grouping; mostly unicellular; includes algae, amoebas, paramecia, slime molds; many are photosynthetic.
• Fungi — heterotrophic; cell walls of chitin; digest food externally (absorptive nutrition); include molds, yeasts, mushrooms; hyphae form mycelium. Important decomposers.
• 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:

Phylum	Key features	Examples
Porifera	No true tissues; filter feeders	Sponges
Cnidaria	Radial symmetry; cnidocytes	Jellyfish, coral, hydra
Platyhelminthes	Flat, acoelomate	Flatworms, tapeworms
Annelida	Segmented; true coelom	Earthworms, leeches
Mollusca	Mantle; often shell	Clams, snails, octopus
Arthropoda	Exoskeleton; jointed appendages	Insects, spiders, crustaceans
Echinodermata	Spiny skin; water vascular system; deuterostome	Sea stars, urchins
Chordata	Notochord, dorsal nerve cord, pharyngeal slits	Fish, amphibians, reptiles, birds, mammals

• Vertebrate classes — fish (jawless: Agnatha; jawed: Chondrichthyes [cartilaginous], Osteichthyes [bony]); Amphibia (water-dependent reproduction); Reptilia (amniotic egg, ectothermic); Aves (birds: amniotic egg, endothermic, feathers); Mammalia (hair, mammary glands, endothermic).
• Marsupials (Marsupialia) — a clade of mammals that give birth to underdeveloped young which complete development in a pouch (marsupium); ~330 living species; predominantly in Australia and New Guinea (kangaroos, wallabies, wombats, koalas, Tasmanian devil) plus opossums in the Americas; lack a true placenta; pouched young attach to nipples for extended development.
• Monotremata (monotremes) — the most basal order of living mammals; lay eggs but nurse young with milk secreted through skin patches (no nipples); retain a cloaca (single posterior opening); only five living species: the platypus (Ornithorhynchus anatinus) and four species of echidna; found in Australia and New Guinea.
• Platypus — a semiaquatic monotreme mammal (Ornithorhynchus anatinus) native to eastern Australia; famous for having a duck-like bill (electroreceptors detect prey), beaver-like tail, otter-like feet, and laying eggs; males have venomous spurs on hind ankles; the platypus’s genome is a mosaic of mammalian, reptilian, and avian features.
• Equidae — the family of odd-toed ungulates (order Perissodactyla) that includes modern horses (Equus caballus), donkeys, zebras, and their extinct relatives; all living members are genus Equus; characterized by a single functional toe (the hoof) on each foot; horses were domesticated ~5,500 years ago in the Eurasian steppe.
• Bivalvia (bivalves) — a class of molluscs with two hinged shells (valves) joined by a ligament and adductor muscles; include clams, oysters, mussels, scallops, and shipworms; mostly filter feeders using gills (ctenidia); no distinct head; the largest bivalve is the giant clam (Tridacna gigas); economically important as food and as bioindicators.
• Pterosauria (pterodactyls) — extinct flying reptiles of the Mesozoic Era; the first vertebrates to achieve powered flight; characterized by a wing membrane (patagium) supported by an elongated fourth finger; include small Pterodactylus and giant Quetzalcoatlus (wingspan ~10–11 m, the largest known flying animal); not dinosaurs but closely related archosaurs.
• Honeybee (Apis mellifera) — the western honey bee; a eusocial insect (order Hymenoptera) living in colonies of up to ~60,000 workers, one queen, and seasonal drones; workers are sterile females performing foraging, nursing, and defense; famous for the waggle dance (Karl von Frisch, 1973 Nobel Prize) by which foragers communicate distance and direction of food sources; produce honey and beeswax; critical pollinators of crops; colonies can suffer colony collapse disorder (CCD); females have a barbed stinger and die after stinging.
• Mosquito — insects of the family Culicidae (order Diptera); only females bite, requiring a blood meal for egg development; females detect hosts via CO₂, heat, and body odor; medically the most deadly animals on Earth, transmitting malaria (Plasmodium, via Anopheles mosquitoes), dengue fever, yellow fever, Zika, and West Nile virus (Aedes and Culex species); larvae are aquatic; the Anopheles genus is the sole vector of human malaria.
• Octopoda (octopuses) — an order of soft-bodied cephalopod molluscs with eight arms lined with suckers; highly intelligent invertebrates capable of problem-solving, tool use, and camouflage via chromatophores; have three hearts, blue copper-based blood (hemocyanin), and a short lifespan; can squirt ink as a defense; the common octopus is Octopus vulgaris; closely related to squids and cuttlefish in class Cephalopoda.
• Penis — the male external genitourinary organ present in many vertebrates; in humans, composed of erectile tissue (two corpora cavernosa and one corpus spongiosum) that fills with blood during erection; serves dual functions of urination (via the urethra) and copulation; the glans penis is the distal end; homologous to the clitoris; development is driven by androgens (testosterone/DHT) during fetal development.
• Bioluminescence — the production and emission of light by a living organism through a chemical reaction, typically involving a luciferin substrate oxidized by a luciferase enzyme with ATP; found in fireflies, deep-sea fish (anglerfish), dinoflagellates, jellyfish, and bacteria; functions in predation, defense (counter-illumination), and communication; the most common form of light production in the deep ocean.
• Echolocation — a biological sonar system used by certain animals to orient and hunt by emitting ultrasonic pulses and interpreting the returning echoes; well-developed in bats (Chiroptera) and toothed whales (Odontoceti), including dolphins; also found in some shrews, tenrecs, and oilbirds; bats emit calls through the mouth or nose and use specialized ears and neural processing to build a three-dimensional acoustic picture of their surroundings.
• Gastrulation — the embryonic process by which the blastula is reorganized into the gastrula, establishing the three primary germ layers: ectoderm (skin, nervous system), mesoderm (muscles, skeleton, circulatory system), and endoderm (gut, lungs, liver); involves coordinated cell movements (invagination, ingression, epiboly); sets up the basic body plan; follows cleavage and blastulation; the outcome is an embryo with a primitive gut (archenteron) and an opening called the blastopore.
• Meristem — regions of undifferentiated, actively dividing cells in plants analogous to animal stem cells; apical meristems at root and shoot tips drive primary (elongation) growth; lateral meristems (vascular cambium, cork cambium) drive secondary (girth) growth; intercalary meristems (in grasses) allow regrowth after grazing; meristematic cells are totipotent — each can give rise to a complete plant.
• Keratin — a family of fibrous structural proteins that form the main structural component of hair, nails, horns, hooves, claws, scales, and the outer layer of skin (epidermis); alpha-keratins (in mammals) form coiled-coil intermediate filaments; beta-keratins (in birds and reptiles) form beta-sheet structures; provide mechanical protection and waterproofing; synthesized by keratinocytes.
• Karyotype — the complete set of chromosomes in a cell, displayed as an ordered image by size, shape, and banding pattern (typically after G-banding); normal human somatic cells have 46 chromosomes (23 pairs, including 1 pair of sex chromosomes); used to diagnose chromosomal abnormalities (e.g., Down syndrome/trisomy 21, Turner syndrome/45,X); produced from cells arrested in metaphase using colchicine.
• Multinucleate cells (syncytia) — cells containing two or more nuclei; arise by nuclear division without cytokinesis, or by cell fusion; examples: skeletal muscle fibers (syncytia formed by myoblast fusion), osteoclasts (bone-resorbing cells), fungal hyphae, and the plasmodium of slime molds; also produced during viral infection when cell membranes fuse (syncytium formation is characteristic of paramyxoviruses including measles and RSV).

Plant Biology

Vascular tissue and water movement

• Xylem — dead, hollow cells (tracheids and vessel elements) that conduct water and dissolved minerals from roots to leaves; movement driven by transpiration pull (cohesion-tension theory); also provides structural support (wood is secondary xylem).
• Phloem — living cells (sieve tube elements + companion cells) that transport sugars (sucrose) and other organic solutes from sources (leaves) to sinks (roots, fruits, growing tissue); bidirectional; driven by pressure-flow (bulk flow) mechanism.
• Transpiration — evaporation of water from leaf surfaces, primarily through stomata; creates negative pressure that pulls water up through the xylem; accounts for most of the water absorbed by roots.
• Stomata — pores in the leaf epidermis flanked by guard cells; open (allowing CO₂ in and water out) when guard cells are turgid (K⁺ uptake → water follows by osmosis); close under drought or darkness; regulated by abscisic acid (ABA).

Plant hormones and tropisms

• Auxin (IAA) — promotes cell elongation at low concentrations; produced at the shoot apex and transported downward; responsible for phototropism (light causes auxin to redistribute toward the shaded side, elongating those cells) and apical dominance (suppresses lateral buds).
• Other key plant hormones — gibberellins (stem elongation, seed germination, fruit development); cytokinins (promote cell division, delay leaf senescence); ethylene (gaseous; promotes fruit ripening, leaf abscission); abscisic acid/ABA (stress response, stomatal closure, seed dormancy).
• Tropisms — directional growth responses: phototropism (toward light; mediated by auxin redistribution); gravitropism (roots grow downward, shoots upward; starch-filled amyloplasts act as statoliths); thigmotropism (growth response to touch; e.g., tendrils of climbing plants).

Ecology

Ecological organization

• Levels — individual → population → community → ecosystem → biome → biosphere.
• Population ecology — studies size, density, distribution, growth; exponential growth (J-curve; unlimited resources); logistic growth (S-curve; limited by carrying capacity K).
• r vs K strategists — r-strategists: small body size, many offspring, little parental care, colonizers (e.g., insects, annual plants); K-strategists: large body size, few offspring, high parental investment, near carrying capacity (e.g., elephants, humans). Concept from MacArthur and Wilson.
• Survivorship curves — Type I (most survive to old age; humans, large mammals); Type II (constant mortality rate; birds); Type III (high early mortality; fish, insects).
• Ecological succession — directional change in community composition over time; primary succession starts on bare substrate (e.g., after glacier retreat; pioneer species: lichens, mosses); secondary succession follows disturbance with soil intact (faster; e.g., after fire or farming); culminates in a climax community.
• Island biogeography — MacArthur and Wilson (1967); species richness on islands is a balance between immigration rate and extinction rate; larger islands and those closer to mainland have more species; the equilibrium theory; applied to habitat islands and conservation biology.
• Niche — the ecological role and space occupied by a species; fundamental niche (full potential range in absence of competitors); realized niche (actual range due to competitive exclusion and other interactions). Hutchinson formalized the n-dimensional hypervolume concept.

Species interactions

• Predation — (+/−) predator benefits, prey harmed; drives coevolution (mimicry, camouflage).
• Competition — (−/−) interspecific competition; competitive exclusion principle (Gause): two species competing for identical resources cannot coexist indefinitely.
• Symbiosis — mutualism (+/+; e.g., mycorrhizae, nitrogen-fixing bacteria in legume root nodules); commensalism (+/0); parasitism (+/−).
• Keystone species — a species with a disproportionately large effect on ecosystem structure relative to its abundance (e.g., sea otters controlling sea urchins/kelp).

Trophic levels and energy flow

• Trophic levels — producers (autotrophs) → primary consumers (herbivores) → secondary consumers → tertiary consumers → decomposers (bacteria, fungi).
• 10% rule — approximately 10% of energy is transferred from one trophic level to the next; explains why food chains are short.
• Gross primary productivity (GPP) vs net primary productivity (NPP) — GPP = total photosynthesis; NPP = GPP minus plant respiration; represents energy available to consumers.
• Decomposers — break down dead organic matter, returning nutrients to the abiotic environment.

Biogeochemical cycles

• Carbon cycle — photosynthesis fixes CO₂; respiration, decomposition, combustion release it; oceans act as a carbon sink.
• Nitrogen cycle — atmospheric N₂ fixed by bacteria (Rhizobium, Azotobacter); nitrification (NH₃ → NO₂⁻ → NO₃⁻); denitrification returns N₂ to atmosphere.
• Phosphorus cycle — no atmospheric phase; phosphate released by weathering of rock; limiting nutrient in many freshwater ecosystems.
• Water cycle — evaporation, transpiration (evapotranspiration), condensation, precipitation, runoff, infiltration.

Biomes

• Tropical rainforest — high temperature and rainfall; highest biodiversity; nutrient-poor soils; near the equator.
• Temperate deciduous forest — four seasons; fertile soils; broadleaf trees lose leaves in fall.
• Taiga (boreal forest) — coniferous; long cold winters; thin acidic soils; large area of North America, Russia.
• Tundra — treeless; permafrost; Arctic and alpine; very low productivity.
• Grassland / savanna — fire-maintained; large grazers; high animal diversity (savanna in tropics, temperate grassland/prairie in mid-latitudes).
• Desert — low precipitation (<25 cm/yr); extreme temperatures; CAM plants dominant.
• Chaparral (Mediterranean) — dry hot summers, mild wet winters; fire-adapted shrubs.
• Freshwater / marine biomes — rivers, lakes, wetlands; marine includes intertidal, pelagic, benthic, coral reefs, and open ocean.

Human Physiology

• Digestive system — mouth (amylase) → esophagus → stomach (pepsin, HCl) → small intestine (most digestion/absorption; villi/microvilli increase surface area; pancreatic enzymes, bile from liver) → large intestine (water absorption) → rectum.
• Circulatory system — heart (4 chambers: right atrium/ventricle handle pulmonary circulation; left handle systemic); blood vessels (arteries, capillaries, veins); red blood cells carry O₂ via hemoglobin; ABO and Rh blood types.
• Cardiac cycle — systole (ventricles contract, blood pumped out) and diastole (ventricles relax, fill with blood); SA node (sinoatrial node, the pacemaker) initiates the electrical impulse, which passes through the AV node, bundle of His, and Purkinje fibers; the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization) are the ECG components.
• Respiratory system — air enters via nasal passages → trachea → bronchi → bronchioles → alveoli (gas exchange by diffusion); diaphragm and intercostal muscles drive breathing; hemoglobin has cooperative O₂ binding (sigmoid O₂-dissociation curve).
• Action potential — an all-or-nothing electrical signal in neurons; resting potential ~−70 mV; depolarization: Na⁺ channels open → inside becomes positive (~+30 mV); repolarization: K⁺ channels open → inside returns negative; refractory period prevents backward propagation; propagates along unmyelinated axons or jumps between nodes of Ranvier (saltatory conduction) in myelinated axons.
• Neurotransmitters — chemical signals released at synapses: acetylcholine (neuromuscular junction, autonomic ganglia; also in CNS), dopamine (reward, motor control; deficient in Parkinson’s), serotonin (mood, sleep; target of SSRIs), GABA (main inhibitory CNS transmitter), glutamate (main excitatory CNS transmitter), norepinephrine (sympathetic system).
• Nervous system — CNS (brain and spinal cord) + PNS; neurons: dendrites receive signals, axon transmits action potential; synaptic transmission via neurotransmitters; autonomic NS: sympathetic (“fight or flight,” norepinephrine) vs parasympathetic (“rest and digest,” acetylcholine).
• Endocrine system — glands secrete hormones into the bloodstream; key glands: pituitary (“master gland”; anterior/posterior), thyroid (T3/T4, metabolism), adrenal (cortisol, epinephrine), pancreas (insulin lowers blood glucose; glucagon raises it).
• Immune system — innate (non-specific; barriers, phagocytes, NK cells, inflammation) and adaptive (specific; B cells produce antibodies; T cells: cytotoxic T cells kill infected cells, helper T cells coordinate response; immunological memory).
• Antibodies (immunoglobulins) — Y-shaped proteins with two heavy and two light chains; variable regions at tips form the antigen-binding sites; five classes (IgG, IgM, IgA, IgD, IgE); clonal selection: antigen selects and activates B cell clones bearing matching receptors, producing plasma cells and memory cells.
• MHC (major histocompatibility complex) — cell-surface proteins that present peptide antigens to T cells; MHC class I (on all nucleated cells; presents to CD8⁺ cytotoxic T cells); MHC class II (on antigen-presenting cells; presents to CD4⁺ helper T cells); called HLA (human leukocyte antigen) in humans; key to transplant compatibility.
• Musculoskeletal system — skeletal muscle: sarcomere is the contractile unit (actin/myosin); sliding-filament model; troponin-tropomyosin regulation by Ca²⁺; ATP required for both contraction and relaxation.
• Nephron — functional unit of the kidney; glomerulus (filtration of blood under pressure into Bowman’s capsule) → proximal convoluted tubule (reabsorption of glucose, amino acids, Na⁺) → loop of Henle (creates osmotic gradient; descending limb permeable to water, ascending to salt) → distal convoluted tubule → collecting duct; ADH controls water reabsorption in collecting duct; aldosterone controls Na⁺ reabsorption.
• Renal system — kidneys filter blood; nephron is functional unit; glomerular filtration → tubular reabsorption → secretion; ADH (antidiuretic hormone) controls water reabsorption; aldosterone controls Na⁺/K⁺ balance.
• Reproductive system — male: testosterone, spermatogenesis; female: estrogen, progesterone, oogenesis, menstrual cycle (~28 days); fertilization in the fallopian tube; embryo implants in uterus.
• Adipose tissue — specialized connective tissue that stores energy as triglycerides in lipid droplets; brown adipose tissue generates heat via uncoupling protein 1 (UCP1/thermogenin) in the inner mitochondrial membrane (non-shivering thermogenesis); white adipose tissue is the primary energy reservoir and secretes adipokines including leptin (signals satiety) and adiponectin; distributed subcutaneously and viscerally.
• Cranium (skull) — bony structure encasing and protecting the brain; composed of 8 cranial bones (frontal, parietal ×2, temporal ×2, occipital, sphenoid, ethmoid) joined by immovable joints called sutures (e.g., coronal, sagittal, lambdoid); the foramen magnum is the large opening at the base through which the brainstem connects to the spinal cord; fontanelles are the soft membranous gaps present in infant skulls that close by ~2 years of age.
• Spleen — largest lymphoid organ, located in the upper left abdomen; red pulp filters blood by destroying old or damaged red blood cells and recycling iron from hemoglobin; white pulp contains lymphocytes and mounts immune responses to blood-borne pathogens; stores platelets and acts as a blood reservoir; a ruptured spleen (from trauma) is a surgical emergency; the spleen can be surgically removed (splenectomy) with increased susceptibility to encapsulated bacterial infections afterward.
• Umbilical cord — connects the fetus to the placenta; contains two umbilical arteries (carrying deoxygenated fetal blood to the placenta) and one umbilical vein (returning oxygenated, nutrient-rich blood to the fetus); surrounded by Wharton’s jelly (a gelatinous connective tissue); the placenta acts as the site of maternal-fetal exchange of oxygen, nutrients, and waste; the umbilical cord normally closes off at birth, leaving the navel (umbilicus).
• Vagina — muscular, fibromuscular canal connecting the uterus (via the cervix) to the external genitalia; serves as the birth canal during delivery and as the receptive organ during intercourse; has an acidic pH (~3.8–4.5) maintained by Lactobacillus species that protect against infection; lined by stratified squamous epithelium; the hymen is a thin mucosal fold at the vaginal opening.
• Vestigial structures — anatomical remnants of features that were functional in ancestral species but have reduced or no current function; classic examples in humans: the coccyx (vestigial tail), the plantaris muscle, the palmaris longus (absent in ~15% of people), the arrector pili muscles, wisdom teeth (third molars), and the appendix; used as evidence for evolution and shared ancestry; vestigiality is relative — structures may retain minor secondary functions.
• Giraffe (recurrent laryngeal nerve) — canonical example in evolutionary biology and anatomy; the recurrent laryngeal nerve (a branch of the vagus nerve, cranial nerve X) travels from the brain down the neck, loops under the aortic arch (left side) or subclavian artery (right side), and returns up the neck to innervate the larynx; in the giraffe this detour is ~4–5 meters despite the larynx being centimeters from the brain; cited as a striking example of evolutionary constraint — anatomical pathways reflect evolutionary history (the nerve took this route in common fish ancestors and was retained through vertebrate evolution); also cited in discussions of suboptimal design in nature.
• Planaria — free-living flatworms (phylum Platyhelminthes, class Turbellaria) that are the classic model organism for regeneration biology; can regenerate a complete individual from a tiny fragment, including regenerating the head and brain; regeneration is mediated by totipotent stem cells called neoblasts (the only dividing somatic cells); used to study the molecular basis of body patterning (Wnt signaling sets anterior-posterior axis) and stem cell biology; Dugesia tigrina and Schmidtea mediterranea are common research species.

Landmark Biologists and Discoveries

• Gregor Mendel — laws of heredity (segregation and independent assortment) from pea plant experiments (published 1866); work ignored until 1900 when De Vries, Correns, and Tschermak rediscovered it.
• Charles Darwin and Alfred Russel Wallace — natural selection as the mechanism of evolution (1858–1859).
• Rosalind Franklin — produced X-ray crystallography Photo 51 of DNA; critical to Watson and Crick’s 1953 structure; did not share the 1962 Nobel (awarded posthumously is not accurate — she died 1958 before the Nobel was given).
• James Watson and Francis Crick — double-helix model of DNA (Nature, April 1953); shared 1962 Nobel Prize in Physiology or Medicine with Maurice Wilkins.
• Frederick Griffith — transformation experiment (1928) demonstrated that genetic material could be transferred between bacteria.
• Avery, MacLeod, and McCarty — demonstrated DNA (not protein) is the transforming material (1944).
• Hershey and Chase — bacteriophage experiment (1952) confirmed DNA is the genetic material using radioactive labels.
• Meselson and Stahl — confirmed semi-conservative DNA replication via cesium-chloride density gradient (1958).
• Linus Pauling — described alpha-helix and beta-sheet protein structures; also proposed (incorrectly) a triple-stranded DNA model.
• Barbara McClintock — discovered transposable elements (“jumping genes”) in maize; 1983 Nobel Prize.
• Lynn Margulis — endosymbiotic theory: mitochondria and chloroplasts derived from ancient bacterial endosymbionts.
• Frederick Sanger — first to sequence a protein (insulin, 1951) and developed chain-termination DNA sequencing (Sanger sequencing, 1977); two Nobel Prizes.
• Kary Mullis — invented PCR (polymerase chain reaction, 1983); 1993 Nobel Prize in Chemistry.
• Jennifer Doudna and Emmanuelle Charpentier — developed CRISPR-Cas9 as a gene-editing tool; 2020 Nobel Prize in Chemistry.
• Hugo de Vries, Carl Correns, Erich Tschermak — independently rediscovered Mendel’s laws in 1900.
• Thomas Hunt Morgan — demonstrated that genes are carried on chromosomes using Drosophila melanogaster; discovered sex-linked inheritance and genetic linkage; 1933 Nobel Prize in Physiology or Medicine; established chromosome theory of heredity.
• George Beadle and Edward Tatum — “one gene, one enzyme” hypothesis (1941) from Neurospora mutants defective in single metabolic steps; established that genes encode enzymes; 1958 Nobel Prize; later refined to “one gene, one polypeptide.”
• Robert Koch — formulated Koch’s postulates (1884), the criteria for establishing that a specific microorganism causes a specific disease; identified the causative agents of tuberculosis (Mycobacterium tuberculosis, 1882) and cholera; 1905 Nobel Prize in Physiology or Medicine.
• Robert Hooke — observed cells in cork and coined the term “cell” (1665).
• Antonie van Leeuwenhoek — first to observe living microorganisms (“animalcules”) using a microscope.
• Louis Pasteur — disproved spontaneous generation; germ theory of disease; pasteurization.
• Ernst Haeckel — coined “ecology”; also proposed (incorrectly) ontogeny recapitulates phylogeny.
• Carl Woese — microbiologist who used 16S rRNA sequencing (1977) to establish the three-domain system; forced recognition of Archaea as a separate domain from Bacteria; considered one of the most important contributions to microbiology of the 20th century.
• E.O. Wilson — Harvard biologist; founded sociobiology (the study of the biological basis of social behavior); co-developed island biogeography theory with MacArthur; wrote Sociobiology (1975) and On Human Nature (1978, Pulitzer); also a leading figure in biodiversity conservation; coined “biophilia.”
• Richard Dawkins — Oxford evolutionary biologist; popularized the gene-centric view of evolution in The Selfish Gene (1976); introduced the concept of the “meme”; wrote The Extended Phenotype (1982) and The Blind Watchmaker (1986).
• Niles Eldredge and Stephen Jay Gould — jointly proposed punctuated equilibrium (1972) in their paper “Punctuated Equilibria: An Alternative to Phyletic Gradualism”; Gould also developed the concept of spandrels (with Lewontin) to critique pure adaptationism.
• Robert MacArthur and E.O. Wilson — formulated the theory of island biogeography in The Theory of Island Biogeography (1967); equilibrium model balances immigration and extinction rates; island size and isolation predict species richness.
• Peter Mitchell — British biochemist; proposed the chemiosmotic hypothesis (1961) to explain how the proton gradient across the inner mitochondrial membrane drives ATP synthesis; won the 1978 Nobel Prize in Chemistry; initially met with resistance before becoming the accepted mechanism.

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.
• 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.
• 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).
• 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.
• 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.

Additional Molecular Biology

• 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.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.

Additional Genetics and Genomics

• 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.

Additional Model Organisms

• 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.
• 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.
• 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.

Additional Evolution Concepts

• 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.
• 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).

Additional Immunology

• Toll-like receptors (TLRs) — pattern-recognition receptors of the innate immune system; recognize conserved pathogen-associated molecular patterns (PAMPs; e.g., LPS, flagellin, dsRNA); discovery by Jules Hoffmann, Bruce Beutler, and Ralph Steinman won the 2011 Nobel Prize; TLR signaling activates NF-κB and interferon pathways.
• Complement system — innate immune cascade of ~30 serum proteins that opsonize pathogens, recruit phagocytes, and lyse bacteria via the membrane attack complex (MAC); three pathways: classical (antibody-triggered), lectin, and alternative; converge at C3 cleavage.
• Adaptive immunity: clonal selection — antigen-specific lymphocytes are selected and clonally expanded; B cells mature in bone marrow; T cells mature in the thymus (positive selection: must recognize self-MHC; negative selection: must not react to self-antigen); naive B cells activated by antigen + T-cell help → plasma cells (antibody secretion) + memory B cells.
• Vaccines — confer immunological memory without disease; killed/inactivated, live-attenuated, subunit, mRNA (COVID-19 mRNA vaccines by Katalin Karikó and Drew Weissman, 2023 Nobel Prize), and viral-vector types; herd immunity threshold depends on R₀.

Additional Ecology Concepts

• Trophic cascade — indirect effects of predators on lower trophic levels; classic example: wolf reintroduction in Yellowstone reduced elk grazing, allowing riparian vegetation to recover (a trophic cascade altering the physical landscape); sea otter–urchin–kelp is another canonical example.
• Eutrophication — nutrient enrichment (especially nitrogen and phosphorus) of aquatic ecosystems; triggers algal blooms; decomposition of dead algae depletes oxygen (hypoxia/dead zones); common in coastal waters receiving agricultural runoff.
• Ecological footprint — a measure of human demand on the biosphere; the area of productive land and water needed to produce the resources consumed and absorb the waste generated; developed by Rees and Wackernagel in the 1990s.
• Metapopulation — a network of semi-isolated local populations connected by dispersal; a local population may go extinct but the patch can be recolonized; Levins metapopulation model; relevant to conservation biology and fragmented habitats (verify: Levins 1969).

Additional Physiology

• Sliding-filament model of muscle contraction — Huxley and Hanson (1954) proposed that actin (thin) and myosin (thick) filaments slide past each other without changing length; myosin heads (cross-bridges) bind actin, undergo a power stroke using ATP hydrolysis, and release; Ca²⁺ released from the sarcoplasmic reticulum exposes actin-binding sites (by moving tropomyosin via troponin).
• Resting membrane potential — ~−70 mV in neurons; results from the differential distribution of ions (high K⁺ inside, high Na⁺ and Cl⁻ outside) maintained by the Na⁺/K⁺-ATPase and selective membrane permeability primarily to K⁺; the Goldman equation describes the combined contribution of all permeable ions.
• Renin-angiotensin-aldosterone system (RAAS) — kidney releases renin (in response to low blood pressure or low Na⁺) → cleaves angiotensinogen to angiotensin I → ACE converts it to angiotensin II → constricts arterioles (raises blood pressure) + stimulates aldosterone release (increases Na⁺ reabsorption in nephron) → raises blood pressure and volume; target of ACE inhibitors and ARBs.
• Hormone classes — peptide/protein hormones (insulin, glucagon, GH; cannot cross membrane; bind surface receptors; fast, use second messengers); steroid hormones (cortisol, estrogen, testosterone; derived from cholesterol; lipid-soluble; cross membrane; bind nuclear receptors; slower, alter gene expression); thyroid hormones (T3/T4) are iodinated tyrosine derivatives that cross membrane and act like steroids.