- Central dogma
- The flow of genetic information in cells: DNA → RNA → protein, via transcription and translation.
- Hydrogen bond
- A weak attraction between a slightly positive hydrogen and a slightly negative atom (like O or N). Holds water molecules and DNA strands together.
- Polarity of water
- Water's bent shape gives it a partially negative oxygen and partially positive hydrogens, making it polar and able to form hydrogen bonds.
- High specific heat of water
- Water resists temperature change because hydrogen bonds absorb heat; this buffers organisms and oceans against swings.
- Why ice floats
- In solid water, hydrogen bonds lock molecules into a spacious lattice that is less dense than liquid water, so ice floats and insulates the water below.
- Hydrophilic
- Water-loving — polar or charged molecules that dissolve readily in water.
- Hydrophobic
- Water-fearing — nonpolar molecules (like lipids) that do not dissolve in water and cluster away from it.
- Dehydration synthesis
- A reaction joining two monomers into a polymer by removing one molecule of water; also called condensation.
- Hydrolysis
- A reaction breaking a polymer into monomers by adding one molecule of water across the bond — the reverse of dehydration synthesis.
- Carbohydrate
- A macromolecule of monosaccharide monomers; provides quick energy (glucose) and structure (cellulose, starch, glycogen).
- Monosaccharide
- A single sugar unit, such as glucose or fructose; the monomer of carbohydrates.
- Lipid
- A nonpolar, hydrophobic macromolecule (fats, phospholipids, steroids) used for energy storage, membranes, and signaling. Not a true polymer.
- Saturated vs unsaturated fat
- Saturated fats have no carbon–carbon double bonds (solid at room temp); unsaturated fats have one or more double bonds with kinks (liquid).
- Phospholipid
- A lipid with a hydrophilic phosphate head and two hydrophobic fatty-acid tails; forms the cell membrane bilayer.
- Protein
- A macromolecule of amino acid monomers that serves as enzymes, structure, transport, and signals; its shape determines its function.
- Amino acid
- The monomer of proteins; has an amino group, a carboxyl group, and a variable R group that sets its properties.
- Peptide bond
- The covalent bond linking amino acids in a protein, formed by dehydration synthesis.
- Levels of protein structure
- Primary (sequence) → secondary (alpha helices and beta sheets) → tertiary (3D fold) → quaternary (multiple subunits).
- Denaturation
- Loss of a protein's functional shape (and activity) when heat or pH disrupts its weak bonds — its sequence is unchanged.
- Nucleic acid
- A macromolecule of nucleotide monomers that stores and transmits genetic information — DNA and RNA.
- Nucleotide
- The monomer of nucleic acids: a phosphate group, a five-carbon sugar, and a nitrogenous base.
- DNA base pairing
- Adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G) by hydrogen bonds.
- Antiparallel strands
- The two strands of a DNA double helix run in opposite directions — one 5' to 3', the other 3' to 5'.
- DNA vs RNA
- DNA is double-stranded with deoxyribose and thymine; RNA is single-stranded with ribose and uracil (replacing thymine).
- Directionality (5' and 3')
- Nucleic-acid strands have a 5' phosphate end and a 3' hydroxyl end; new nucleotides are added only to the 3' end.
- pH
- A measure of hydrogen-ion concentration; below 7 is acidic, 7 is neutral, above 7 is basic. Enzymes work best near a specific pH.
- Polymer
- A large molecule built from repeating monomer subunits joined by covalent bonds.
- Cell theory
- All living things are made of cells, the cell is the basic unit of life, and all cells come from preexisting cells.
- Prokaryotic cell
- A small cell with no membrane-bound nucleus or organelles; DNA is a single circular molecule in the cytoplasm (bacteria and archaea).
- Eukaryotic cell
- A cell with a membrane-bound nucleus and organelles (plants, animals, fungi, protists).
- Phospholipid bilayer
- The two-layer sheet of phospholipids forming the cell membrane: hydrophilic heads face the water, hydrophobic tails face inward.
- Fluid mosaic model
- The membrane is a fluid bilayer with embedded proteins, cholesterol, and carbohydrates that move laterally.
- Selective permeability
- The membrane lets small nonpolar molecules cross freely while controlling or blocking ions and large polar molecules.
- Simple diffusion
- Movement of small nonpolar molecules across a membrane down their concentration gradient, requiring no energy.
- Facilitated diffusion
- Passive movement of substances down their gradient through channel or carrier proteins; no energy needed.
- Osmosis
- The diffusion of water across a selectively permeable membrane toward the side with more solute.
- Active transport
- Movement of a substance against its concentration gradient using energy (ATP), e.g., the sodium-potassium pump.
- Hypertonic solution
- A solution with more solute than the cell; water leaves the cell, which shrinks (crenates).
- Hypotonic solution
- A solution with less solute than the cell; water enters the cell, which swells and may burst (lyse).
- Isotonic solution
- A solution with equal solute concentration to the cell; no net movement of water.
- Nucleus
- The organelle that stores the cell's DNA and controls gene expression; surrounded by a double-membrane nuclear envelope.
- Ribosome
- The site of protein synthesis (translation); free in the cytoplasm or bound to the rough ER. Made of rRNA and protein.
- Rough endoplasmic reticulum
- ER studded with ribosomes that synthesizes and folds proteins destined for membranes or secretion.
- Smooth endoplasmic reticulum
- ER without ribosomes that synthesizes lipids and detoxifies drugs and poisons (abundant in liver cells).
- Golgi apparatus
- The organelle that modifies, sorts, packages, and ships proteins and lipids in vesicles.
- Mitochondrion
- The site of aerobic cellular respiration and most ATP production; has a double membrane and its own DNA.
- Chloroplast
- The site of photosynthesis in plants and algae; contains chlorophyll, thylakoids, and stroma, and its own DNA.
- Lysosome
- A membrane-bound sac of digestive enzymes that breaks down waste, debris, and worn-out organelles at acidic pH.
- Vacuole
- A storage organelle; the large central vacuole of plant cells maintains turgor pressure and stores water and ions.
- Cell wall
- A rigid layer outside the membrane (cellulose in plants) that provides support and protection.
- Endosymbiotic theory
- Mitochondria and chloroplasts arose from free-living prokaryotes engulfed by a host cell; evidence: own DNA, ribosomes, double membranes.
- Surface-area-to-volume ratio
- As a cell grows, volume rises faster than surface area; a high ratio (small cells) speeds exchange across the membrane.
- Cholesterol in membranes
- Wedged between phospholipids, cholesterol buffers membrane fluidity — keeping it fluid in cold and stable in heat.
- Enzyme
- A biological catalyst (usually a protein) that speeds a reaction by lowering its activation energy without being consumed.
- Activation energy
- The energy barrier that must be overcome for a reaction to start; enzymes lower it.
- Active site
- The region of an enzyme that binds the substrate; its shape gives the enzyme its specificity.
- Substrate
- The reactant molecule an enzyme acts on, binding at the active site.
- Induced fit
- The active site changes shape slightly to grip the substrate more tightly once it binds.
- Competitive inhibition
- An inhibitor binds the active site and competes with the substrate; can be overcome by adding more substrate.
- Noncompetitive (allosteric) inhibition
- An inhibitor binds away from the active site, changing the enzyme's shape so it works less well; not reversed by adding substrate.
- Effect of temperature and pH on enzymes
- Each enzyme has an optimal temperature and pH; beyond it, activity drops and the enzyme can denature.
- ATP
- Adenosine triphosphate, the cell's energy currency; energy is released when its terminal phosphate bond is broken to form ADP.
- Photosynthesis equation
- 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. Converts light energy into chemical energy stored in sugar.
- Light-dependent reactions
- Occur in the thylakoid membranes; split water (releasing O₂) and produce ATP and NADPH using light energy.
- Calvin cycle
- The light-independent reactions in the stroma that use ATP and NADPH to fix CO₂ into sugar.
- Source of oxygen in photosynthesis
- The O₂ released comes from splitting water (H₂O) in the light reactions, not from CO₂.
- Chlorophyll
- The green pigment in chloroplasts that absorbs light (mainly red and blue) to drive the light reactions.
- Cellular respiration equation
- C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP. Releases the energy stored in glucose.
- Glycolysis
- The first stage of respiration, in the cytoplasm: splits glucose into two pyruvate with a small net gain of ATP and NADH. No oxygen needed.
- Krebs cycle (citric acid cycle)
- Occurs in the mitochondrial matrix; oxidizes pyruvate, releases CO₂, and loads NADH and FADH₂.
- Electron transport chain
- On the inner mitochondrial membrane; electrons drive proton pumping, and chemiosmosis powers ATP synthase. Oxygen is the final electron acceptor.
- Final electron acceptor
- In aerobic respiration, oxygen accepts electrons at the end of the chain, forming water.
- Chemiosmosis
- Using a proton (H⁺) gradient across a membrane to drive ATP synthase and make ATP — in both respiration and photosynthesis.
- ATP synthase
- The enzyme that makes ATP as protons flow through it down their gradient (chemiosmosis).
- Fermentation
- Anaerobic pathway that regenerates NAD⁺ so glycolysis can continue without oxygen; yields little ATP (lactic acid or alcohol).
- Aerobic vs anaerobic
- Aerobic respiration uses oxygen and yields much ATP; anaerobic processes (fermentation) do not use oxygen and yield little.
- NADH and FADH₂
- Electron carriers that shuttle high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain.
- Pyruvate
- The three-carbon product of glycolysis; enters the mitochondrion for the Krebs cycle when oxygen is present.
- Cell signaling stages
- Reception (signal binds receptor) → transduction (signal relayed and amplified inside) → response (change in cell activity).
- Ligand
- A signaling molecule that binds a specific receptor to trigger a cellular response.
- Receptor
- A protein that binds a specific signal molecule; only cells with the right receptor respond to a given signal.
- Signal transduction
- The relay of a signal inside a cell after a ligand binds, often through a cascade that amplifies the message.
- Second messenger
- A small intracellular molecule (such as cyclic AMP or calcium ions) that spreads a signal rapidly through the cytoplasm.
- Phosphorylation cascade
- A chain of protein kinases that activate one another by transferring phosphate groups, amplifying a signal.
- Intracellular vs membrane receptor
- Hydrophobic signals (steroids) cross the membrane to intracellular receptors; hydrophilic signals bind surface receptors.
- Endocrine signaling
- Hormones secreted into the bloodstream act on distant target cells that carry the matching receptor.
- Negative feedback
- A loop that counteracts a change to restore a set point and maintain homeostasis (cooling when overheated).
- Positive feedback
- A loop that amplifies a change to drive a process to completion (blood clotting, childbirth contractions).
- Homeostasis
- The maintenance of a stable internal environment despite external change, mostly by negative feedback.
- Apoptosis
- Programmed cell death — a controlled, signal-driven self-destruction used in development and to remove damaged cells.
- Cell cycle
- The ordered sequence a cell follows to divide: interphase (G₁, S, G₂) followed by mitosis and cytokinesis.
- Interphase
- The longest phase of the cell cycle: the cell grows (G₁), copies its DNA (S), and prepares to divide (G₂).
- Mitosis
- Nuclear division producing two genetically identical diploid daughter cells, used for growth and repair.
- Phases of mitosis
- Prophase → metaphase → anaphase → telophase, followed by cytokinesis.
- Cytokinesis
- The division of the cytoplasm that splits one cell into two after mitosis.
- Cell-cycle checkpoints
- Control points (G₁, G₂, M) that verify each stage is complete before the cell proceeds, driven by cyclins and Cdks.
- Cancer
- Uncontrolled cell division caused by mutations that disable cell-cycle checkpoints and apoptosis.
- Sister chromatids
- The two identical copies of a chromosome joined at the centromere after DNA replication, separated in anaphase.
- G protein-coupled receptor
- A common membrane receptor that activates a G protein when a signal binds, triggering second messengers like cyclic AMP.
- Mitosis vs meiosis
- Mitosis makes 2 identical diploid cells (growth/repair); meiosis makes 4 genetically varied haploid gametes (reproduction).
- Diploid number restoration
- Fertilization fuses two haploid gametes (n + n) to restore the diploid (2n) chromosome number in the zygote.
- Meiosis
- Cell division producing four haploid gametes, halving the chromosome number and generating genetic variation.
- Diploid vs haploid
- Diploid (2n) cells have two sets of chromosomes; haploid (n) gametes have one set.
- Homologous chromosomes
- A matched pair of chromosomes (one from each parent) carrying the same genes, that pair up in meiosis I.
- Crossing over
- The exchange of segments between homologous chromosomes in prophase I, creating new allele combinations.
- Independent assortment
- Homologous pairs line up and separate randomly in meiosis I, mixing alleles of different genes into gametes.
- Meiosis I vs meiosis II
- Meiosis I separates homologous chromosomes (reduction to haploid); meiosis II separates sister chromatids.
- Gamete
- A haploid reproductive cell (sperm or egg) that fuses with another in fertilization to form a diploid zygote.
- Nondisjunction
- The failure of chromosomes to separate properly in meiosis, producing gametes with extra or missing chromosomes.
- Allele
- One of the alternative versions of a gene that can occupy a given locus.
- Genotype vs phenotype
- Genotype is the genetic makeup (alleles); phenotype is the observable trait that results.
- Homozygous vs heterozygous
- Homozygous = two identical alleles (TT or tt); heterozygous = two different alleles (Tt).
- Dominant vs recessive
- A dominant allele is expressed in the heterozygote; a recessive allele is masked unless homozygous.
- Law of segregation
- Mendel's principle that the two alleles of a gene separate during gamete formation, one per gamete.
- Law of independent assortment
- Mendel's principle that alleles of different genes (on different chromosomes) are inherited independently.
- Punnett square
- A grid that predicts offspring genotypes and phenotypes from the parents' alleles.
- Monohybrid cross ratio
- Crossing two heterozygotes (Tt × Tt) gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio.
- Dihybrid cross ratio
- Crossing two double heterozygotes (RrYy × RrYy) gives a 9:3:3:1 phenotype ratio.
- Test cross
- Crossing an individual of unknown genotype with a homozygous recessive to reveal whether it is homozygous or heterozygous.
- Incomplete dominance
- The heterozygote shows a blended, intermediate phenotype (red × white snapdragons → pink).
- Codominance
- Both alleles are fully and separately expressed in the heterozygote (AB blood type shows both A and B antigens).
- Multiple alleles
- A gene with more than two possible alleles in the population, such as the ABO blood group (Iᴬ, Iᴮ, i).
- Polygenic trait
- A trait controlled by many genes, producing continuous variation (skin color, height).
- Sex-linked trait
- A trait whose gene is on a sex chromosome (usually X); X-linked recessive traits appear more often in males.
- Why X-linked recessive traits favor males
- Males have one X chromosome, so a single recessive allele is enough to show the trait (e.g., color blindness).
- Pedigree
- A chart tracking a trait through generations of a family, used to deduce inheritance patterns.
- Semiconservative replication
- DNA replication in which each new molecule keeps one original (template) strand and one newly made strand.
- DNA polymerase
- The enzyme that builds a new DNA strand by adding nucleotides to the 3' end of the growing strand.
- Helicase
- The enzyme that unwinds and separates the two DNA strands at the replication fork.
- Okazaki fragments
- Short DNA segments synthesized on the lagging strand and later joined by DNA ligase.
- Transcription
- The synthesis of messenger RNA from a DNA template by RNA polymerase, the first step of gene expression.
- RNA polymerase
- The enzyme that builds an mRNA strand from a DNA template during transcription.
- Translation
- The assembly of a protein at the ribosome, reading mRNA codons and adding the matching amino acids via tRNA.
- Codon
- A sequence of three mRNA nucleotides that specifies one amino acid or a start/stop signal.
- Anticodon
- The three-base sequence on a tRNA that pairs with a complementary mRNA codon during translation.
- mRNA, tRNA, rRNA
- Messenger RNA carries the code; transfer RNA brings amino acids; ribosomal RNA forms part of the ribosome.
- Start and stop codons
- AUG (methionine) starts translation; UAA, UAG, and UGA signal it to stop.
- RNA processing (introns/exons)
- In eukaryotes, a primary transcript has introns removed (splicing) and exons joined before leaving the nucleus.
- Operon
- A cluster of bacterial genes controlled together by one promoter and operator, such as the lac operon.
- lac operon
- An inducible operon in E. coli; lactose (allolactose) inactivates the repressor, turning on lactose-digesting genes.
- Promoter
- The DNA sequence where RNA polymerase binds to begin transcription of a gene.
- Transcription factor
- A protein that binds DNA to turn the transcription of specific genes up or down in eukaryotes.
- Epigenetics
- Heritable changes in gene expression without changes to the DNA sequence — DNA methylation and histone modification.
- DNA methylation
- Adding methyl groups to DNA, which typically silences gene expression by making the gene less accessible.
- Cell differentiation
- How cells with identical DNA become specialized by expressing different subsets of genes.
- Point mutation
- A change in a single DNA base; may be silent, missense (changes an amino acid), or nonsense (creates a stop).
- Frameshift mutation
- An insertion or deletion that shifts the reading frame, usually altering every codon downstream.
- Mutations as variation
- Mutations are the ultimate source of new alleles and genetic variation on which selection acts.
- PCR
- Polymerase chain reaction — a technique that amplifies (copies) a target DNA sequence through repeated heating and cooling.
- Genetic code is universal
- Nearly all organisms use the same codons for the same amino acids — strong evidence of common ancestry.
- Natural selection
- The process by which individuals with heritable advantageous traits survive and reproduce more, shifting allele frequencies.
- Fitness
- An individual's reproductive success — how many offspring it leaves relative to others in the population.
- Adaptation
- A heritable trait that improves survival or reproduction in a given environment, shaped by natural selection.
- Evolution
- A change in the heritable allele frequencies of a population over generations (descent with modification).
- Directional selection
- Selection favoring one extreme phenotype, shifting the population's trait distribution toward it.
- Stabilizing selection
- Selection favoring the intermediate phenotype and against extremes (e.g., human birth weight).
- Disruptive selection
- Selection favoring both extremes over the intermediate, which can split a population.
- Sexual selection
- Selection driven by differences in mating success, producing traits like elaborate plumage or large antlers.
- Genetic drift
- A random change in allele frequencies due to chance, with the greatest effect in small populations.
- Bottleneck effect
- A form of genetic drift in which a population is sharply reduced by a chance event, narrowing its gene pool.
- Founder effect
- A form of genetic drift in which a few individuals start a new population, carrying only a subset of the alleles.
- Gene flow
- The movement of alleles between populations through migration; tends to make populations more similar.
- Hardy-Weinberg equilibrium
- A non-evolving state where p + q = 1 and p² + 2pq + q² = 1, given no mutation, no selection, random mating, no gene flow, and a large population.
- Hardy-Weinberg: p and q
- p is the frequency of the dominant allele and q the recessive; p² is homozygous dominant, 2pq heterozygous, q² homozygous recessive.
- Five Hardy-Weinberg conditions
- No mutation, no natural selection, random mating, no gene flow, and a large population.
- Speciation
- The formation of new species, usually when populations become reproductively isolated and diverge genetically.
- Reproductive isolation
- Barriers (prezygotic or postzygotic) that prevent populations from interbreeding, driving speciation.
- Homologous structures
- Structures shared by related species due to common ancestry (the forelimb bones of mammals); evidence for evolution.
- Phylogenetic tree
- A branching diagram showing evolutionary relationships and common ancestry among species.
- Cladogram
- A diagram grouping organisms by shared derived characters to show their branching evolutionary relationships.
- Molecular evidence for evolution
- Similarities in DNA and protein sequences across species reveal shared ancestry; more similarity means closer relationship.
- Convergent evolution
- Unrelated species independently evolve similar traits because they face similar environments (analogous structures).
- Common ancestry
- The principle that all life descends from a shared ancestor, supported by the universal genetic code and shared biochemistry.
- Antibiotic resistance
- An example of natural selection: resistant bacteria survive antibiotics and reproduce, so resistance spreads.
- Heritable variation
- Inherited differences among individuals — the raw material natural selection acts on; without it, no evolution.
- Producer (autotroph)
- An organism that makes its own food, usually by photosynthesis; the base of the food chain (plants, algae).
- Consumer (heterotroph)
- An organism that gets energy by eating other organisms — primary (herbivores), secondary, and tertiary consumers.
- Decomposer
- An organism (fungi, bacteria) that breaks down dead matter and waste, returning nutrients to the environment.
- Trophic level
- A feeding position in a food chain — producers, primary consumers, secondary consumers, and so on.
- 10% rule
- Only about 10% of the energy at one trophic level is passed to the next; the rest is lost as heat and metabolism.
- Food web
- A network of interconnected food chains showing the many feeding relationships in an ecosystem.
- Energy flow vs matter cycling
- Energy flows one way through an ecosystem and is lost as heat; matter (carbon, nitrogen) is recycled repeatedly.
- Carbon cycle
- Carbon moves from the atmosphere into organisms by photosynthesis and back out by respiration and combustion.
- Nitrogen fixation
- The conversion of atmospheric nitrogen gas into usable compounds (ammonia) by bacteria, including those in legume root nodules.
- Nitrogen cycle
- The movement of nitrogen through fixation, nitrification, assimilation, and denitrification among air, soil, and organisms.
- Exponential growth
- Accelerating population growth with unlimited resources, producing a J-shaped curve.
- Logistic growth
- Population growth that slows as it nears the carrying capacity, producing an S-shaped curve.
- Carrying capacity (K)
- The maximum population size an environment can sustain given its resources; logistic growth levels off here.
- Density-dependent factors
- Limits that intensify as a population grows — competition, predation, and disease.
- Density-independent factors
- Limits that affect a population regardless of its size — weather, natural disasters, fire.
- Mutualism
- A symbiosis in which both species benefit (+/+), such as bees pollinating flowers.
- Commensalism
- A symbiosis in which one species benefits and the other is unaffected (+/0).
- Parasitism
- A symbiosis in which one species benefits at the expense of the host (+/−), such as a tapeworm.
- Competition
- An interaction in which organisms vie for the same limited resource, harming both (−/−).
- Predation
- An interaction in which one organism (predator) kills and eats another (prey).
- Ecological succession
- The gradual, predictable change in a community over time, from a disturbance toward a stable climax community.
- Biodiversity
- The variety of life in an ecosystem; greater biodiversity generally increases stability and resilience.
- Population
- All the individuals of one species living in the same area and interbreeding.
- Community
- All the populations of different species living and interacting in the same area.
- Ecosystem
- A community of organisms together with the nonliving (abiotic) factors of their environment.
