GACE® Biology (6–12) (710)
Practice Test

Chemistry of biological molecules and macromolecules: major elements composing living things — CHNOPS (Carbon: backbone of all organic molecules; Hydrogen: found in all organic molecules and water; Nitrogen: in amino acids, nucleic acids, ATP; Oxygen: in most organic molecules, water, and cellular respiration; Phosphorus: in nucleic acids, ATP, and phospholipids; Sulfur: in some amino acids like cysteine and methionine); the physical and chemical properties of water that make it essential to life: polarity (unequal sharing of electrons between O and H atoms creates a partial negative charge on O and partial positive charges on H; this makes water a polar molecule); hydrogen bonding (the attraction between the partial negative charge on one water molecule’s oxygen and the partial positive charge on a neighboring molecule’s hydrogen; responsible for water’s unique properties); cohesion (water’s tendency to stick to itself — enables water to be pulled up tall trees through xylem without breaking the water column); adhesion (water’s tendency to stick to other polar surfaces — enables capillary action); high specific heat capacity (water absorbs and releases large amounts of heat before changing temperature — stabilizes body temperature and climate); high heat of vaporization (evaporation of water absorbs large amounts of heat — enables sweating for cooling); density of ice less than liquid water (ice floats — allows aquatic ecosystems to survive winter); biological macromolecules, their monomers, and functions: carbohydrates (monomers: monosaccharides such as glucose, fructose, galactose; polymers: disaccharides like sucrose and lactose; polysaccharides like starch, glycogen, cellulose, and chitin; functions: primary energy source, structural components in cell walls and exoskeletons); proteins (monomers: amino acids — 20 types, each with a central carbon, amino group, carboxyl group, and variable R group; polymers: polypeptides and proteins; peptide bonds link amino acids; four levels of structure: primary = amino acid sequence; secondary = alpha helices and beta pleated sheets from hydrogen bonding; tertiary = 3D folding from R-group interactions; quaternary = two or more polypeptide chains; functions: enzymes [catalysts], structural support [collagen, keratin], transport [hemoglobin], signaling [hormones like insulin], defense [antibodies]); lipids (not polymers; triglycerides = 3 fatty acids + glycerol; saturated fats: straight chains, solid at room temperature; unsaturated fats: double bonds, kinks in chain, liquid at room temperature; phospholipids: two fatty acid tails + phosphate head group; amphipathic — spontaneously form bilayers in water; steroids: ring structure; cholesterol is the backbone of steroid hormones; functions: long-term energy storage, membrane structure, insulation, hormones); nucleic acids (monomers: nucleotides, each with a sugar, phosphate group, and nitrogenous base; DNA: deoxyribose sugar; double-stranded; bases A, T, G, C; A-T base pairs have 2 hydrogen bonds; G-C base pairs have 3 hydrogen bonds; RNA: ribose sugar; single-stranded; bases A, U, G, C; types: mRNA [messenger], tRNA [transfer], rRNA [ribosomal]).

DNA structure, replication, and protein synthesis: DNA double helix structure (Watson and Crick, 1953; anti-parallel strands running 5′ to 3′ and 3′ to 5′; bases on the inside held together by hydrogen bonds; sugar-phosphate backbone on the outside); DNA replication is semi-conservative: each new DNA molecule has one original strand and one newly synthesized strand; key enzymes: helicase (unwinds the double helix at the replication fork), primase (synthesizes a short RNA primer to start replication), DNA polymerase III (adds new nucleotides in the 5′ to 3′ direction; has proofreading ability), DNA polymerase I (removes RNA primers and replaces with DNA), DNA ligase (seals the gaps between Okazaki fragments on the lagging strand); leading strand (synthesized continuously in the direction of the replication fork); lagging strand (synthesized discontinuously as Okazaki fragments running away from the fork); protein synthesis: transcription (in the nucleus: RNA polymerase unwinds DNA and synthesizes mRNA in the 5′ to 3′ direction; the template strand is read 3′ to 5′; mRNA is processed: 5′ cap and poly-A tail added; introns spliced out; exons joined by spliceosomes) and translation (at ribosomes in the cytoplasm: mRNA is read in codons of 3 bases; tRNA brings the corresponding amino acid; start codon = AUG [codes for methionine]; stop codons = UAA, UAG, UGA; the genetic code is degenerate [multiple codons for most amino acids] and universal [same in virtually all organisms]).

Cell structure, membrane, transport, and cell theory: cell theory: (1) all living things are composed of one or more cells; (2) the cell is the basic structural and functional unit of life; (3) all cells arise from pre-existing cells (Virchow, 1855); prokaryotic cells (bacteria and archaea): no membrane-bound nucleus; circular DNA in the nucleoid region; 70S ribosomes; cell wall (peptidoglycan in bacteria); may have flagella, pili, capsule; binary fission for reproduction; eukaryotic cells (plants, animals, fungi, protists): membrane-bound nucleus containing linear DNA; 80S ribosomes; membrane-bound organelles; cell membrane structure (fluid mosaic model: phospholipid bilayer with embedded proteins; phospholipid heads face outward [hydrophilic]; tails face inward [hydrophobic]; integral membrane proteins span the bilayer; peripheral membrane proteins attach to the surface; cholesterol provides fluidity at low temperatures and stability at high temperatures; glycoproteins and glycolipids on the outer surface serve as cell-identity markers); key organelles and their functions: nucleus (contains DNA; site of DNA replication and transcription; nucleolus produces rRNA), mitochondria (site of aerobic cellular respiration; double membrane; ATP synthesis via oxidative phosphorylation; own DNA; arose by endosymbiosis), chloroplasts (site of photosynthesis; in plant and algal cells; double membrane; contain thylakoids and stroma; own DNA; arose by endosymbiosis), endoplasmic reticulum (rough ER: studded with ribosomes; site of protein synthesis and folding for secretion; smooth ER: no ribosomes; site of lipid synthesis and detoxification), Golgi apparatus (modifies, packages, and ships proteins and lipids to their destinations; creates vesicles), lysosomes (in animal cells; contain hydrolytic enzymes; break down waste, damaged organelles, and foreign material; work at pH ~4.8), ribosomes (site of protein synthesis; made of rRNA and protein; 70S in prokaryotes; 80S in eukaryotes; polyribosomes for efficient translation), cell wall (cellulose in plants; chitin in fungi; rigid; provides structural support and protection), central vacuole (in plant cells; stores water, maintains turgor pressure, stores pigments and waste products), cytoskeleton (microfilaments of actin, intermediate filaments, microtubules; provide structure, enable cell movement, cell division, and organelle positioning); active and passive transport: passive transport moves molecules DOWN their concentration gradient (no ATP required): simple diffusion (O&sub2;, CO&sub2;, lipid-soluble molecules pass directly through the phospholipid bilayer), facilitated diffusion (polar molecules and ions move through channel or carrier proteins), osmosis (water moves through aquaporins from low solute concentration [hypotonic] to high solute concentration [hypertonic]); active transport moves molecules AGAINST their concentration gradient (requires ATP): protein pumps (Na⁺/K⁺ pump; the proton pump), endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis — membrane surrounds material and brings it in), exocytosis (vesicles fuse with membrane to release contents).

Photosynthesis, cellular respiration, thermodynamics, and reproduction: photosynthesis (6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂): light-dependent reactions in the thylakoid membranes (absorb light energy through Photosystem II and I; split water molecules releasing O₂ and H⁺; energize electrons to produce NADPH; use chemiosmosis and ATP synthase to produce ATP; create a proton gradient); Calvin cycle/light-independent reactions in the stroma (use ATP and NADPH from light reactions to fix CO₂ into organic molecules; RuBisCO is the key enzyme; produce G3P; regenerate RuBP; 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P); cellular respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36–38 ATP): glycolysis (in cytoplasm; glucose [6C] → 2 pyruvate [3C]; net 2 ATP; 2 NADH; does not require O₂), pyruvate oxidation (pyruvate enters mitochondrial matrix; becomes acetyl-CoA; releases 1 CO₂; produces 1 NADH per pyruvate), Krebs/citric acid cycle (in mitochondrial matrix; 2 turns per glucose; releases 2 CO₂ per turn; produces 2 ATP total, 3 NADH and 1 FADH₂ per turn), oxidative phosphorylation/electron transport chain and chemiosmosis (in inner mitochondrial membrane; NADH and FADH₂ donate electrons; O₂ is the final electron acceptor forming water; the electron transport chain pumps H⁺ into intermembrane space; protons flow back through ATP synthase [chemiosmosis] producing ~32–34 ATP); laws of thermodynamics: first law (energy cannot be created or destroyed, only transformed — photosynthesis converts light energy to chemical energy; cellular respiration converts chemical energy to ATP and heat), second law (entropy increases in isolated systems; energy conversions are never 100% efficient — explains why energy is lost as heat at each trophic level); mitosis produces 2 genetically identical diploid daughter cells from one parent cell (IPMAT: Interphase, Prophase, Metaphase, Anaphase, Telophase + Cytokinesis); cell cycle checkpoints (G1: cell size and nutrient check; G2: DNA replication check; spindle assembly checkpoint: all chromosomes attached to spindle); cancer = uncontrolled cell cycle; meiosis produces 4 genetically unique haploid gametes; meiosis I separates homologous chromosomes (crossing over in Prophase I is the source of genetic recombination between non-sister chromatids of homologous chromosomes; increases genetic diversity); meiosis II separates sister chromatids; types of reproduction: sexual (meiosis + fertilization; genetic variation through crossing over, independent assortment, random fertilization; zygote is diploid), asexual (binary fission in prokaryotes; budding; fragmentation and regeneration; vegetative propagation; parthenogenesis; offspring are genetically identical clones).

Ecological concepts, biomes, and population dynamics: key ecological terms and levels of organization: individual (one organism), population (all individuals of the same species in an area), community (all populations of different species in an area), ecosystem (the community plus its abiotic environment), biome (a large geographic area characterized by a particular climate and plant community), biosphere (all of Earth’s living things and their environments); habitat vs. niche: habitat is where an organism lives (the physical address); niche is the organism’s functional role in its community, including everything it eats, how it reproduces, when it’s active, and its interactions (the occupation); the competitive exclusion principle (Gause, 1934): two species with identical niches cannot coexist indefinitely in the same place — one will outcompete and replace the other; species may coexist through niche differentiation (resource partitioning); types of terrestrial biomes and their defining characteristics: tropical rainforest (high temperature and rainfall year-round; highest species diversity of any biome; closed canopy; emergent layer; Panama, Congo Basin, Amazon Basin), savanna/tropical grassland (warm temperature; seasonal rainfall with a distinct dry season; scattered trees; large grazing mammals; Africa, South America, Australia), desert (low precipitation (<25 cm/year); extreme temperature fluctuations; sparse vegetation; specialized adaptations (CAM photosynthesis, nocturnal behavior); Sahara, Mojave, Gobi), temperate grassland/prairie (moderate precipitation; cold winters; hot summers; few trees; deep fertile soils; American Midwest, Argentinian Pampas), temperate deciduous forest (moderate precipitation; four distinct seasons; deciduous trees that lose leaves in fall; Eastern North America, Western Europe, East Asia), boreal forest/taiga (cold temperatures; low precipitation; coniferous trees (spruce, fir, pine); largest land biome by area; Canada, Russia, Scandinavia), tundra (extremely cold; permafrost (permanently frozen soil) preventing tree root growth; very low precipitation; Arctic and alpine; short growing season; low biodiversity); population dynamics: population growth equation dN/dt = rN (r = intrinsic growth rate; N = population size; exponential growth when resources are unlimited produces a J-shaped curve); logistic growth (S-shaped sigmoid curve): dN/dt = rN[(K-N)/K] where K = carrying capacity (the maximum population size a given environment can sustainably support); factors that regulate population size: density-dependent limiting factors (increase in effect as population density increases: predation, disease, competition for food and territory, starvation, parasitism) and density-independent limiting factors (affect populations regardless of density: floods, droughts, fires, volcanic eruptions, severe weather); r-selected species (high growth rate, many small offspring with little parental care, short generation time, pioneer species — dandelions, insects, mice) vs. K-selected species (slow growth rate, few large offspring with extensive parental care, long generation time — elephants, humans, whales).

Ecological interactions, energy flow, biogeochemical cycles, and ecosystem resilience: types of ecological interactions between species: competition (both species harmed: −/−; may lead to competitive exclusion or niche differentiation; intraspecific: same species; interspecific: different species), predation (+/−; predator benefits, prey harmed; predator-prey cycles: Lotka-Volterra model — lynx-hare cycle is the classic example), herbivory (+/−; like predation but of plants by animals), symbiosis: mutualism (+/+; both benefit: mycorrhizae — fungi and plant roots; nitrogen-fixing bacteria in legume root nodules; pollinator and flower), commensalism (+/0; one benefits, other unaffected: barnacles on whales; orchids growing on trees for light), parasitism (+/−; parasite benefits, host harmed: tapeworms, ticks, mistletoe); keystone species: a species that has a disproportionately large effect on ecosystem structure relative to its biomass (sea otters control sea urchin populations, which allows kelp forests to thrive; wolves in Yellowstone control elk grazing, allowing riparian vegetation to recover — trophic cascade); energy flow in ecosystems: energy enters through producers (autotrophs: photosynthesis or chemosynthesis); energy flows through trophic levels: producers (first trophic level) → primary consumers (second trophic level: herbivores) → secondary consumers (third trophic level: carnivores) → tertiary consumers (fourth trophic level) → detritivores and decomposers (break down dead organic matter, recycling nutrients); the 10% rule: approximately 10% of energy at one trophic level is available to the next (the rest is lost as heat through metabolic processes and respiration); energy pyramids (number, biomass, energy) — energy pyramids are always upright; biogeochemical cycles — how matter is cycled through biotic and abiotic reservoirs: carbon cycle (producers fix CO₂ through photosynthesis; consumers obtain carbon by eating; decomposers return carbon through decomposition; burning fossil fuels and deforestation release stored carbon, increasing atmospheric CO₂); nitrogen cycle (nitrogen fixation by bacteria such as Rhizobium converts N₂ to NH₃; nitrification converts NH₃ to NO₂− and NO₃−; assimilation: plants absorb NO₃− and incorporate it into amino acids; decomposition; denitrification returns N₂ to the atmosphere; nitrogen is the most common limiting nutrient for plant growth); water cycle (evaporation, transpiration, condensation, precipitation, runoff, infiltration, groundwater flow); phosphorus cycle (no atmospheric phase; phosphorus moves from rock (weathering) to soil to organisms through food chains and returns via decomposition; phosphorus is the most common limiting nutrient in aquatic systems; eutrophication — excess phosphorus [and nitrogen] in water bodies causes algal blooms, which deplete dissolved oxygen [hypoxic dead zones] when they decompose); ecosystem equilibrium, resistance (how well an ecosystem resists disturbance), and resilience (how quickly it recovers); succession: primary succession (begins in an area with no soil — bare rock after a glacier retreats or volcanic eruption; pioneer species are the first to colonize — typically lichens and mosses; extremely slow; ends in climax community); secondary succession (begins where a community has been destroyed but the soil remains intact — after a forest fire, flood, or abandoned farmland; much faster than primary succession).

Human impacts on climate and biodiversity; sustainability: the impact of human activities on climate change: burning fossil fuels (coal, oil, natural gas) releases CO₂ and other greenhouse gases that have been sequestered for millions of years; deforestation removes trees that absorb CO₂, releasing stored carbon and reducing the forest’s carbon sink capacity; agriculture (methane from livestock and rice paddies; nitrous oxide from nitrogen fertilizers); cement production; enhanced greenhouse effect: the additional warming caused by elevated greenhouse gas concentrations (beyond the natural greenhouse effect that keeps Earth habitable); consequences of human-driven climate change: rising global average temperatures (+1.1°C since pre-industrial times per IPCC Sixth Assessment Report 2021), melting ice caps and glaciers (sea level rise threatening low-lying coastal areas), more frequent and intense extreme weather events, ocean acidification (CO₂ dissolves in seawater to form carbonic acid, threatening marine organisms with calcium carbonate shells — coral bleaching and reef dissolution), shifting biome boundaries and phenological changes (earlier spring events), increased frequency and duration of droughts and wildfires; the impact of climate change on species and ecosystems: polar bears losing sea ice habitat; coral reefs bleaching due to thermal stress; poleward and upward elevation shifts in species ranges; disruption of food web timing (e.g., insect hatches and bird migrations no longer synchronized); increased extinction risk for already-threatened species; renewable resources (resources that regenerate naturally at a rate that meets or exceeds human use: solar energy, wind energy, hydroelectric power, geothermal energy, sustainably harvested timber and fisheries) vs. nonrenewable resources (resources that form much slower than humans consume them: fossil fuels — coal, oil, natural gas — took millions of years to form; uranium for nuclear power); types and sources of environmental pollution: air pollution (combustion of fossil fuels: particulate matter [PM2.5 and PM10]; nitrogen oxides [NOx] from vehicle exhaust → acid rain; sulfur dioxide [SO₂] from coal burning → acid rain; greenhouse gases; ozone [O₃] formed by photochemical reactions in smog; carbon monoxide [CO] from incomplete combustion; indoor air pollution: radon, formaldehyde, second-hand smoke), water pollution (agricultural runoff carrying fertilizers [eutrophication], pesticides, and sediment; industrial discharge of heavy metals [mercury, lead, cadmium] and toxic chemicals; oil spills; plastic pollution; thermal pollution from power plant cooling water), soil pollution (pesticide accumulation, heavy metal contamination from mining and industrial activity, landfill leachate), noise pollution (disrupts wildlife communication, navigation, and reproduction), light pollution (disrupts nocturnal animal behavior and human circadian rhythms); ecological consequences of human activities: habitat destruction (the leading cause of biodiversity loss worldwide — deforestation for agriculture accounts for the largest share; urbanization; wetland draining), habitat fragmentation (roads, power lines, and development divide continuous habitat into isolated patches, increasing edge effects and reducing gene flow), invasive species (non-native species introduced to new ecosystems where they lack natural predators or competitors; examples: kudzu vine in the American South; zebra mussels in the Great Lakes; cane toads in Australia; often outcompete native species), overexploitation (overfishing, poaching, commercial collection — Atlantic cod, passenger pigeon [extinct], bluefin tuna [endangered]), pollution (directly kills organisms or disrupts reproduction through endocrine disruption); sustainability: meeting the needs of the present without compromising the ability of future generations to meet their own needs (Brundtland Commission definition, 1987); sustainability strategies include reducing greenhouse gas emissions, transitioning to renewable energy, sustainable agriculture (crop rotation, integrated pest management, precision agriculture), sustainable fisheries management, marine and terrestrial protected areas, and reducing plastic waste.

Mendelian genetics, molecular genetics, and evolution: Mendel’s Law of Segregation (each organism carries two alleles for each trait; during gamete formation, the two alleles separate so each gamete receives one allele) and Law of Independent Assortment (genes for different traits assort independently of one another during gamete formation — holds when genes are on different chromosomes or far apart on the same chromosome); dominant alleles mask recessive alleles in heterozygotes; Punnett squares predict genotype and phenotype ratios; monohybrid cross (Aa × Aa: 1 AA : 2 Aa : 1 aa genotype ratio; 3 dominant : 1 recessive phenotype ratio); dihybrid cross (AaBb × AaBb: 9:3:3:1 phenotype ratio); exceptions to simple dominance: codominance (both alleles expressed simultaneously — ABO blood type; AB individuals express both A and B antigens), incomplete dominance (heterozygote shows blended phenotype — red × white = pink in four o’clocks), multiple alleles (ABO blood type has three alleles: I⊃A, I⊃B, i), polygenic traits (multiple genes affect one trait — skin color, height, weight — produce a continuous distribution), pleiotropy (one gene affects multiple traits — sickle-cell allele causes HbS in all RBCs); sex-linked traits (genes on sex chromosomes: X-linked recessive traits such as color blindness and hemophilia are more common in males [XY] because males have only one X chromosome and cannot be carriers); sources and types of mutations: gene mutations (point mutations: substitution — missense [changes amino acid], nonsense [premature stop codon], silent [no change in amino acid]; insertions and deletions causing frameshift mutations shifting the reading frame, potentially altering every subsequent amino acid); chromosomal mutations (deletion, duplication, inversion, translocation of chromosome segments; aneuploidy: incorrect chromosome number — monosomy: 2n−1; trisomy: 2n+1 — Down syndrome: trisomy 21; Turner syndrome: 45,X; Klinefelter syndrome: 47,XXY; nondisjunction during meiosis); factors that affect gene expression: epigenetics (DNA methylation silences gene expression; histone acetylation promotes transcription; imprinting; X-chromosome inactivation in female mammals), transcription factors (proteins that bind to promoter regions and enhance or inhibit transcription), hormones (act as gene expression regulators — steroid hormones enter cells and bind to DNA directly), environmental factors (temperature in Himalayan rabbits [cool extremities develop dark fur]; diet, stress, toxin exposure); basic methods in molecular biology research: PCR (polymerase chain reaction — amplifies specific DNA sequences by cycling through denaturation, primer annealing, and extension; essential for DNA profiling, disease diagnosis, ancestry testing), gel electrophoresis (separates DNA, RNA, or proteins by size through an electric field in an agarose or polyacrylamide gel; smaller fragments migrate farther), DNA sequencing (Sanger sequencing; next-generation sequencing; critical for genomics, phylogenetics, and disease research), CRISPR-Cas9 (a gene-editing tool adapted from a bacterial immune defense system; allows precise cuts at specific DNA sequences, enabling gene knockouts and insertions; potential for treating genetic diseases), recombinant DNA technology (restriction enzymes cut DNA at specific recognition sequences; DNA ligase joins fragments; plasmids are used as vectors to introduce foreign genes into host organisms; used to produce insulin, vaccines, and agricultural GMOs); theory of natural selection: Darwin’s four postulates: (1) variation: individuals in a population vary in their heritable traits; (2) heritability: traits are passed from parents to offspring; (3) differential survival and reproduction: individuals with certain traits are more likely to survive and reproduce; (4) adaptation: over generations, favorable traits become more common in the population; directional selection (one extreme phenotype favored — antibiotic resistance; industrial melanism in peppered moths), stabilizing selection (intermediate phenotype favored — human birth weight; eliminates extremes), disruptive/diversifying selection (both extreme phenotypes favored; intermediate phenotypes eliminated — can lead to speciation; Grants’ finch beak studies); speciation and reproductive isolation: allopatric speciation (geographic isolation separates a population into two groups that evolve independently until they can no longer interbreed; most common mode of speciation); sympatric speciation (speciation without geographic isolation — polyploidy in plants is the most common mechanism); pre-zygotic isolating mechanisms (prevent mating or fertilization: habitat isolation, temporal isolation [different breeding seasons], behavioral/ethological isolation [different mating signals], mechanical isolation [incompatible reproductive structures], gametic isolation [sperm cannot fertilize eggs]) and post-zygotic isolating mechanisms (prevent viable, fertile offspring: hybrid inviability, hybrid sterility [mule = horse × donkey], hybrid breakdown); Hardy-Weinberg equilibrium: the allele frequencies in a population will remain constant from generation to generation if: (1) no mutations, (2) random mating, (3) no natural selection, (4) no gene flow (migration), (5) large population size (no genetic drift); equations: p² + 2pq + q² = 1 (genotype frequencies) and p + q = 1 (allele frequencies), where p = frequency of dominant allele, q = frequency of recessive allele, p² = frequency of homozygous dominant, 2pq = frequency of heterozygous, q² = frequency of homozygous recessive; any deviation from Hardy-Weinberg conditions signals that evolution is occurring; basic taxonomy: Linnaean classification hierarchy (Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species; mnemonic: “Dear King Philip Came Over For Good Soup”); three domains of life: Bacteria (prokaryotes; peptidoglycan cell walls; diverse metabolic pathways), Archaea (prokaryotes; no peptidoglycan; found in extreme environments; more closely related to eukaryotes than to bacteria), Eukarya (eukaryotes; kingdoms: Protista, Fungi, Plantae, Animalia); evidence of common ancestry and diversity: homologous structures (same underlying anatomy, different functions: whale flipper, human arm, bat wing, cat leg — evidence of common ancestry); analogous structures (different underlying anatomy, same function: insect wing and bird wing — evidence of convergent evolution, not common ancestry); molecular evidence: the more similar two species’ DNA and amino acid sequences, the more recently they shared a common ancestor; the fossil record provides a chronological sequence of organisms; biogeography (related species tend to occur on adjacent landmasses; island biogeography); phylogenetic trees (cladograms) and the concept of clades (monophyletic groups: an ancestor and all its descendants).

The central dogma: DNA → RNA → Protein — the most tested Testlet 206 concept at depth: the central dogma describes the directional flow of genetic information: DNA is transcribed into mRNA, which is translated into protein; the reverse flow (protein → DNA) does not occur under normal circumstances (retroviruses use reverse transcriptase to go RNA → DNA, which is an exception); transcription: RNA polymerase binds to the promoter sequence on the template (antisense) strand and reads the template strand in the 3′ → 5′ direction, synthesizing a complementary mRNA strand in the 5′ → 3′ direction; in eukaryotes, the pre-mRNA undergoes processing before leaving the nucleus: a 5′ methylguanosine cap is added (protects mRNA from degradation and aids ribosome binding); a poly-A tail is added to the 3′ end (protects from degradation and signals for nuclear export); RNA splicing removes introns (non-coding sequences) and joins exons (coding sequences) via spliceosomes; translation: occurs at ribosomes in the cytoplasm; the mRNA sequence is read in codons (triplets of nucleotides); tRNA molecules carry amino acids to the ribosome, matching their anticodon to the mRNA codon; ribosomes have three sites: A site (aminoacyl — incoming tRNA with amino acid), P site (peptidyl — growing polypeptide chain), E site (exit — empty tRNA leaves here); start codon = AUG (codes for methionine; signals translation start); stop codons = UAA, UAG, UGA (do not code for an amino acid; signal release of the polypeptide chain); properties of the genetic code: degenerate (most amino acids have multiple codons), universal (virtually all organisms use the same code), non-overlapping (each nucleotide belongs to only one codon); exam questions test your ability to predict the effect of a specific mutation on the resulting protein: silent mutation (synonymous codon — same amino acid; no change in protein); missense mutation (non-synonymous codon — different amino acid; may alter protein function depending on R-group similarity and location in the protein); nonsense mutation (codon becomes a stop codon; premature termination; protein usually nonfunctional); insertion or deletion (frameshift — all downstream codons shifted; protein almost certainly nonfunctional); point mutation in the promoter or Shine-Dalgarno/Kozak sequence (may reduce or eliminate transcription or translation initiation; less mRNA or protein produced).

Mitosis vs. meiosis — the cell division comparison most tested in Testlet 206: mitosis produces 2 daughter cells that are genetically identical to the parent cell (barring mutation); cells remain diploid (2n → 2n); purpose: growth, repair, and asexual reproduction; phases: Prophase (chromatin condenses; spindle begins to form; nuclear envelope breaks down), Metaphase (chromosomes align at the metaphase plate; spindle fibers attach to kinetochores of sister chromatids), Anaphase (sister chromatids are pulled apart to opposite poles by kinetochore microtubules; each pole gets a complete set of chromosomes), Telophase (nuclear envelopes re-form around each set of chromosomes; chromosomes begin to decondense), Cytokinesis (the cytoplasm divides; cleavage furrow in animals; cell plate in plants); meiosis produces 4 daughter cells (gametes or spores) that are genetically unique; cells become haploid (2n → n); purpose: sexual reproduction; meiosis consists of two sequential divisions: Meiosis I separates homologous chromosome pairs (reductional division): Prophase I = crossing over (recombination) at chiasmata between non-sister chromatids of homologous chromosomes — the SINGLE MOST IMPORTANT source of genetic variation in meiosis; Metaphase I: tetrads (bivalents) of homologous chromosomes align at the metaphase plate; the orientation of each homologous pair is random (independent assortment generates 2ⁿ possible chromosome combinations); Anaphase I: homologous chromosomes (not sister chromatids) are pulled to opposite poles; Telophase I and Cytokinesis I: two haploid cells form (each still with sister chromatids joined); Meiosis II is similar to mitosis but occurs in haploid cells: sister chromatids are separated; result is 4 haploid cells each with unique combinations of genetic material; three sources of genetic variation in sexual reproduction: crossing over (Prophase I) — creates new combinations of alleles on the same chromosome; independent assortment (Metaphase I) — determines which chromosome from each homologous pair goes to which pole; random fertilization — any egg can be fertilized by any sperm, vastly multiplying genetic combinations.

Photosynthesis vs. cellular respiration — the two energy transformation processes: photosynthesis equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂; occurs in two stages: light-dependent reactions (in thylakoid membranes of chloroplasts): Photosystem II absorbs light at 680 nm and splits water molecules (photolysis), releasing O₂ as a byproduct and energizing electrons; energized electrons pass through an electron transport chain in the thylakoid membrane, pumping H⁺ ions into the thylakoid lumen (creating a proton gradient); H⁺ ions flow back through ATP synthase via chemiosmosis, generating ATP; Photosystem I (absorbs 700 nm light) re-energizes electrons and uses them to reduce NADP⁺ to NADPH; products: ATP, NADPH, O₂; Calvin cycle (light-independent reactions, in stroma of chloroplasts): CO₂ is fixed by the enzyme RuBisCO onto a 5-carbon acceptor (RuBP) to form an unstable 6-carbon intermediate that immediately splits into two 3-carbon molecules (3-phosphoglycerate); ATP and NADPH from the light reactions are used to reduce 3-phosphoglycerate to G3P (glyceraldehyde-3-phosphate); G3P is the precursor for glucose and other organic molecules; RuBP is regenerated; net: 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P; cellular respiration equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36–38 ATP; occurs in four stages: glycolysis (cytoplasm — does NOT require oxygen; glucose [6C] split into 2 pyruvate [3C]; net gain 2 ATP, 2 NADH; can proceed during anaerobic respiration/fermentation); pyruvate oxidation (mitochondrial matrix — each pyruvate converted to acetyl-CoA; releases 1 CO₂, produces 1 NADH per pyruvate); Krebs/citric acid cycle (mitochondrial matrix — 2 turns per glucose; produces 2 ATP, 6 NADH, and 2 FADH₂ total per glucose; releases 4 CO₂ total per glucose); oxidative phosphorylation/ETC and chemiosmosis (inner mitochondrial membrane — NADH and FADH₂ donate electrons; electron transport chain pumps H⁺ into the intermembrane space; O₂ is the final electron acceptor, forming water; protons flow back through ATP synthase producing ~32–34 ATP; this is where the vast majority of ATP is produced); key contrasts: photosynthesis is anabolic (builds complex molecules from simple ones) and endergonic (requires energy input — light); cellular respiration is catabolic (breaks down complex molecules) and exergonic (releases energy); both processes occur in plant cells; only respiration occurs in animal cells (which have mitochondria but not chloroplasts).

Biological macromolecules: structure-function relationships for all four types: carbohydrates: monomers are monosaccharides (glucose, fructose, galactose); joined by glycosidic bonds (condensation/dehydration synthesis; water is released); starch (plant energy storage: amylose and amylopectin, both α-1,4 glycosidic bonds; amylopectin has α-1,6 branch points) and glycogen (animal energy storage in liver and muscle, highly branched) are both readily digestible; cellulose (plant cell wall, β-1,4 glycosidic bonds that create straight, hydrogen-bonded fibrils; most animals lack the enzyme cellulase and cannot digest cellulose); chitin (fungal cell walls and arthropod exoskeletons); proteins: 20 amino acids, each with a central carbon bonded to an amino group (−NH₂), a carboxyl group (−COOH), a hydrogen, and a variable R group (side chain) that determines the amino acid’s chemical properties; amino acids are linked by peptide bonds (between the carboxyl of one and the amino group of the next; water is released); protein structure: primary = the sequence of amino acids (determined by the gene); secondary = alpha helix (coiled, stabilized by hydrogen bonds between backbone C=O and N-H groups) or beta pleated sheet (zigzag strands, also hydrogen-bonded); tertiary = 3D folding of the full polypeptide from R-group interactions (hydrophobic interactions, disulfide bridges between cysteine residues, ionic bonds, hydrogen bonds); quaternary = assembly of two or more polypeptide subunits; enzyme active site is a 3D pocket that binds a specific substrate; denaturation (disruption of secondary/tertiary/quaternary structure by heat, pH change, or chemicals) destroys function because the active site changes shape; lipids: not polymers (not made of monomers joined by repeated bonds); triglycerides = 3 fatty acid chains esterified to a glycerol backbone; saturated fats: no C=C double bonds; straight chains pack tightly; solid at room temperature (butter, lard; associated with cardiovascular disease); unsaturated fats: one (monounsaturated) or more (polyunsaturated) C=C double bonds that introduce kinks in the chain, preventing tight packing; liquid at room temperature (olive oil, fish oil; healthier profile); trans fats: unsaturated fats with the hydrogen atoms on opposite sides of the double bond (partial hydrogenation of vegetable oils); more solid than cis unsaturated fats; associated with cardiovascular disease; phospholipids: glycerol backbone + 2 fatty acid tails + a phosphate-containing head group; amphipathic (hydrophilic head, hydrophobic tails); spontaneously arrange into bilayers in water — the basis of all biological membranes; steroids: four fused carbon rings; cholesterol is the precursor for all steroid hormones (estrogen, testosterone, cortisol, aldosterone) and bile salts; nucleic acids: A-T base pairs in DNA are held by 2 hydrogen bonds; G-C base pairs are held by 3 hydrogen bonds (G-C pairs are stronger — DNA with a higher G-C content requires more energy to denature [separate] and has a higher melting temperature Tm); RNA uses uracil (U) in place of thymine (T); DNA is double-stranded; RNA is generally single-stranded.

Energy flow through ecosystems and the 10% rule — highest-priority Testlet 207 concept: all energy in an ecosystem originates from the sun (or, in chemosynthetic ecosystems, from inorganic chemical energy); producers (autotrophs) capture solar energy through photosynthesis and convert it into chemical energy stored in organic molecules; energy flows in one direction through the trophic levels: producers (first trophic level) → primary consumers / herbivores (second trophic level) → secondary consumers / carnivores (third trophic level) → tertiary consumers (fourth trophic level) → quaternary consumers (fifth trophic level); detritivores and decomposers (bacteria and fungi) break down dead organic matter at all levels, recycling nutrients back to the abiotic environment; the 10% rule (Lindeman’s efficiency): approximately 10% of the energy stored at one trophic level is transferred to and stored in the next trophic level; the remaining ~90% is lost as metabolic heat (through cellular respiration), used in the organism’s own life processes, or passed to decomposers; worked example: if producers fix 100,000 kcal of energy, then primary consumers receive ~10,000 kcal, secondary consumers receive ~1,000 kcal, tertiary consumers receive ~100 kcal, quaternary consumers receive ~10 kcal; this rapid energy decline explains why: food chains rarely have more than 4–5 trophic levels (too little energy to support a sixth level); predators at the top of long food chains must eat enormous amounts to meet their energy needs; a plant-based (lower trophic level) diet is more energy-efficient and can support a larger human population than a meat-based diet; energy pyramids (number, biomass, and energy) are always upright (more energy/biomass at the base than at the top) — a biomass pyramid can be temporarily inverted in some aquatic ecosystems where phytoplankton (small biomass per unit area but fast-reproducing) support a larger total biomass of zooplankton, but energy pyramids are ALWAYS upright.

Population growth models and carrying capacity — frequently tested ecological mathematics: exponential growth (J-shaped curve): occurs when resources are unlimited; the population grows at a constant per capita rate; equation: dN/dt = rN, where N = population size, r = per capita growth rate (birth rate − death rate); the larger the population, the faster it grows (compounding effect); rare in nature for extended periods; examples: bacterial growth in ideal lab conditions; invasive species immediately after introduction before predators and competitors catch up; logistic growth (S-shaped / sigmoid curve): occurs when resources are limited and the environment has a maximum sustainable population size called the carrying capacity (K); equation: dN/dt = rN[(K−N)/K]; when N is small relative to K, [(K−N)/K] ≈ 1 and growth is approximately exponential; as N approaches K, [(K−N)/K] approaches 0 and growth rate slows; at N = K, growth rate = 0 (births = deaths); when N > K, the growth rate becomes negative (deaths > births) and the population declines back toward K; the inflection point (where growth rate is fastest) occurs at N = K/2; real populations oscillate around K rather than reaching it exactly; what determines K: food/nutrients, water, space/territory, light, nesting sites, disease, predation — all density-dependent factors; r-selected vs. K-selected species: r-selected species (high r, low K, fast-reproducing — insects, annuals, mice — do well in variable environments and early succession); K-selected species (low r, high K, slow-reproducing — elephants, whales, humans — do well in stable environments at carrying capacity).

Types of ecological interactions and their notation — the six key symbiotic and competitive relationships: ecological interactions are classified by their effect on each species involved (+ = benefit, − = harm, 0 = no effect): competition (−/−): both species are harmed; intraspecific competition (between members of the SAME species) is typically the strongest form because individuals have identical resource requirements; interspecific competition (between members of DIFFERENT species) may lead to competitive exclusion (one species outcompetes and eliminates the other from the habitat — Gause’s competitive exclusion principle) or to niche differentiation/resource partitioning (species evolve to use slightly different resources, reducing overlap and allowing coexistence — Darwin’s finch beak differences is a classic example); predation (+/−): the predator benefits; the prey is harmed; predator-prey interactions often produce cyclical population fluctuations modeled by the Lotka-Volterra equations; classic example: snowshoe hare and Canadian lynx cycle from Hudson’s Bay Company data; as prey populations increase, predator populations follow (with a lag); as prey decline (from predation), predators decline, allowing prey to recover; herbivory (+/−): a special case of predation where the consumer (herbivore) eats plants; mutualism (+/+): both species benefit; mycorrhizal fungi and plant roots (fungi provide water/minerals; plant provides photosynthate); pollinator and flowering plant; nitrogen-fixing Rhizobium bacteria in legume root nodules; clownfish and sea anemone; commensalism (+/0): one species benefits; the other is neither helped nor harmed; barnacles on whale skin (barnacles get transported and fed; whale unaffected); orchids growing on tree branches for access to light; parasitism (+/−): the parasite benefits; the host is harmed (but usually not killed quickly, which distinguishes it from predation); tapeworms in intestines; ticks; mistletoe on tree branches; keystone species: a species that has a disproportionately large effect on community structure relative to its numerical abundance (if removed, the community changes dramatically); sea otters (control sea urchin populations; without otters, urchins overgraze kelp, destroying kelp forests); gray wolves reintroduced to Yellowstone (controlled elk grazing — trophic cascade allowing riparian vegetation to recover, stabilizing stream banks, changing river morphology).

Biogeochemical cycles — carbon and nitrogen as the most tested cycles: the carbon cycle: carbon enters ecosystems as CO₂ from the atmosphere (or dissolved carbonate from water); producers fix CO₂ via photosynthesis into organic compounds (glucose, etc.); carbon moves through food chains as organisms eat one another; carbon is returned to the atmosphere (or water) as CO₂ through aerobic cellular respiration by all organisms and through decomposition of dead organic matter; in the ocean, phytoplankton fix CO₂ and marine organisms build shells of calcium carbonate (CaCO₃), which settle as limestone when organisms die (long-term carbon sequestration); over geologic time, organic matter compressed under sediment formed fossil fuels (coal, oil, natural gas) — another long-term carbon sink; human activities — primarily burning fossil fuels and deforestation — add approximately 10 billion metric tons of carbon per year to the atmosphere; atmospheric CO₂ has risen from pre-industrial ~280 ppm to 420+ ppm today; the nitrogen cycle: nitrogen gas (N₂) makes up ~78% of the atmosphere but is unusable by most organisms in this form; nitrogen fixation converts N₂ to ammonia (NH₃) — performed by free-living bacteria such as Azotobacter and Cyanobacteria, and by symbiotic bacteria (Rhizobium) in legume root nodules; lightning also fixes small amounts of nitrogen; nitrification is a two-step process: Nitrosomonas bacteria convert NH₃ to nitrite (NO₂−); Nitrobacter bacteria convert NO₂− to nitrate (NO₃−); plants absorb NO₃− from the soil and incorporate nitrogen into amino acids, nucleotides, and chlorophyll (assimilation); when organisms die, decomposers break down nitrogen-containing organic compounds back to NH₃ (ammonification/decomposition); denitrification by bacteria such as Pseudomonas and Paracoccus converts NO₃− back to N₂ gas in anaerobic conditions, returning nitrogen to the atmosphere; excess nitrogen (from agricultural fertilizer runoff) enters water bodies and causes eutrophication: algal blooms consume excess nutrients → algae die and are decomposed by bacteria → bacterial respiration depletes dissolved oxygen → hypoxic or anoxic dead zones form (e.g., the Gulf of Mexico dead zone below the Mississippi River outflow).

Natural selection mechanisms and their effect on phenotypic distributions: natural selection operates on phenotypic variation in a population that has a genetic basis; three modes of natural selection describe how selection pressure changes the distribution of phenotypes in a population over time: directional selection: one extreme of the phenotypic distribution is favored; the population mean shifts in the direction of the favored extreme; examples: antibiotic resistance (bacteria with resistance alleles survive treatment and reproduce; susceptible bacteria are killed; over generations, the population shifts toward resistance); industrial melanism in peppered moths (Biston betularia — before industrialization, light-colored moths on lichen-covered bark were camouflaged; dark-colored moths were rare; after industrial pollution killed lichens and darkened tree bark, dark moths were better camouflaged; selection favored dark moths and the population shifted toward melanism; frequency of dark moths increased from <1% to >90% near industrial cities in England within decades); DDT resistance in insects; stabilizing selection: intermediate phenotypes are favored; both extremes are selected against; the distribution narrows (variance decreases) but the mean stays the same; examples: human birth weight (very low birth weight: high mortality from developmental complications; very high birth weight: high mortality from complications during birth; intermediate birth weight ~3.5 kg: highest survival; this selection pressure has maintained the mean birth weight relatively stable over human history); egg clutch size in birds (too few eggs: underutilization of resources; too many eggs: parents cannot provision all offspring adequately; intermediate clutch size maximized offspring survival); disruptive or diversifying selection: both extreme phenotypes are favored; intermediate phenotypes are selected against; the distribution becomes bimodal (two peaks); increases phenotypic and genetic variation in the population; can eventually lead to speciation if the two groups stop interbreeding; example: Grant’s research on Geospiza finches in the Galápagos during a drought — medium-sized seeds became rare; only very small soft seeds (eaten by small-billed finches) and very large hard seeds (cracked by large-billed finches) remained; medium-billed birds could eat neither efficiently and their frequency declined; for exam questions, be prepared to: identify which type of selection is occurring from a description; sketch the expected change in the phenotypic distribution (bell curve shifts directionally, narrows and becomes taller, or splits into bimodal) before vs. after selection.

Mendelian genetics and Punnett squares with exceptions to simple dominance: Mendel’s Law of Segregation: each organism carries two alleles for each trait; the two alleles separate during gamete formation so that each gamete contains only one allele; each offspring receives one allele from each parent; Mendel’s Law of Independent Assortment: genes for different traits (located on different chromosomes or far apart on the same chromosome) assort independently of one another during gamete formation; this is the genetic basis of the 9:3:3:1 dihybrid ratio; monohybrid cross (Aa × Aa): genotype ratio 1 AA : 2 Aa : 1 aa; phenotype ratio 3 dominant : 1 recessive (only offspring with two recessive alleles [aa] express the recessive phenotype); dihybrid cross (AaBb × AaBb): 16 possible genotypic outcomes; phenotype ratio 9 (A​_B​_) : 3 (A​_bb) : 3 (aaB​_) : 1 (aabb); exceptions to simple dominance: codominance: both alleles are fully expressed simultaneously in the heterozygote; ABO blood type is the classic example — the I⊃A allele causes A antigens on red blood cells; I⊃B allele causes B antigens; I⊃AI⊃B heterozygotes have BOTH A and B antigens on their RBCs (type AB blood — universal recipient); heterozygotes do NOT show a blended phenotype; incomplete dominance: the heterozygote shows a phenotype intermediate between the two homozygous phenotypes (blended, NOT codominant); in four o’clocks and snapdragons: red (RR) × white (WW) → pink (RW) offspring; the pink color results because the single R allele produces only enough pigment to make the flower pink rather than fully red; sex-linked traits: genes on sex chromosomes; X-linked recessive traits such as red-green color blindness and hemophilia A: males (XY) have only ONE X chromosome, so a single copy of the recessive allele is sufficient to express the trait (males cannot be “carriers”); females (XX) need TWO copies of the recessive allele to express it; females with ONE recessive allele are unaffected carriers; the trait is much more common in males; sex-linked pedigree solving: a male with an X-linked trait always inherited the X with the trait from his mother (cannot get it from his father); all daughters of an affected male are at least carriers; additional exceptions: multiple alleles (the ABO locus has THREE alleles: I⊃A, I⊃B, i, though any individual has only two); polygenic inheritance (multiple genes contribute to a single quantitative trait — skin color, height; produces a continuous/normal distribution); pleiotropy (a single gene affects multiple traits — the sickle-cell allele causes abnormal hemoglobin in ALL red blood cells, leading to oxygen delivery problems, sickle-cell crises, spleen damage, and susceptibility to infections — multiple phenotypic effects from a single locus).

Hardy-Weinberg equilibrium as a null hypothesis for evolution: the Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary forces; it provides a mathematical baseline against which real populations are compared — deviations from Hardy-Weinberg equilibrium indicate that evolution is occurring; five conditions required for Hardy-Weinberg equilibrium (no evolution): (1) no mutations (no new alleles being introduced or changed); (2) random mating (all genotypes have equal reproductive partners; no sexual selection or assortative mating); (3) no natural selection (all genotypes have equal survival and reproductive success); (4) no gene flow (no immigration or emigration of alleles into or out of the population); (5) very large (effectively infinite) population size (no genetic drift — random sampling errors); if ANY of these conditions is violated, the population’s allele or genotype frequencies will change — i.e., evolution is occurring; Hardy-Weinberg equations: p + q = 1 (where p = frequency of the dominant allele and q = frequency of the recessive allele); p² + 2pq + q² = 1 (where p² = frequency of homozygous dominant individuals, 2pq = frequency of heterozygous individuals, q² = frequency of homozygous recessive individuals); to solve: if you know q² (the proportion of recessive-phenotype individuals in the population, which can be observed directly), take the square root to get q; then p = 1 − q; then calculate p² and 2pq; genetic drift — the random change in allele frequencies that occurs in small populations: founder effect (a small group of individuals colonizes a new area; the founding population has a random sample of alleles from the original population that may not represent the original allele frequencies; this can lead to reduced genetic diversity and overrepresentation of alleles that happened to be common in the small founding group; example: Ellis-van Creveld syndrome in the Amish community of Pennsylvania, founded by a small group of settlers); bottleneck effect (a catastrophic event dramatically reduces population size; the surviving individuals are a random, non-representative sample of the original allele frequencies; the population’s genetic diversity is permanently reduced even after the population recovers in size; example: Northern elephant seal population was reduced to ~20 individuals by hunting in the 19th century and has recovered numerically but has very low genetic diversity).

Human impacts and climate change — the distinctive half of Testlet 208: habitat destruction is the single most important cause of biodiversity loss worldwide; tropical deforestation (primarily for agriculture — cattle ranching and soy production in the Amazon Basin, palm oil in Southeast Asia) is the largest contributor; the Amazon rainforest has lost approximately 20% of its original forest cover; habitat fragmentation (dividing large continuous habitat into isolated patches) increases edge effects, reduces interior habitat, isolates populations, and impedes gene flow — often as damaging as outright destruction; invasive species represent the second leading cause of biodiversity loss: the brown tree snake (Boiga irregularis) arrived in Guam via military cargo from the Admiralty Islands post-World War II; in the absence of natural predators, the population exploded; the snake caused the extinction or near-extinction of 12 of Guam’s 22 native forest bird species within decades, and has eliminated populations of native lizards and small mammals; Guam is often cited as the worst case of island bird extinctions caused by a single invasive predator; climate change and coral reefs: ocean surface water temperatures are rising due to the enhanced greenhouse effect; when sea surface temperatures rise even 1–2°C above the summer maximum for a sustained period, coral polyps expel their photosynthetic symbiotic algae (zooxanthellae); without zooxanthellae, the coral loses its color (bleaching) and its primary food source; if temperatures return to normal quickly, zooxanthellae may return; if the bleaching persists, the coral starves and dies; the Great Barrier Reef experienced its most widespread and severe mass bleaching event on record in 2024; the Brundtland Commission (World Commission on Environment and Development, 1987) defined sustainable development as: “development that meets the needs of the present without compromising the ability of future generations to meet their own needs;” renewable vs. nonrenewable energy: renewable (solar, wind, hydroelectric, geothermal, sustainably harvested biomass — replenish on human timescales; low or zero direct greenhouse gas emissions during operation; variable/intermittent availability is a challenge for solar and wind; infrastructure and land use impacts must be managed) vs. nonrenewable (fossil fuels: coal, oil, natural gas formed over millions of years from compressed organic matter; uranium for nuclear power; finite supply; fossil fuel combustion is the primary driver of current climate change; nuclear produces no direct CO₂ but generates radioactive waste).