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MCAT Biology AQA EXAM REVISION
PAPER WITH QUESTIONS &
ANSWERS/ 2026/
GUARANTEED PASS/ RATED A+
Central Dogma - ✔✔✔ DNA → RNA → Protein Purines
- which bases?
- how many rings? - ✔✔✔ Pyrimidines
- which bases?
- how many rings? - ✔✔✔ Remember: "Pyrimidine" has a "y" in it; so does "cytosine" and "thymine." Phosphodiester Bond - ✔✔✔ Joins one nucleotide to the next; between the 3rd C of one ribose and the 5th C of the other to create the sugar-phosphate backbone Directionality of DNA (convention) - ✔✔✔ 5' → 3' (but strands are antiparallel, so other strand is opposite) 3' of DNA attached to ... - ✔✔✔ OH 5' of DNA attached to ... - ✔✔✔ Phosphate group DNA composition - ✔✔✔ 1. Phosphate group
- 5-Carbon sugar
- Nitrogenous base (A, T, G, C) Which nitrogenous bases form 2 hydrogen bonds? - ✔✔✔ A, T Which nitrogenous bases form 3 hydrogen bonds? - ✔✔✔ G, C Replisome - ✔✔✔ Proteins that govern the replication process Origin of replication - ✔✔✔ Where replication begins. Prokaryotes have 1; eukaryotes have many on each chromosome
DNA synthesis: direction of synthesis - ✔✔✔ 5' → 3' (the DNA is read 3' → 5') Steps of replication (5) - ✔✔✔ 1. Helicase unzips double helix
- RNA polymerase builds a primer
- DNA polymerase adds leading/lagging strands
- Primers removed
- Okazaki fragments joined RNA vs. DNA: differences - ✔✔✔ RNA:
- C2 is oxygenated (has OH)
- Single stranded
- Uses uracil instead of thymine
- Can move through nuclear pores out of nucleus DNA
- C2 is deoxygenaged (has H)
- Double stranded (double helix)
- Uses thymine
- Stuck in the nucleus Transcription - ✔✔✔ Process of making RNA (rRNA, mRNA, tRNA) Promoter - ✔✔✔ Required for transcription. Sequence of DNA nucleotides that signals beginning point for transcription. Primer - ✔✔✔ Required for DNA replication Consensus sequence - ✔✔✔ Most common promoter sequences; closer the DNA nucleotides are to the consensus sequence, the more tightly the RNA polymerase can bind, which leads to more frequent transcription (and vice versa) RNA polymerase - ✔✔✔ Synthesizes RNA in transcription Transcription: Steps - ✔✔✔ 1. Initiation - transcription factors, transcription initiation complex (including RNA pol) finds promoter
- Elongation - Template/antisense DNA strand is read and complementary RNA synthesized in 5' → 3' direction (same as DNA synthesis); DNA is read in 3' → 5' (also same as DNA synthesis)
- Termination - temination sequence marks end, special proteins dissociate RNA pol from DNA Gene regulation - ✔✔✔ Most occurs at transcription via repressors and activators, which bind near promoter and affect activity of RNA polymerase
Northern Blot - ✔✔✔ Identifies specific sequences of RNA through nucleic acid hybridization. Western Blot - ✔✔✔ Detects protein levels with antibodies. Genetic code: 3 major features - ✔✔✔ 1. Degenerative: More than one codon for each amino acid
- Unambiguous: Each codon encodes for only ONE amino acid
- Universality: nearly every living organism uses same code Codon - ✔✔✔ Three consecutive nucleotides on mRNA Stop codons - ✔✔✔ UAA, UAG, UGA Start codon (and methoinine) - ✔✔✔ AUG Convention for writing RNA - ✔✔✔ 5' → 3' Translation - ✔✔✔ Process of protein synthesis. Three steps: Initiation, elongation, termination. Translation (initiation) - ✔✔✔ mRNA has left nucleus via nuclear pores, and is now in cytosol; initiation factors (proteins) help the 5' end of mRNA attach to the small ribosome subunit. tRNA with 5'-CAU-3' anticodon gets methionine and goes into P site. This together is called the "initiation complex." Ribosome
- prokaryote
- eukaryote - ✔✔✔ In both prok and euk, made of rRNA and protein; small and large subunits.
- prokaryote: 30S and 50S subunits (70S total)
- eukaryote: 40S and 60S subunits (80S total); made in nucleolus; exported separately to the cytoplasm (prok. do not have nucleolus, but synthesis process is similar) Translation (elongation) - ✔✔✔ tRNA with corresponding amino acid attaches to the A site (2 GTPs expended). Translocation occurs: ribosome shifts 3 nucleotide units along mRNA toward 3' end (1 more GTP expended), new tRNA now at P site. tRNA previously in P site moves to E (exits). Repeats until stop codon reaches P site. A site - ✔✔✔ Aminoacyl site of translation
- site where tRNA bearing amino acid first attaches to ribosome P site - ✔✔✔ Peptidyl site of translation
- site where first tRNA bearing methionine goes
- site where amino acid chain is elongated E site - ✔✔✔ Exit site of translation
- tRNA (now w/o amino acid) leaves the ribosomal complex Translocation (translation)
- purpose
- energy expenditure - ✔✔✔ Translocation is a step of elongation; the ribosome shifts 3 nucleotide units toward 3' end of mRNA so that the next codon is exposed. 3 GTPs are used in total. Termination (translation) - ✔✔✔ Translation ends when a stop codon reaches the P site; release factor protein binds to stop codon and ribosom disassembles. Post-translational modifications of peptides - ✔✔✔ - Other groups may be added to the peptide chain (sugars, lipids, phosphates)
- May be cleaved in one or more places Where does translation take place? - ✔✔✔ Two possibilities:
- Free-floating ribosome in the cytosol
- Ribosome that attaches itself to the rough ER during translation and injects proteins into ER lumen How / why do some ribosomes attach to rough ER? - ✔✔✔ A 20-amino acid sequence (signal peptide) near front of polypeptide is recognized by protein-RNA signal- recognition particle (SRP) that carries ribosome complex to a receptor protein on the ER. Gene mutation - ✔✔✔ Alteration in the sequence of DNA in a single gene Chromosomal mutation - ✔✔✔ The structure of a chromosome is changed Mutagens - ✔✔✔ Chemical or physical agents that can cause mutations (induced) Point mutation - ✔✔✔ A single base-pair of nucleotides in a double strand of DNA is changed Base-pair substitution mutation - ✔✔✔ Type of point mutation. One base pair is replaced by another. Missense mutation - ✔✔✔ Base-pair mutation that occurs in amino acid coding sequence of a gene. Does not always affect the a.a. or the protein:
- No change in protein function: neutral mutation
- amino acid is not changed: silent mutation
(homologues) - ✔✔✔ Two chromosomes that encode for the same traits Cell life cycle: steps (diagram) - ✔✔✔ G₁, S, G₂, M Cell life cycle: Interphase - ✔✔✔ G₁ (first growth phase) S (synthesis) G₂ (second growth phase) Cell life cycle: G₁ (characteristics) - ✔✔✔ - cell has just split
- new organelles, proteins produced = growth
- G₁ checkpoint: appropriate ratio of cytoplasm to DNA = cell enters S (division) phase; otherwise goes to G₀
- normal longest stage of cycle Cell life cycle: G₀ (characteristics) - ✔✔✔ - nongrowing phase (NOT interphase)
- allows for differences in length of cell cycle; i.e., mature neurons permanently in G₀ Cell life cycle: S (characteristics) - ✔✔✔ - cell's energy devoted to replicating DNA
- technically maintains same chromosome number, but now each chromosome has identical sister chromatid Cell life cycle: G₂ (characteristics) - ✔✔✔ - cell prepares to divide
- G₂ checkpoint: when there is enough mitosis promoting factor (MPF), mitosis is triggered Mitosis: steps - ✔✔✔ Prophase Metaphase Anaphase Telophase (Remember: PMAT) Prophase: characteristics (mitosis) - ✔✔✔ - condensation of chromatin into chromosomes
- centrioles move to opposide ends of cells
- nucleolus and nucleus disappear
- spindle apparatus forms Spindle apparatus - ✔✔✔ - aster fibers (microtubules radiating from centrioles)
- kinetochore microtubules growing from centromeres of chromosomes
- spindle microtubules connecting centrioles
Metaphase: characteristics (mitosis) - ✔✔✔ Chromosomes align at the equator of the cell Anaphase: characteristics (mitosis) - ✔✔✔ - sister chromatids split at centromeres and move to opposite ends of cells
- cytokinesis may start toward end of anaphase Telophase: characteristics (mitosis) - ✔✔✔ - nuclear membrane reforms
- nucleolus reforms
- chromosomes decondense
- cytokinesis continues Meiosis: definition - ✔✔✔ Double nuclear division that produces four haploid (n) gametes from one (2n) cell. Which cells undergo meiosis?
- male
- female - ✔✔✔ 1) male: spermatogonium (spermatogenesis)
- oogonium (oogenesis) After S phase (prior to meiosis): which cells undergo, and what do they become?
- male
- female - ✔✔✔ 1) male: spermatogonium (2n) → primary spermatocyte (2n)
- female: oogonium (2n) → primary oocyte (2n) Meiosis I: division characteristics (general), and:
- male
- female - ✔✔✔ Halves ploidy (in most from 2n → n); reduction division. Splits up homologous pairs.
- male: 1 primary spermatocyte → 2 secondary spermatocytes
- female: 1 primary oocyte → 1 secondary oocyte + 1 polar body Meiosis II: division characteristic (general), and:
- male
- female - ✔✔✔ Preserves ploidy (n → n). Splits up sister chromatids.
- male: 2 secondary spermatocytes → 4 spermatids → spermatozoa (sperm)
- female: 1 secondary oocyte (requires insemination to proceed into meiosis II) → 1 fertilized ovum (egg) / zygote + 1 polar body Meisosis:
- male vs.
One cell has an extra chromatid; other cell is missing a chromatid. Meiosis I: diagram - ✔✔✔ Meiosis II: diagram - ✔✔✔ Hydrogen bond - ✔✔✔ Type of bonding that gives water its special characteristics:
- liquid at room temperature
- cohesive forces between molecules Hydrophobic - ✔✔✔ Do not dissolve easily in water because are squeezed away from water and aggregated. Hydrophilic - ✔✔✔ Dissolve easily in water because negatively charged ends attract positively charged ends of water and vice versa. Hydrolysis reaction - ✔✔✔ Reaction by which macromolecules are separated. (lysis = separation) Catabolic reaction - ✔✔✔ Type of reaction that breaks down a molecule into smaller parts. Done by hydrolysis. Dehydration reaction - ✔✔✔ Reaction by which macromolecules are formed. Anabolic reaction - ✔✔✔ Type of reaction that builds a molecule from smaller parts (think "anabolic steroids" that build muscle). Done by dehydration. Lipid - ✔✔✔ Biological molecule that has low solubility in water and high solubility in nonpolar organic solvents. Types of Lipids - ✔✔✔ 1. fatty acids
- triacylglycerols
- phospholipids
- glycolipids
- steroids
- terpenes Fatty acids - ✔✔✔ Building blocks for most complex lipids. Composition: long chains of carbons with carboxylic acid at one end. Can be saturated (a) or unsaturated (b). Some fatty acids (eicosanoids) can also serve as local hormones. Saturated fatty acids - ✔✔✔ Only single carbon bonds
Unsaturated fatty acids - ✔✔✔ Contain one or more double carbon-carbon bonds Glycerol - ✔✔✔ 3 - carbon backbone Triacylglycerols (aka triglycerides) - ✔✔✔ Composition: Glycerol + 3 fatty acids Function: store metabolic energy, thermal insulation, padding Adipocytes - ✔✔✔ A.k.a. "fat cells." Cells whose cytoplasm contains nothing but triglycerides. Phospholipids - ✔✔✔ Composition: Glycerol + 2 fatty acids + 1 phosphate group (polar) on opposite end from fatty acids. Amphipathic - both polar and nonpolar. Function: membranes. Glycolipids - ✔✔✔ Composition: Glycerol + 2 fatty acids + carbohydrates Also amphipathic. Function: Membranes of myelinated nervous system cells. Steroids - ✔✔✔ Composition: 4-ringed structures. Include some hormones, vitamin D, and cholesterol (important membrane component) Function: Regulate metabolic activities Terpenes - ✔✔✔ Include vitamin A, important for vision. Proteins - ✔✔✔ Polypeptides built from amino acids, linked by peptide bonds. Amino acid - ✔✔✔ Amino group, alpha carbon, carbonyl, and R group (side chain). 10 of the 20 amino acids are essential to humans; i.e., must be directly ingested because body cannot manufacture them. Primary structure (protein) - ✔✔✔ Number of polypeptide chains, number/sequence of amino acids, location of disulfid bonds. Secondary structure (protein) - ✔✔✔ Alpha-helix (twisting formation) or Beta-pleated sheet (where the it lies alongside itself). Reinforced by hydrogen bonds between the carbonyl oxygen and the hydrogen on the amino group. Tertiary structure (protein) - ✔✔✔ Peptide chain curline into a globular shape. Five forces at work:
- phosphate group Nucleic acids - ✔✔✔ Polymers of nucleotides - DNA and RNA. Phosphodiester bonds - ✔✔✔ Join nucleotides. Enzymes - ✔✔✔ Act as catalysts - lowering the activation energy for a reaction and increasing the rate of the reaction. Do NOT alter the equilibrium of the reaction. Substrates - ✔✔✔ Reactant/s upon which an enzyme works. Active site - ✔✔✔ Where on the enzyme that the substrate binds, usually with noncovalent bonds, to form the enzyme-substrate complex. Lock and key theory - ✔✔✔ Theory that states that the active site of an enzyme has a shape that only fits a specific substrate. Explains some but not all enzymes. Induced fit model - ✔✔✔ Shape of both enzyme and substrate are altered upon bonding to increase specificity and help reaction proceed. Rxn rate and [substrate] - effects of substrate concentration - ✔✔✔ As the relative concentration of the substrate increases, the rate of the reaction also increases, but to a lesser and lesser degree until a maximum rate is achieved. Rxn rate and temperature - effect of increasing temperature - ✔✔✔ At first, temperature increases will increase rxn rate, but then the enzyme will denature and rxn rate drops off. Rxn rate and pH - effects of changing pH - ✔✔✔ Enzymes work best within specific pH ranges. Cofactor - ✔✔✔ Many enzymes require this - can be a coenzyme or a metal ion - to get to optimal activity. Coenzyme - ✔✔✔ Divided into two groups:
- Cosubstrates: reersibly bind to the enzyme and transfer a chemical group to another substrate. Then are reverted to the original form by another enzymatic rxn. Example- ATP.
- Prosthetic groups: remain covalently bound to the enzyme through the rxn, and remain unchanged. Example-vitamins, heme. Metal ions (as cofactors) - ✔✔✔ Can act alone or with a prosthetic group. Irreversible inhibitors
(of enzymes) - ✔✔✔ Something which binds covalently (sometimes nonocovalently) to the enzyme and disrupts its function. Tend to be highly toxic, i.e., penicillin. Competitive inhibitors (of enzymes) - ✔✔✔ Compete with the substrate by binding reversible with noncovalent bonds to the active site. Often resemble the substrate. Rxn rate can be increased by increasing substrate concentration. Noncompetitive inhibitors (of enzymes) - ✔✔✔ Bind noncovalently to the enzyme at a spot OTHER than the active site and change the conformation of the enzyme. Often act on more than one enzyme. Example: cyanide. Enzyme regulation - allosteric interactions - ✔✔✔ Modification of enzyme configuration resulting from the binding of an activator or inhibitor at a specific binding site. Can be positive or negative regulation. Negative feedback inhibition - ✔✔✔ When a downstream product comes back and inhibits an enzyme earlier in a series of reactions. Positive feedback - ✔✔✔ When a downstream product returns to activate an enzyme. Metabolism - ✔✔✔ All cellular chemical reactions. Anabolism - ✔✔✔ Molecular synthesis (through dehydration rxns). Catabolism - ✔✔✔ Molecular degradation (through hydrolysis rxns). Glycolysis (summary and net products) - ✔✔✔ Glucose (6-C sugar) is broken down in the cytosol into 2 pyruvates (3-C molecules) via several enzymatic rxns and using 2 ATP. Products:
- 2 ATP (2 used, 4 produced)
- 2 NADH
- 2 pyruvate Fermentation - ✔✔✔ Fermentation includes glycolysis, as well as oxidizes NADH back to NAD+. Pyruvate is converted to lactic acid or ethanol and expelled as waste. Because includes glycolysis, 2 ATP are considered to be formed! Substrate level phosphorylation - ✔✔✔ Method of formation of ATP in glycolysis and also in the Krebs cycle. O₂ not used.
Heterozygous - ✔✔✔ One dominant and one recessive allele (hybrid) Law of Segregation - ✔✔✔ Alleles separate independently of each other when forming gametes. Partial / incomplete dominance - ✔✔✔ When a heterozygous offspring shows a phenotype that is intermediate between homozygous ones. I.e., mate red and white flowers and get pink flowers. Codominance - ✔✔✔ When a heterozygote offspring displays both phenotypes. I.e., human blood types (A and B antigens can both be displayed at the same time in a heterozygote). Law of Independent Assortment - ✔✔✔ Genes located on different chromosomes assort independently of each other. (But when genes are on the same chromosome, the likelihood that they will remain together is indirectly proportional to the distance separating them - closer genes are more likely to stay together.) Dihybrid cross ratio - ✔✔✔ 9:3:3: Sex-linked trait - ✔✔✔ Found on sex chromosome (usually X). Example: hemophilia. Gene pool - ✔✔✔ Total of all alleles in a population. (Does not refer to phenotype; refers to genotype.) Evolution - ✔✔✔ Simply put: a change in the gene pool. Taxonomical classification - ✔✔✔ Domain King Phylum Class Order Family Genus Species Naming organism - ✔✔✔ Genus and species are given in order; italicized. Genus capitalized; species is not. Species: definition - ✔✔✔ Organisms that can reproduce fertile offspring with each other.
r-selection - ✔✔✔ Reproductive strategy in which parents produce large numbers of offspring that mature rapidly with little/no parental care. High brood mortality rate. Population growth curve is exponential. Found in unpredictable environments. K-selection - ✔✔✔ Reproductive strategy in which parents produce small number of offspring that mature slowly with much parently care. Sigmoidal growth cure which levels off at carrying capacity (maximum # of organisms that the environment can maintain). Speciation - ✔✔✔ Process by which new species are formed. Geographic, seasonal, and behavioral isolation are some factors that can bring this about. Adaptive radiation - ✔✔✔ When several separate species arise from a single ancestor (i.e., Galapagos finches). Evolutionary bottleneck - ✔✔✔ When a species faces a crisis that causes a shift in the allelic frequencies of the survivors. Divergent evolution - ✔✔✔ Two or more species evolving maintain a similar structure from a common ancestor (homologous structure). Convergent evolution - ✔✔✔ Two species independently evolve a similar structure (analagous/homoplastic). Example: birds/bats both have wings, but did not evolve from a common ancestor. Genetic drift - ✔✔✔ When an allele is permamently lost because all members with that allele die. Hardy-Weinberg equilibrium: factors (no change in gene pool if these factors are present) - ✔✔✔ 1. large population
- mutational equilibrium
- immigration/emigration must not change the gene pool
- random mating
- no selection for fittest organism Hardy-Weinberg equilibrium equations - ✔✔✔ 1. p² + 2pq +q²
- p + q = 1 Types of muscle tissue - ✔✔✔ 1. Skeletal
- Cardiac
- Smooth What are the possible functions of muscle contraction? - ✔✔✔ 1. Body movement
- Stabilization of body position
- Movement of substances through body
- Ca²⁺ binds to troponin (on thin/actin filament), exposing binding site for myosin (active site is normally blocked by tropomyosin). Myosin has ADP + Pi attached to it at this point.
- Myosin head binds to actin, expels ADP + Pi, myosin head swivels and actin is pulled toward center of sarcomere ("power stroke"; contraction)
- ATP attaches to myosin head, releasing it from active site (and tropomyosin covers site again)
- ATP splits into ADP + Pi again on the myosin, repositioning the myosin for another "power stroke"
- Ca²⁺ actively pumped back into SR after each cycle Neuromuscular synapse - ✔✔✔ Neuron attached to muscle cell T-tubules - ✔✔✔ Small channels in the sarcolemma (muscle cell membrane) that allow for uniform muscle contraction because action potential spreads more quickly Motor unit - ✔✔✔ Between 2 - 2,000 muscle fibers innervated by 1 neuron to form a motor unit (they are independent). Smaller motor units activated first; larger ones as needed = smooth increase in force generated by muscle. Intricate movements controlled by smaller motor units; muscles that need greater force have larger motor units. Muscle cell: reproduction - ✔✔✔ Does NOT happen! No mitosis in muscle cells. They just get bigger after repetitive activity (hypertrophy). Myoglobin - ✔✔✔ Protein similar to Hb, but can only hold one O₂ molecule. Type I muscle fibers - ✔✔✔ Aka: slow oxidative / slow-twitch muscle fibers. (skeletal muscle) Contain lots of myoglobin (and therefore appear red) and mitochondria. Split ATP slowly = slow fatigue but also slow contraction. Type II A muscle fibers - ✔✔✔ Aka: fast oxidative / fast-twitch A muscle fibers. (skeletal muscle) Also red (contain myoglobin). Split ATP at a high rate = contract rapidly. Resistant to fatigue, but not as much as Type I fibers. Type II B muscle fibers - ✔✔✔ Aka: fast glycolytic / fast-twitch B muscle fibers. (skeletal muscle) Low myoglobin (appear white). High glycogen content. Contract VERY rapidy. How do muscles cause movements at joints? - ✔✔✔ They decrease in length, bringing the muscle's origin and insertion closer together.
Cardiac muscle (diagram) - ✔✔✔ Cardiac muscle - ✔✔✔ Striated (composed of sarcomeres), single nucleus. Larger and more numerous mitochondria. Involuntary. Cells separated by intercalated disc. Cardiac muscle forms a net which contracts in on itself like a squeezing fist. Growth: hypertrophy (no mitosis). Intercalated disc - ✔✔✔ Region with gap junctions (desmosomes) that allow an action potential to spread from one cardiac cell to another via electrical synapses. Cardiac muscle: action potential characteristics - ✔✔✔ Action potential exhibits a plateau after depolarization, which lengthens contraction time. This plateau is cased by slow voltage-gated Ca²⁺ channels. Smooth muscle: characteristics - ✔✔✔ Involuntary, innervated by autonomic nervous system. Single nucleus, NO sarcomeres (no striations). Contain thin and thick filaments; also intermediate filaments, which are attached to dense bodies throughout the cell. Intermediate filaments (muscle) - ✔✔✔ Smooth muscle only. Attached to both thin/thick filaments and dense bodies throughout muscle cell. Intermediate filaments pull dense bodies together during contraction. (Smooth muscle shrinks lengthwise.) Single-unit smooth muscle - ✔✔✔ Aka visceral. Most common. Cells connected by gap junctions that spread action potential from a neuron through many cells so cells contract as a unit. Found in: small arteries and veins, stomach, intestines, uterus, bladder Multiunit smooth muscle - ✔✔✔ Each muscle fiber attached directly to a neuron. So can contract independently. Found in: large arteries, bronchioles, pili muscles (hair follicles), iris. Smooth muscle contraction: what causes it? - ✔✔✔ Neural stimulus, hormones, changes in pH, O₂, CO₂, temperature, ion concentrations. Bone: functions - ✔✔✔ 1. Support of soft tissue
- Protection of internal organs
- Help with movement of body
- Mineral storage
- Blood cell production
- Energy storage (adipose cells in bone marrow) Bone tissue: cell types - ✔✔✔ 1. Osteoprogenetor cells
- Osteoblasts
