Genetics & Evolution High-yield
- Mendel's Law of Segregation and Law of Independent Assortment underlie the classic monohybrid 3:1 and dihybrid 9:3:3:1 ratios — practice Punnett squares until they're automatic.
- Incomplete dominance blends both traits (e.g. snapdragon flower colour); codominance expresses both traits fully at once (e.g. IA/IB alleles in the ABO blood group).
- ABO blood groups are controlled by three alleles (IA, IB, i) at one locus — IA and IB are codominant, i is recessive to both.
- Sex-linked recessive traits (colour blindness, haemophilia) are carried on the X chromosome — carrier mothers (XAXa) pass the allele to 50% of sons, who are affected.
- Linked genes (on the same chromosome) don't assort independently; crossing over in meiosis I creates recombinant gametes — recombination frequency estimates map distance.
- Mutation types: point mutations (substitution, insertion, deletion) alter single bases; frameshift mutations shift the whole reading frame; chromosomal aberrations include deletion, duplication, inversion and translocation.
- Darwin's natural selection rests on four observations: overproduction, variation, struggle for existence, and survival/reproduction of the best-adapted ("fitness").
- Evidence for evolution: homologous structures (same origin, different function — e.g. forelimbs of vertebrates) vs analogous structures (different origin, same function — e.g. wings of insects and birds); also fossil records, biogeography, and molecular/DNA similarity.
- Hardy–Weinberg equilibrium: p² + 2pq + q² = 1 and p + q = 1, valid only when there's no mutation, no migration, random mating, an infinitely large population, and no natural selection.
- Speciation: allopatric speciation occurs via geographic isolation; sympatric speciation occurs without geographic separation (e.g. via polyploidy in plants).
💡
Exam tip: Genetics MCQs are almost always calculation-based — a quick, error-free Punnett square or pedigree read is worth more than memorising definitions.
Coordination & Control High-yield
- A neuron transmits impulses via a resting potential (~-70mV, maintained by the Na+/K+ pump) that reverses into an action potential when threshold is reached.
- Synaptic transmission is chemical: neurotransmitters cross the synaptic cleft and bind receptors on the postsynaptic membrane — acetylcholine is the classic exam example.
- Reflex arc pathway: receptor → sensory neuron → relay neuron in spinal cord → motor neuron → effector — note it bypasses the brain for speed.
- Major endocrine glands and hormones: pituitary (the 'master gland' — GH, TSH, ACTH, FSH/LH), thyroid (thyroxine — metabolic rate; calcitonin — lowers blood Ca2+), parathyroid (PTH — raises blood Ca2+), adrenal cortex/medulla (cortisol, adrenaline), pancreas (insulin lowers, glucagon raises blood glucose).
- Hormonal control works mainly through negative feedback loops — e.g. rising thyroxine suppresses further TSH release from the pituitary.
- Plant tropisms are directional growth responses to stimuli: phototropism (light), geotropism/gravitropism (gravity), thigmotropism (touch) — all mediated chiefly by auxin redistribution.
- Key plant hormones: auxins (cell elongation, apical dominance), gibberellins (stem elongation, seed germination), cytokinins (cell division), abscisic acid (dormancy, stress/stomatal closure), ethylene (fruit ripening).
- Eye: light is focused by the cornea and lens onto the retina, where rods (dim light, monochrome) and cones (colour, bright light) transduce it into nerve impulses.
- Ear: the cochlea (hearing) and the semicircular canals/vestibule (balance) are both housed in the inner ear.
- Diabetes mellitus Type 1 = insufficient insulin production (autoimmune destruction of β-cells); Type 2 = insulin resistance — a frequently tested distinction.
Reproduction High-yield
- Asexual reproduction methods: binary fission (bacteria, amoeba), budding (yeast, hydra), fragmentation, and spore formation (fungi) — no gamete fusion, offspring are genetically identical to the parent.
- Spermatogenesis occurs in the seminiferous tubules of the testes; a diploid spermatogonium ultimately yields four haploid sperm.
- Oogenesis occurs in the ovaries; a diploid oogonium yields one functional haploid ovum plus polar bodies (unequal cytoplasm division).
- Menstrual cycle phases: follicular phase (FSH stimulates follicle growth, rising estrogen), ovulation (LH surge triggers egg release, ~day 14), luteal phase (corpus luteum secretes progesterone to maintain the uterine lining).
- If fertilisation doesn't occur, the corpus luteum degenerates, progesterone falls, and menstruation begins.
- Fertilisation restores the diploid number; the zygote divides to form a morula, then a blastocyst, which implants in the uterine wall.
- The placenta handles gas exchange, nutrient transfer, and waste removal between mother and foetus, and secretes hormones (hCG, progesterone) that maintain pregnancy.
- Flower structure: the stamen (anther + filament) is the male part producing pollen; the carpel/pistil (stigma, style, ovary) is the female part.
- Double fertilisation in angiosperms is unique: one sperm fertilises the egg (→ diploid zygote), the other fuses with two polar nuclei (→ triploid endosperm, the seed's food store).
- Lower plants (bryophytes, pteridophytes) show alternation of generations between a haploid gametophyte and a diploid sporophyte — know which stage dominates in mosses (gametophyte) vs ferns (sporophyte).
Bioenergetics High-yield
- Photosynthesis has two stages: light-dependent reactions in the thylakoid membrane (photosystems I & II split water, generate ATP and NADPH, release O2) and the light-independent Calvin cycle in the stroma (CO2 fixed by the enzyme RuBisCO into sugar).
- Limiting factor concept: at any moment, photosynthesis rate is capped by whichever of light intensity, CO2 concentration, or temperature is in shortest supply.
- C3 plants fix CO2 directly via RuBisCO (most plants); C4 plants (e.g. maize, sugarcane) first fix CO2 into a 4-carbon compound to minimise photorespiration in hot climates; CAM plants (e.g. cacti) fix CO2 at night to conserve water.
- Cellular respiration has three main stages: glycolysis (cytoplasm, glucose → 2 pyruvate, net 2 ATP, doesn't need O2), the Krebs cycle (mitochondrial matrix, releases CO2 and electron carriers), and the electron transport chain (inner mitochondrial membrane, oxidative phosphorylation — the major ATP-generating step, ~34 ATP).
- Total ATP yield from one glucose molecule via aerobic respiration is commonly cited as ~36–38 ATP.
- Without oxygen, cells ferment pyruvate: lactic acid fermentation in muscle/some bacteria; alcoholic fermentation (ethanol + CO2) in yeast.
- ATP is the universal energy currency — energy is stored/released in its phosphoanhydride bonds (ADP + Pi ⇌ ATP).
- Enzymes lower activation energy via the induced-fit model (the active site moulds around the substrate); activity is affected by temperature, pH, substrate concentration, and inhibitors.
- Competitive inhibitors resemble the substrate and bind the active site (overcome by more substrate); non-competitive inhibitors bind elsewhere and change enzyme shape (not overcome by more substrate).
Cell Biology & Cell Division High-yield
- Prokaryotic cells lack a membrane-bound nucleus and most organelles (bacteria, archaea); eukaryotic cells have a true nucleus and membrane-bound organelles (animals, plants, fungi, protists).
- Key organelles: nucleus (genetic control), mitochondria ('powerhouse', site of aerobic respiration), ribosomes (protein synthesis), rough ER (protein processing, studded with ribosomes) vs smooth ER (lipid synthesis, detoxification), Golgi apparatus (packaging/modification), lysosomes (digestion, contain hydrolytic enzymes), chloroplasts (photosynthesis, plants only).
- The cell membrane follows the fluid mosaic model: a phospholipid bilayer with embedded/peripheral proteins that can move laterally.
- Transport across membranes: diffusion and osmosis are passive (no ATP, move down a gradient); active transport moves substances against a gradient and requires ATP; endocytosis/exocytosis move bulk material via vesicles.
- The cell cycle: Interphase (G1 — growth, S — DNA replication, G2 — preparation) followed by Mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis.
- Mitosis produces two diploid, genetically identical daughter cells (growth/repair); meiosis produces four haploid, genetically varied cells (gamete formation).
- Crossing over between homologous chromosomes occurs in prophase I of meiosis and is the main source of genetic variation, alongside independent assortment.
- Uncontrolled cell division (loss of normal cell-cycle checkpoints, e.g. via oncogene activation or tumour-suppressor gene loss) underlies cancer — a common conceptual MCQ link between cell biology and genetics.