Maharashtra HSC Biology cheat sheet

Mendel's law of segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one allele per gene.

Law of independent assortment applies only to genes on different chromosomes or far apart on the same chromosome; linked genes deviate from 9:3:3:1.

Test cross (cross with homozygous recessive) reveals the genotype of a dominant phenotype; a 1:1 ratio indicates heterozygosity.

Incomplete dominance gives an intermediate phenotype (e.g., Snapdragon pink) and a 1:2:1 genotypic and phenotypic ratio in F2.

Codominance: both alleles express fully (human AB blood group); multiple alleles exist in a population, but a diploid individual carries only two.

DNA structure: antiparallel double helix, 10 bp per turn, 3.4 nm pitch, 0.34 nm rise per base pair; A=T and G≡C with 2 and 3 hydrogen bonds.

Semiconservative replication (Meselson-Stahl, 15N) proceeds 5'→3' with continuous leading strand and discontinuous lagging strand (Okazaki fragments).

Transcription in eukaryotes: RNA polymerase II makes mRNA; introns removed by spliceosomes; monocistronic mRNA, while prokaryotic mRNA is often polycistronic.

Genetic code: triplet, degenerate, non-overlapping, comma-less, universal; AUG is initiator and methionine codon; three stop codons UAA, UAG, UGA.

Restriction endonucleases cut palindromic sequences producing sticky or blunt ends; EcoRI recognizes GAATTC and cuts between G and A.

DNA ligase seals phosphodiester bonds between vector and insert; ligation requires compatible ends and ATP.

Cloning vector essentials: origin of replication, selectable marker, multiple cloning site, and (for expression) a promoter.

pBR322 carries ampicillin and tetracycline resistance; insertional inactivation of a marker identifies recombinants.

Transformation methods include CaCl2 heat shock, electroporation, microinjection, gene gun (biolistics), and Agrobacterium tumefaciens Ti plasmid for plants.

PCR cycle: denaturation ~94°C, primer annealing ~50-60°C, extension ~72°C by Taq polymerase; exponential amplification 2^n.

Gel electrophoresis separates DNA by size; DNA is negatively charged and migrates toward the anode through agarose.

Human Genome Project (completed 2003): ~3.1 billion base pairs, about 20,000-25,000 protein-coding genes, less than 2% coding sequence.

DNA fingerprinting uses VNTR/STR repeats; Southern blotting transfers DNA to membrane for hybridization with a labeled probe.

Interphase: G1 (growth), S (DNA replication, DNA doubles, chromosome number unchanged), G2 (protein synthesis, organelle duplication).

Mitosis stages: prophase (chromatin condenses, spindle forms), metaphase (chromosomes at equator), anaphase (centromeres split, chromatids move to poles), telophase (nuclear envelope reforms).

Cytokinesis in plants occurs by cell plate formation; in animals by cleavage furrow.

Meiosis I is reductional (2n→n); prophase I includes synapsis and crossing over at chiasmata in pachytene.

Meiosis II is equational; four haploid cells result; crossing over and independent assortment generate variation.

Asexual reproduction modes: binary fission, budding, fragmentation, spore formation, vegetative propagation; offspring are genetically identical clones.

In angiosperms, double fertilization produces diploid zygote and triploid primary endosperm nucleus.

Oogenesis produces one functional ovum plus polar bodies; spermatogenesis yields four functional sperm from one spermatogonium.

Spermatogenesis occurs in seminiferous tubules; FSH stimulates Sertoli cells, LH stimulates Leydig cells to secrete testosterone.

Menstrual cycle: menstrual phase (1-5), follicular/proliferative (6-13), ovulation ~day 14, luteal/secretory (15-28).

Ovulation is triggered by LH surge; corpus luteum secretes progesterone, which maintains the endometrium.

Fertilization occurs in the ampullary-isthmic junction of the fallopian tube; implantation occurs about 6-7 days after fertilization.

hCG maintains the corpus luteum in early pregnancy; placenta secretes hCG, hPL, estrogen, and progesterone.

Contraceptives: combined oral pills inhibit ovulation; IUDs (CuT) are spermicidal; condoms also prevent STIs.

ART includes IVF-ET, ZIFT, GIFT, ICSI, and surrogacy; IVF involves fertilization outside the body followed by embryo transfer.

STIs of note: gonorrhoea (Neisseria), syphilis (Treponema), chlamydiasis, genital herpes, and AIDS (HIV).

Water potential (Ψw) = Ψs + Ψp; pure water at atmospheric pressure has Ψw = 0; solutes lower Ψs.

Water moves from higher (less negative) to lower (more negative) water potential; root hair absorption is osmotic.

Transpiration pull-cohesion-tension theory explains xylem ascent; transpiration is regulated by guard cells and stomata.

Macronutrients: N, P, K, Ca, Mg, S; micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.

Nitrogen fixation by Rhizobium (symbiotic) and Azotobacter (free-living); leghaemoglobin protects nitrogenase from oxygen.

Photosynthesis: light reactions in thylakoids produce ATP and NADPH; Calvin cycle in stroma fixes CO2 via RuBisCO.

C4 plants (e.g., maize, sugarcane) use PEP carboxylase and bundle sheath cells; they lack photorespiration and have higher productivity.

Limiting factor: Blackman's law states the slowest factor determines the rate; CO2 is often limiting in C3 plants.

Glycolysis in cytoplasm: glucose → 2 pyruvate, net 2 ATP and 2 NADH; substrate-level phosphorylation.

Link reaction converts pyruvate to acetyl-CoA in mitochondria, releasing CO2 and forming NADH.

Krebs cycle per glucose yields 6 NADH, 2 FADH2, 2 ATP (GTP), and 4 CO2.

Oxidative phosphorylation: electron transport chain yields ~26-28 ATP; total ~30-32 ATP per glucose.

Fermentation: lactic acid (animals) or ethanol + CO2 (yeast); NAD+ regeneration allows glycolysis to continue.

Plant growth regulators: auxin (apical dominance, rooting), gibberellin (stem elongation, bolting), cytokinin (cell division), ABA (stress, stomatal closure), ethylene (ripening).

Photoperiodism: short-day plants flower when night exceeds critical length; phytochrome (Pr/Pfr) mediates the response.

Vernalization: low-temperature treatment promotes flowering (e.g., winter wheat); florigen is the hypothesized flowering signal.

Digestion: carbohydrates begin in mouth (salivary amylase), proteins in stomach (pepsin), fats mainly in small intestine (lipase, bile emulsification).

Absorption of glucose and amino acids occurs in the small intestine villi; fat absorbed as chylomicrons into lacteals.

Respiratory volumes: tidal volume ~500 mL; vital capacity = TV + IRV + ERV; residual volume remains after maximal expiration.

Oxygen transport: hemoglobin binds O2; the dissociation curve shifts right with higher CO2, H+, temperature (Bohr effect).

Human heart: SA node is the pacemaker; cardiac cycle ~0.8 s; normal resting heart rate ~72 beats/min.

Blood pressure: systolic/diastolic ~120/80 mm Hg; hypertension is ≥140/90 mm Hg.

Blood groups: AB has both antigens (universal recipient), O has no antigens (universal donor); Rh factor matters in pregnancy.

Cardiac output = stroke volume × heart rate; about 5 L/min at rest.

Nephron: glomerular filtration in Bowman's capsule, reabsorption in PCT, secretion in DCT, concentration in loop of Henle.

Counter-current mechanism in loop of Henle and vasa recta creates a medullary osmotic gradient; ADH increases water reabsorption.

Gout is uric acid accumulation; uremia is urea retention; dialysis uses a semipermeable membrane.

Skeletal muscle contraction: sliding filament theory; actin slides over myosin; Ca2+ and ATP required.

Sarcomere: A band (myosin), I band (actin only), Z line boundaries; contraction shortens I band and H zone.

Neuron resting potential ~ -70 mV; depolarization by Na+ influx, repolarization by K+ efflux; action potential is all-or-none.

Synapse: neurotransmitter released by Ca2+ influx; acetylcholine is broken down by acetylcholinesterase.

Brain parts: cerebrum (intelligence, memory), cerebellum (balance, coordination), medulla (respiration, heartbeat), hypothalamus (homeostasis).

Hypothalamus releases tropic hormones (TRH, CRH, GnRH) that control the anterior pituitary.

Anterior pituitary hormones: GH, TSH, ACTH, FSH, LH, prolactin; posterior pituitary stores ADH and oxytocin.

Thyroid: thyroxine regulates basal metabolic rate; iodine deficiency causes goitre; calcitonin lowers blood Ca2+.

Insulin lowers blood glucose; glucagon raises it; diabetes mellitus results from insulin deficiency or resistance.

Adrenal: cortisol (stress, gluconeogenesis), aldosterone (Na+ retention), adrenaline (fight-or-flight).

Homeostasis examples: thermoregulation, osmoregulation, blood glucose regulation, and acid-base balance.

Animal behaviour: innate (fixed action patterns, taxes) versus learned (conditioning, imprinting, insight).

Social behaviour: altruism, kin selection, and communication by pheromones, sound, and visual signals.

Miller-Urey experiment (1953) demonstrated abiotic formation of amino acids from CH4, NH3, H2, and water vapor with electric discharge.

Oparin-Haldane hypothesis: life arose from chemical evolution in the primitive ocean (hot dilute soup).

Evidence for evolution: fossils, homologous structures (divergent evolution), analogous structures (convergent), vestigial organs, embryology, molecular homology.

Lamarck: use and disuse with inheritance of acquired characters; disproved, but recognized environmental influence.

Darwin's natural selection: variation, heritability, differential reproductive success; 'Origin of Species' 1859.

Modern synthesis: mutation, recombination, gene flow, genetic drift, and natural selection drive evolution.

Hardy-Weinberg equilibrium: p² + 2pq + q² = 1; allele frequencies constant in absence of evolutionary forces.

Human evolution: Homo habilis, H. erectus, H. neanderthalensis, H. sapiens; bipedalism and enlarged brain are key trends.

Ecosystem components: producers, consumers, decomposers; energy flows unidirectionally, nutrients cycle.

10% law: only about 10% of energy transfers between trophic levels; food chains rarely exceed 4-5 links.

Ecological pyramids: of number, biomass, energy; energy pyramid is always upright.

Primary productivity: gross (GPP) minus respiration equals net (NPP); NPP supports the food chain.

Biogeochemical cycles: carbon, nitrogen, phosphorus, and water; nitrogen cycle includes fixation, nitrification, denitrification.

Biodiversity hotspots: Western Ghats and Sri Lanka, Himalaya, Indo-Burma; India has ~8% of global species.

Environmental issues: greenhouse gases (CO2, CH4, N2O, CFCs), ozone depletion by CFCs, acid rain (SO2, NOx), eutrophication.

Conservation: in-situ (national parks, biosphere reserves) and ex-situ (zoos, seed banks, cryopreservation).