RBSE SS Biology cheat sheet

Double fertilisation in angiosperms: one male gamete fuses with the egg (syngamy → zygote), the second fuses with two polar nuclei (triple fusion → triploid primary endosperm nucleus).

Microsporogenesis: sporogenous tissue → microspore mother cell → meiosis → microspore tetrad → each pollen grain divides mitotically into generative cell + vegetative cell.

Megasporogenesis: one megaspore mother cell → meiosis → four megaspores; only the chalazal one survives and forms the 8-nucleate, 7-celled embryo sac.

Mature embryo sac cells: 2 synergids, 1 egg, 3 antipodals, 2 polar nuclei in the central cell - total 7 cells, 8 nuclei.

Human spermatogenesis: spermatogonium (2n) → primary spermatocyte → meiosis I → 2 secondary spermatocytes → meiosis II → 4 spermatids → spermiogenesis → 4 sperm.

Oogenesis yields one ovum plus polar bodies; meiosis II completes only after sperm penetration, so the secondary oocyte is released arrested at metaphase II.

Menstrual cycle: menstrual phase (days 1–5), follicular/proliferative (6–13), ovulation (~day 14, LH surge), luteal/secretory (15–28) driven by corpus luteum progesterone.

hCG, hPL and relaxin are produced by the placenta; hCG maintains the corpus luteum and is the target of pregnancy tests.

Contraception mechanisms: barrier (condom, diaphragm), hormonal (pills inhibit ovulation), IUD (CuT releases Cu ions, spermicidal), surgical (vasectomy, tubectomy).

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

Test cross = cross with homozygous recessive; a 1:1 ratio in offspring indicates the test parent was heterozygous.

Dihybrid F2 phenotypic ratio 9:3:3:1 assumes independent assortment, i.e. genes on different chromosomes or far apart on the same one.

Incomplete dominance (e.g. Snapdragon flower colour) gives F2 phenotype ratio 1:2:1; codominance (AB blood group) shows both alleles expressed.

Human ABO: I^A and I^B are codominant, both dominant over i; genotypes give 4 phenotypes from 6 genotypes.

Sex-linked recessive (haemophilia, colour blindness): affected males far outnumber females because males are hemizygous (XY).

Linkage: genes on the same chromosome tend to be inherited together; recombination frequency rises with distance and is used to map genes.

Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1; allele frequencies stay constant in a large, randomly mating population without selection, mutation, migration or drift.

Natural selection types: directional (one extreme favoured), stabilising (mean favoured), disruptive (both extremes favoured); adaptive radiation = divergent evolution from a common ancestor.

Pathogen–disease pairs: Plasmodium → malaria; Wuchereria → filariasis; Ascaris → ascariasis; Entamoeba → amoebiasis; HIV → AIDS; Salmonella typhi → typhoid.

Malaria transmission: female Anopheles mosquito injects sporozoites; parasite multiplies in liver then RBCs, releasing haemozoin causing chill and fever.

Immunity: innate (non-specific, present at birth) vs acquired (specific, memory cells); active = antibodies made after exposure, passive = ready-made antibodies given.

Vaccination uses antigenic proteins or weakened/inactivated pathogens to generate memory B and T cells; colostrum provides passive immunity to newborns.

Antibiotics such as penicillin block bacterial cell-wall synthesis; they are ineffective against viruses because viruses lack a cell wall and their own metabolism.

Plant breeding steps: collection of variability → evaluation and selection → cross hybridisation → selection of superior recombinants → testing and release of variety.

Green Revolution relied on semi-dwarf, high-yield wheat and rice; Sonalika and Kalyan Sona are semi-dwarf wheat varieties.

Biofortification raises nutrient content of crops, e.g. Atlas 66 wheat (protein), iron-rich bajra, vitamin A-rich carrot and spinach.

Single Cell Protein uses microbes (Spirulina, Methylophilus) grown on cheap substrates as a protein-rich feed or food supplement.

Restriction endonucleases cut DNA at specific palindromic recognition sequences; EcoRI cuts G↓AATTC leaving sticky ends.

DNA ligase joins insert to vector; alkaline phosphatase and ligase are used to prevent vector self-ligation.

pBR322 features: ori for replication, ampR and tetR selectable markers, and unique restriction sites for cloning.

Steps of recombinant DNA technology: isolation of DNA → cutting with restriction enzymes → ligation into vector → transformation into host → selection → large-scale culture.

PCR: denaturation (~94 °C), annealing of primers (~50–60 °C), extension by Taq polymerase (~72 °C); repeated cycles amplify DNA exponentially.

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

Bt cotton carries cry genes (cryIAc, cryIIAb) from Bacillus thuringiensis; the toxin is inactive protoxin until activated in the alkaline insect gut.

RNA interference is used against Meloidogyne incognita in tobacco by expressing a complementary dsRNA that silences the nematode's mRNA.

Insulin production: proinsulin has a C-peptide removed; Eli Lilly's Humulin was the first recombinant human insulin made in E. coli.

Population growth: exponential N_t = N_0 e^{rt}; logistic dN/dt = rN(K−N)/K with carrying capacity K.

Population interactions: mutualism (+,+), competition (−,−), predation and parasitism (+,−), commensalism (+,0), amensalism (−,0).

Ecological pyramids: number, biomass and energy; energy pyramids are always upright because ~10% of energy transfers between trophic levels.

Primary succession starts on bare rock or lava; pioneer species include lichens and mosses; climax community is the stable end point.

Nutrient cycles: gaseous (carbon, nitrogen) have atmospheric reservoirs; sedimentary (phosphorus) have rock reservoirs and no gaseous phase.

Ozone depletion is caused by CFCs releasing chlorine radicals that break O3 into O2; the Montreal Protocol limits CFC production.

Eutrophication: nutrient enrichment → algal bloom → oxygen depletion → fish kill; measured as BOD, which rises with organic pollution.

In-situ conservation: biosphere reserves, national parks, sanctuaries; ex-situ: zoos, seed banks, botanical gardens, cryopreservation.

Species–area relationship: log S = log C + Z log A, where Z is the slope (regression coefficient) of the line.

Prokaryotic cell: 70S ribosomes, no membrane-bound organelles, nucleoid with circular DNA, cell wall of peptidoglycan.

Eukaryotic organelles: mitochondria (ATP, own DNA, 70S ribosomes), chloroplast (photosynthesis), ER (protein/lipid synthesis), Golgi (packaging and secretion).

Fluid mosaic model (Singer and Nicolson): phospholipid bilayer with proteins, cholesterol and carbohydrates; membrane is selectively permeable.

Mitochondria and chloroplasts are semi-autonomous: they contain circular DNA and 70S ribosomes and divide by fission, supporting endosymbiotic theory.

Cell cycle: interphase G1, S (DNA replication), G2, then M phase; DNA content doubles in S phase while chromosome number stays constant.

Mitosis stages: prophase (chromosome condensation), metaphase (alignment at equator), anaphase (centromeres split, chromatids move to poles), telophase (nuclear envelope reforms).

Meiosis I is reductional (2n → n) with crossing over in pachytene of prophase I; meiosis II is equational.

Enzymes lower activation energy; they are specific, and activity rises with temperature and pH up to an optimum, then denatures.

Plasmolysis occurs when a plant cell loses water in a hypertonic solution; the protoplast shrinks away from the cell wall.

Photosynthesis: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O; light reactions in thylakoid membranes, Calvin cycle in stroma.

Light reaction products: ATP, NADPH and O2; O2 comes from photolysis of water at photosystem II.

Calvin cycle: CO2 fixed by RuBisCO onto RuBP → 3-PGA → reduction by ATP and NADPH → regeneration of RuBP.

C4 plants (maize, sugarcane) use PEP carboxylase with no photorespiration and have Kranz anatomy; C3 plants (rice, wheat) use RuBisCO and photorespire.

C4 pathway: mesophyll fixes CO2 into OAA → malate → bundle-sheath cells release CO2 for the Calvin cycle.

Transpiration pull (cohesion–tension theory) drives water up the xylem; transpiration rate falls with high humidity and rises with wind and temperature.

Stomatal opening: K+ ions enter guard cells, water follows osmotically, turgor rises and the stoma opens; ABA closes stomata under water stress.

Plant hormones: auxin (apical dominance, elongation), gibberellin (stem elongation, bolting), cytokinin (cell division), ABA (stress, dormancy), ethylene (ripening).

Mineral deficiency symptoms: N and Mg cause chlorosis (Mg is central to chlorophyll); Fe deficiency also causes chlorosis; Ca and B affect meristems.

Blood: plasma without clotting factors is serum; RBCs live ~120 days; platelets (thrombocytes) are cell fragments for clotting.

Cardiac cycle: SA node is the pacemaker; systole raises ventricular pressure and closes AV valves, producing the first heart sound.

Oxygen transport: haemoglobin binds O2 cooperatively giving a sigmoid dissociation curve; a right shift (higher CO2, lower pH, higher temperature) releases more O2 to tissues.

Nephron: glomerular filtration in the glomerulus → reabsorption in PCT → loop of Henle creates a medullary osmotic gradient → DCT and collecting duct respond to ADH.

ADH increases water reabsorption in the collecting duct, concentrating urine; aldosterone increases Na+ reabsorption; ANF lowers blood pressure.

Neuron: resting potential ~−70 mV from Na+/K+ pump; depolarisation by Na+ influx → action potential → repolarisation by K+ efflux; conduction is saltatory in myelinated axons.

Digestion: salivary amylase digests starch; pepsin (stomach, acidic) digests protein; trypsin and lipase act in the alkaline small intestine; bile emulsifies fats.

Respiration: inspiration is active (diaphragm and external intercostals contract); tidal volume ~500 mL, vital capacity ~3500–4500 mL in adults.

Hormones: insulin lowers blood glucose, glucagon raises it; thyroxine regulates basal metabolic rate; adrenaline triggers fight-or-flight responses.