BSEH SS Biology cheat sheet

Human spermatogenesis: spermatogonia (2n) → primary spermatocyte (2n) → secondary spermatocyte (n) after meiosis I → spermatids (n) after meiosis II → sperm; one primary spermatocyte yields four sperm.

Oogenesis is unequal: one primary oocyte yields one ovum plus three polar bodies; meiosis II completes only after fertilization, and the second polar body is extruded at that point.

Menstrual cycle: menstrual phase (days 1-5), follicular/proliferative phase driven by FSH and estrogen, LH surge triggers ovulation near day 14, luteal/secretory phase driven by progesterone from the corpus luteum.

Double fertilization in angiosperms: one male gamete fuses with the egg (2n zygote) and the second fuses with two polar nuclei to form triploid (3n) primary endosperm nucleus.

Embryo sac of a typical angiosperm is 8-nucleate and 7-celled: 3 antipodals, 2 synergids, 1 egg, and a central cell with 2 polar nuclei.

Placenta hormones: hCG maintains the corpus luteum in early pregnancy; hPL, estrogen, and progesterone are produced later, with progesterone maintaining the endometrium.

Contraceptive modes: barrier (condom, diaphragm), hormonal (combined pills suppress the LH surge), IUD (CuT releases copper ions that reduce sperm motility), surgical (vasectomy, tubal ligation).

Gametogenesis timing: spermatogenesis begins at puberty and is continuous; oogenesis begins in the fetus, arrests at prophase I, and resumes at each ovulation.

Apomixis produces seeds without fertilization and preserves hybrid genotype; parthenocarpy is fruit development without fertilization, as in banana.

Mendel's law of segregation: alleles separate during gamete formation; the law of independent assortment applies only to genes on different chromosomes or far apart on the same chromosome.

Test cross: cross an unknown dominant phenotype with a homozygous recessive to reveal its genotype; a 1:1 ratio indicates heterozygosity.

Dihybrid F2 phenotypic ratio 9:3:3:1 requires independent assortment; linkage distorts this ratio toward parental classes.

Hardy-Weinberg: p + q = 1 and p² + 2pq + q² = 1; allele frequencies stay constant only with no mutation, no migration, random mating, no selection, and large population.

Sex-linked inheritance: a carrier mother (X^A X^a) × normal father gives 50% of sons affected; fathers pass X-linked alleles to all daughters and no sons.

Human genetic disorders: Down syndrome is trisomy 21; Turner is 45,X; Klinefelter is 47,XXY; sickle-cell anemia is autosomal recessive (Glu→Val at position 6 of β-globin).

Evolution evidence classes: paleontological (fossils), anatomical (homologous vs analogous organs), embryological, and molecular (DNA/protein sequence similarity).

Natural selection requires heritable variation, differential reproductive success, and change in allele frequency across generations; it acts on phenotypes, not directly on genotypes.

Speciation: allopatric requires geographic isolation; sympatric occurs without geographic separation via polyploidy or behavioral/habitat isolation.

Pathogen-disease pairs: Plasmodium → malaria; Wuchereria bancrofti → filariasis; Ascaris → ascariasis; Trichophyton → ringworm; HIV → AIDS; Salmonella typhi → typhoid.

Immunity types: innate (barriers, phagocytes, inflammation) is non-specific; acquired is specific and split into antibody-mediated (B cells) and cell-mediated (T cells).

Vaccination induces active acquired immunity by exposing the body to weakened or inactivated antigen, generating memory B and T cells.

Antibiotics such as penicillin target bacterial cell wall synthesis and are ineffective against viruses, which lack cell walls and their own metabolism.

Plant breeding steps: collection of variability, evaluation and selection of parents, cross-hybridization, selection of superior recombinants, and testing/release.

Green Revolution relied on semi-dwarf, high-yield wheat and rice varieties with increased harvest index and responsiveness to fertilizer and irrigation.

Biofortification raises micronutrient content of crops, for example iron-rich bajra, vitamin A-rich carrot, and protein-rich beans.

Apiculture and pisciculture: Apis indica is a common honeybee species; composite fish culture mixes surface, column, and bottom feeders to use pond resources fully.

Single-cell protein and biogas: Spirulina is a protein-rich microbial biomass; biogas from methanogens (e.g., Methanobacterium) is mainly methane.

Recombinant DNA core steps: isolate DNA, cut with restriction endonuclease, ligate into a vector with DNA ligase, transform a host, and select transformants.

Restriction enzymes such as EcoRI cut palindromic sequences producing sticky ends; DNA ligase joins compatible sticky ends.

pBR322 features: origin of replication, ampicillin-resistance gene, tetracycline-resistance gene, and unique restriction sites used as selectable markers.

Agrobacterium tumefaciens Ti plasmid is the standard vector for dicot transformation; the disarmed plasmid delivers genes without tumor formation.

PCR cycle: denaturation (~94 °C), primer annealing (~50-60 °C), extension by Taq polymerase (~72 °C); each cycle doubles the target DNA.

Bt cotton carries cry genes (e.g., cryIAc, cryIIAb) encoding insecticidal proteins active against bollworm larvae.

RNA interference: introducing a complementary RNA silences a specific nematode gene, used against Meloidogyne incognita in tobacco.

Insulin production: proinsulin is expressed in E. coli as fused chains, then the C-peptide is removed to yield mature human insulin.

Gene therapy delivers a functional gene to correct a defect; the first approved case was ADA (adenosine deaminase) deficiency, using lymphocytes.

Population growth: exponential is dN/dt = rN; logistic is dN/dt = rN(K−N)/K, where K is carrying capacity and growth slows as N approaches K.

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

Ecological pyramid types: number, biomass, and energy; the energy pyramid is always upright because of the 10% transfer rule between trophic levels.

Primary productivity is the rate of organic matter synthesis by producers; net primary productivity equals gross primary productivity minus respiration.

Nutrient cycles: carbon is gaseous (atmosphere), phosphorus is sedimentary (rock), and nitrogen fixation converts N₂ to ammonia via nitrogenase.

Succession: primary succession starts on bare substrate with pioneer species such as lichens; secondary succession starts where soil already exists.

Biodiversity hotspots are regions with high endemism and high habitat loss; India contains the Western Ghats and the Eastern Himalayas hotspots.

Ozone depletion is driven by chlorofluorocarbons releasing chlorine that breaks O₃; the Montreal Protocol phased out such substances.

In situ conservation includes national parks, biosphere reserves, and sacred groves; ex situ includes zoos, seed banks, and botanical gardens.

Prokaryotic cells lack a membrane-bound nucleus and organelles; they have 70S ribosomes, a nucleoid, and often a peptidoglycan cell wall.

Eukaryotic organelles: mitochondria and chloroplasts are double-membraned; lysosomes are single-membraned and contain hydrolytic enzymes at acidic pH.

Fluid mosaic model: the plasma membrane is a phospholipid bilayer with embedded and peripheral proteins; cholesterol modulates fluidity.

Enzymes lower activation energy without altering reaction equilibrium; they are specific, and their activity depends on pH and temperature.

Competitive inhibitors bind the active site and are overcome by raising substrate concentration; non-competitive inhibitors bind elsewhere and are not.

Glycolysis occurs in the cytoplasm and nets 2 ATP and 2 NADH per glucose; the Krebs cycle runs in the mitochondrial matrix.

Oxidative phosphorylation: electron transport chain on the inner mitochondrial membrane pumps protons; ATP synthase makes roughly 34 ATP per glucose.

Photosynthesis: light reactions in thylakoid membranes split water and produce ATP and NADPH; the Calvin cycle in the stroma fixes CO₂ via RuBisCO.

Cell cycle: interphase has G1, S (DNA replication), and G2; mitosis separates sister chromatids in anaphase, and cytokinesis follows telophase.

Water potential: ψ = ψs + ψp; water moves from higher to lower water potential, and pure water at atmospheric pressure has ψ = 0.

Transpiration pull drives the ascent of sap through xylem; cohesion and adhesion maintain the continuous water column.

Stomatal regulation: guard cells open when turgid (K⁺ influx) and close under water stress via abscisic acid.

Mineral deficiency symptoms: Mg and N deficiency cause chlorosis; Ca deficiency causes necrosis of young leaves because Ca is phloem-immobile.

Photosynthetic pathways: C4 plants fix CO₂ into oxaloacetate via PEP carboxylase and avoid photorespiration; CAM plants fix CO₂ at night.

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

Photoperiodism: short-day plants flower when the night exceeds a critical length; long-day plants flower when the night is shorter than the critical length.

Nitrogen fixation requires nitrogenase, which is oxygen-sensitive; leghaemoglobin in root nodules protects the enzyme from oxygen.

Seed germination requires water imbibition, oxygen, and suitable temperature; gibberellin breaks dormancy and induces α-amylase for starch breakdown.

Nephron function: filtration in the glomerulus, reabsorption of glucose and amino acids in the proximal tubule, and water reabsorption via ADH in the collecting duct.

Counter-current mechanism in the loop of Henle builds the medullary osmotic gradient, allowing urine concentration up to about four times plasma.

Cardiac cycle: SA node sets the rhythm; systole ejects blood, and the cardiac output equals heart rate times stroke volume.

Respiratory transport: about 97% of oxygen is carried by hemoglobin; CO₂ travels mainly as bicarbonate, with carbonic anhydrase catalyzing its formation.

Digestion: salivary amylase acts on starch; pepsin digests protein in acidic stomach; trypsin and lipase act in the alkaline small intestine.

Neural signaling: resting potential is maintained by the Na⁺/K⁺ pump; depolarization opens Na⁺ channels, and repolarization opens K⁺ channels.

Muscle contraction: the sliding filament model; Ca²⁺ binds troponin, exposing actin-binding sites so myosin heads form cross-bridges using ATP.

Hormone axes: the hypothalamus releases tropic hormones, the anterior pituitary releases stimulating hormones, and the target gland secretes its hormone under negative feedback.

Blood glucose regulation: insulin lowers glucose by promoting cellular uptake and glycogen synthesis; glucagon raises it by stimulating glycogenolysis.