Kerala Plus Two Biology (DHSE) cheat sheet

Double fertilisation: one male gamete fuses with egg (syngamy → zygote), the other with two polar nuclei (triple fusion → primary endosperm nucleus, 3n).

Endosperm development: free-nuclear first (in coconut, the liquid is free-nuclear endosperm), then cellular. In albuminous seeds (maize, castor) endosperm persists; in exalbuminous seeds (pea, groundnut) it is consumed.

Perisperm is persistent nucellus (e.g., black pepper, beet); it is diploid and distinct from endosperm.

Apomixis = asexual seed formation without fertilisation (e.g., Citrus, mango polyembryony); parthenocarpy = fruit without fertilisation (banana).

Human spermatogenesis: spermatogonia (2n) → primary spermatocyte (2n) → meiosis I → secondary spermatocyte (n) → meiosis II → spermatids (n) → spermiogenesis → sperm. Sertoli cells nourish; Leydig cells secrete testosterone.

Menstrual cycle: menstrual phase (days 1–5), proliferative/follicular (6–13), ovulatory (LH surge ~day 14), secretory/luteal (15–28). Corpus luteum secretes progesterone; if no pregnancy it degenerates into corpus albicans.

hCG, hPL, oestrogen, progesterone are produced by the placenta; hCG and hPL are placenta-specific. Relaxin is secreted by the corpus luteum near term.

Reproductive health: Saheli is a non-steroidal oral contraceptive (centchroman); MTP is safe in the first trimester (up to 12 weeks); ART includes IVF-ET, ZIFT, GIFT, and ICSI.

Law of segregation: alleles separate during gamete formation; test cross (cross with recessive homozygote) reveals genotype. Law of independent assortment applies only to genes on different chromosomes (or far apart on the same chromosome).

Incomplete dominance: heterozygote shows intermediate phenotype (Snapdragon flower colour). Codominance: both alleles expressed (AB blood group). Multiple alleles: ABO has three alleles (IA, IB, i).

Chromosomal disorders: Down syndrome = trisomy 21 (47 chromosomes); Klinefelter = XXY (47); Turner = XO (45).

DNA structure: antiparallel double helix, 10 bp per turn, pitch 3.4 nm, rise per bp 0.34 nm. A=T (2 H-bonds), G≡C (3 H-bonds).

DNA replication is semi-conservative (Meselson–Stahl); DNA polymerase adds nucleotides 5'→3' and needs a primer; leading strand continuous, lagging strand in Okazaki fragments joined by ligase.

Lac operon: inducer (lactose/allolactose) binds repressor, freeing operator; RNA polymerase transcribes z, y, a genes. Negative regulation by repressor; positive regulation by CAP-cAMP.

Hardy–Weinberg: p² + 2pq + q² = 1 and p + q = 1. A population in equilibrium has constant allele frequencies; disturbing forces are mutation, recombination, natural selection, genetic drift, and gene flow.

Evolution evidence: homologous organs (same origin, different function, e.g., forelimbs) indicate common ancestry; analogous organs (different origin, same function, e.g., wings of butterfly and bird) indicate convergent evolution. Industrial melanism in Biston betularia is a classic example of natural selection.

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

Malaria: female Anopheles is the vector; sporozoites enter via bite, multiply in liver, then infect RBCs; haemozoin release causes chill and fever. Quinine (from Cinchona) and chloroquine are antimalarials.

Immunity: innate (non-specific, present at birth) vs acquired (specific, memory). Active immunity = antibodies produced by the body (infection/vaccine); passive = ready-made antibodies (colostrum, antivenom).

AIDS: HIV is a retrovirus with reverse transcriptase; it infects helper T-lymphocytes (CD4); diagnosis by ELISA; treatment with anti-retroviral drugs; transmission by unsafe sex, infected blood, and mother-to-child.

Cancer: loss of contact inhibition; benign vs malignant (metastasising) tumours; carcinogens (physical, chemical, biological); detection by biopsy, CT, MRI; treatment by surgery, radiotherapy, chemotherapy, and immunotherapy (α-interferon).

Microbes in human welfare: Lactobacillus → curd; Saccharomyces cerevisiae → bread and ethanol; Aspergillus niger → citric acid; Clostridium butylicum → butyric acid; Trichoderma polysporum → cyclosporin A (immunosuppressant); Monascus purpureus → statins.

Sewage treatment: primary (physical removal of solids) and secondary (aerobic sludge digestion by microbes); biogas is produced by methanogens (e.g., Methanobacterium) in anaerobic sludge digesters.

Biocontrol and biofertilisers: Bacillus thuringiensis (Bt) and Trichoderma are biocontrol agents; Rhizobium (symbiotic), Azospirillum/Azotobacter (free-living), and mycorrhiza (Glomus) are biofertilisers.

Restriction enzymes (molecular scissors) cut palindromic sequences; EcoRI cuts G↓AATTC producing sticky ends; DNA ligase joins sticky ends. Vectors need an origin of replication, selectable marker, and recognition sites.

pBR322 has ampicillin and tetracycline resistance genes; insertional inactivation of a marker is used to select recombinants. Ti plasmid of Agrobacterium tumefaciens is a natural vector for plants.

PCR: denaturation (~94 °C), annealing (~50–60 °C), extension (~72 °C) with Taq polymerase; amplifies DNA exponentially. Gel electrophoresis separates DNA by size (towards anode).

Recombinant insulin (Humulin): A and B chains produced separately in E. coli and joined by disulphide bonds; earlier insulin from animal pancreas caused allergies.

Gene therapy: first clinical case was ADA deficiency (SCID) using retroviral vector and bone marrow cells; permanent cure requires bone marrow transplant.

RNA interference (RNAi): dsRNA silences a specific mRNA; used to make tobacco resistant to Meloidogyne incognita (nematode).

Bt crops: cry genes from Bacillus thuringiensis produce protoxin crystals; activated in the alkaline gut of insects (e.g., cryIAc for cotton bollworm, cryIIAb for corn borer).

ELISA and PCR are used for early disease detection; transgenic animals (e.g., Rosie, the first transgenic cow) produce human protein-enriched milk (α-lactalbumin).

Population growth: exponential N_t = N_0 e^{rt} (unlimited resources); logistic dN/dt = rN((K−N)/K) gives S-shaped curve with carrying capacity K.

Population interactions: mutualism (+,+), competition (−,−), predation (+,−), parasitism (+,−), commensalism (+,0), amensalism (−,0). Gause's competitive exclusion principle: two species cannot occupy the same niche indefinitely.

Ecosystem energy flow: 10% law (Lindeman); gross primary productivity (GPP) minus respiration = net primary productivity (NPP). Pyramid of energy is always upright; pyramid of biomass can be inverted (ocean).

Nutrient cycles: gaseous (carbon, nitrogen) and sedimentary (phosphorus, sulphur). Carbon is mostly stored in oceans and fossil fuels; phosphorus cycle has no gaseous phase.

Ecological succession: primary succession starts on bare rock (pioneer species = lichens); secondary succession follows disturbance (e.g., abandoned farmland). Hydrarch succession proceeds from hydric to mesic conditions.

Biodiversity hotspots in India: Western Ghats and Sri Lanka, Indo-Burma, Himalaya, Sundaland (Nicobar Islands). Hotspots have high endemism and habitat loss.

In situ conservation: biosphere reserves, national parks, wildlife sanctuaries, sacred groves. Ex situ: zoos, botanical gardens, seed banks, cryopreservation.

Species–area relationship: log S = log C + Z log A; Z (slope) is ~0.1–0.2 for small areas and ~0.6–1.2 for very large areas (e.g., entire continents).

Cell cycle: interphase (G1, S, G2) and M phase. DNA replicates in S phase; chromosomes are most condensed in metaphase; cytokinesis in animal cells by cleavage furrow, in plant cells by cell plate.

Meiosis I: prophase I has five substages (leptotene, zygotene, pachytene, diplotene, diakinesis); crossing over occurs at pachytene; synaptonemal complex forms at zygotene. Meiosis II separates sister chromatids.

Biomolecules: carbohydrates (mono-, di-, poly-), proteins (amino acids linked by peptide bonds), nucleic acids (nucleotides), lipids (fatty acids + glycerol). Enzymes are mostly proteins; ribozymes are RNA catalysts.

Protein structure: primary (sequence), secondary (α-helix, β-pleated sheet), tertiary (3D folding), quaternary (multiple polypeptides, e.g., haemoglobin). Denaturation destroys 3D structure but not primary sequence.

Enzyme kinetics: Michaelis–Menten constant (K_m) is the substrate concentration at half V_max; low K_m means high affinity. Competitive inhibitors increase apparent K_m; non-competitive inhibitors decrease V_max.

Nucleic acids: DNA has deoxyribose and thymine; RNA has ribose and uracil. DNA is double-stranded (except in some viruses); RNA is usually single-stranded.

Cell organelles: mitochondria (ATP production, own DNA), chloroplast (photosynthesis, own DNA), ribosomes (protein synthesis), Golgi (packaging), lysosomes (hydrolytic enzymes, 'suicidal bags').

Prokaryotic vs eukaryotic: prokaryotes lack a nucleus and membrane-bound organelles; have 70S ribosomes; eukaryotes have 80S ribosomes and a true nucleus.

Water potential (Ψ_w) = solute potential (Ψ_s) + pressure potential (Ψ_p). Pure water has Ψ_w = 0; solutes lower Ψ_s (negative); turgor pressure raises Ψ_p (positive).

Transpiration pull–cohesion–tension theory explains water ascent in xylem; root pressure contributes at night. Stomatal opening: K+ influx lowers Ψ_w, water enters guard cells, they become turgid.

Mineral nutrition: macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Hydroponics is growing plants in nutrient solution. Nitrogen fixation by Rhizobium (symbiotic) and Azotobacter (free-living).

Photosynthesis: light reaction in thylakoids (PSII → PSI, non-cyclic photophosphorylation produces ATP and NADPH); dark reaction (Calvin cycle) in stroma. C4 plants use PEP carboxylase and have Kranz anatomy; photorespiration is absent in C4.

C3 vs C4: C3 plants (rice, wheat) show photorespiration; C4 plants (maize, sugarcane) have higher productivity at high temperature. CAM plants (succulents) fix CO2 at night.

Respiration: glycolysis (cytoplasm, net 2 ATP + 2 NADH), Krebs cycle (mitochondrial matrix, 2 ATP + 6 NADH + 2 FADH2 per glucose), oxidative phosphorylation (inner membrane, ~26–28 ATP). RQ = CO2 released / O2 consumed; RQ = 1 for carbohydrates, <1 for fats, >1 for organic acids.

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

Photoperiodism: short-day plants (flower when day length < critical period, e.g., rice), long-day plants (e.g., wheat), day-neutral plants (e.g., tomato). Vernalisation: cold treatment promotes flowering (e.g., winter wheat).

Plant tissues: meristematic (apical, lateral, intercalary) and permanent (simple: parenchyma, collenchyma, sclerenchyma; complex: xylem, phloem). Xylem conducts water upward; phloem transports food bidirectionally.

Animal tissues: epithelial (covering, lining, glandular), connective (loose, dense, cartilage, bone, blood), muscular (skeletal, smooth, cardiac), neural. Skeletal muscle is striated and voluntary; cardiac is striated and involuntary.

Cockroach: chitinous exoskeleton, 3 pairs of legs, 2 pairs of wings, malpighian tubules for excretion, tracheal respiration, open circulatory system, dioecious with sexual dimorphism.

Frog: amphibian, three-chambered heart, external fertilisation, metamorphosis; Rana tigrina is common. Earthworm: closed circulatory system, nephridia for excretion, hermaphrodite.

Morphology of flowering plants: root (tap vs fibrous), stem (nodes, internodes), leaf (petiole, lamina, venation), flower (calyx, corolla, androecium, gynoecium), fruit, seed.

Placentation types: marginal (pea), axile (China rose, tomato), parietal (mustard, argemone), free central (Dianthus, Primrose), basal (sunflower, marigold).

Inflorescence: racemose (indefinite, main axis continues to grow) vs cymose (definite, main axis terminates in a flower).

Aestivation: valvate (calyx of mustard), twisted (cotton), imbricate (Cassia), vexillary (pea).

Five-kingdom classification (Whittaker): Monera, Protista, Fungi, Plantae, Animalia. Three domains (Carl Woese): Bacteria, Archaea, Eukarya.

Binomial nomenclature rules (ICBN/ICZN): genus capitalised, species lowercase, italicised in print, underlined in handwriting; author name abbreviated after species.

Algae: Chlorophyceae (green, chlorophyll a & b), Phaeophyceae (brown, fucoxanthin), Rhodophyceae (red, phycoerythrin). Bryophytes are amphibians of the plant kingdom; pteridophytes are first vascular plants.

Gymnosperms: naked seeds, no ovary; e.g., Cycas, Pinus, Ginkgo. Angiosperms: enclosed seeds, double fertilisation; dicots vs monocots (cotyledons, venation, vascular bundles).

Fungi: saprophytic, parasitic, symbiotic (lichens = algae + fungi; mycorrhiza = fungi + roots). Cell wall of chitin; reproduce by spores (conidia, sporangiospores, basidiospores).

Animal kingdom phyla: Porifera (sponges, choanocytes), Cnidaria (coelenterata, nematocysts), Platyhelminthes (flatworms), Nematoda (roundworms), Annelida (segmented worms), Arthropoda (largest phylum), Mollusca, Echinodermata (water vascular system), Chordata (notochord, dorsal hollow nerve cord).

Chordates: Urochordata (notochord in larva), Cephalochordata (notochord throughout), Vertebrata (notochord replaced by vertebral column).

Viruses: non-cellular, obligate intracellular parasites; contain DNA or RNA, not both; capsid of protein subunits (capsomeres); e.g., TMV (RNA), bacteriophage (DNA). Viroids: free RNA without capsid (potato spindle tuber).