PSEB SS Biology cheat sheet

Sexual reproduction in angiosperms: microsporogenesis yields pollen grains from microspore mother cells via meiosis; megasporogenesis yields the embryo sac (typically 7 cells, 8 nuclei) inside the ovule.

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

Human spermatogenesis produces four functional sperm per primary spermatocyte; oogenesis produces one ovum plus three polar bodies, and meiosis II completes only after sperm entry.

Menstrual cycle phases: menstrual (days 1–5), follicular/proliferative (rising oestrogen, LH surge ~day 14), ovulation, then luteal/secretory (progesterone from corpus luteum).

Hormonal control: FSH stimulates follicle growth and spermatogenesis-supporting Sertoli cells; LH triggers ovulation and Leydig-cell testosterone secretion.

Reproductive health: ART terms - IVF (fertilisation outside body, embryo transfer), ZIFT (zygote/early embryo into fallopian tube), GIFT (gamete intra-fallopian transfer), IUI (semen into uterus).

Mendelian genetics: monohybrid F2 phenotypic ratio 3:1, genotypic 1:2:1; dihybrid F2 9:3:3:1; test cross with recessive homozygote reveals an unknown dominant genotype.

Human genetics: ABO blood groups show multiple alleles (IA, IB, i) and codominance; sex-linked recessive traits (haemophilia, colour blindness) appear more in males because one X carries the allele.

Molecular basis: DNA replication is semi-conservative; transcription uses RNA polymerase; translation reads mRNA codons 5'→3' against tRNA anticodons, with AUG as start and UAA/UAG/UGA as stop codons.

Evolution: Hardy–Weinberg equilibrium p² + 2pq + q² = 1 holds for allele frequencies p and q in a non-evolving population; deviations indicate selection, drift, mutation, migration or non-random mating.

Evidence for evolution includes homologous organs (common ancestry), analogous organs (convergent evolution), fossils, and molecular homology; Darwin's natural selection requires heritable variation and differential reproductive success.

Human health: pathogens - Plasmodium (malaria, protozoan), Wuchereria (filariasis), Salmonella typhi (typhoid), HIV (AIDS, retrovirus); immunity is antibody-mediated (B cells) or cell-mediated (T cells).

Biotechnology: recombinant DNA steps - restriction enzyme digestion, ligation into vector, transformation, selection of transformants, and expression; EcoRI cuts palindromic GAATTC; Ti plasmid of Agrobacterium is a plant vector.

PCR cycles use denaturation (~94 °C), primer annealing (~50–60 °C) and extension (~72 °C, Taq polymerase); gel electrophoresis separates DNA by size toward the anode (DNA is negatively charged).

Ecology: population growth - exponential dN/dt = rN; logistic dN/dt = rN(K−N)/K; energy flow is unidirectional with roughly 10% transfer between trophic levels; pyramids may be inverted in aquatic ecosystems.

Biodiversity: hotspots are regions of high endemism and high threat; in-situ conservation includes national parks and biosphere reserves; ex-situ includes zoos, seed banks and cryopreservation.

Microbes in human welfare: Lactobacillus for curd, Saccharomyces for bread/ethanol, Aspergillus niger for citric acid, Penicillium for penicillin, and Trichoderma/Glomus as biofertilisers or biocontrol agents.