NEB Class 12 Biology cheat sheet

Biology (Grade 12) - Core Content Reference

• Reproduction: asexual (binary fission, budding, fragmentation, spore formation, vegetative propagation) produces genetically identical offspring; sexual reproduction involves meiosis and syngamy and generates variation.

• Human male reproductive system: testes contain seminiferous tubules (spermatogenesis) and Leydig cells (testosterone); Sertoli cells nourish developing sperm; epididymis, vas deferens, seminal vesicles, prostate and Cowper's glands contribute to semen.

• Human female cycle: FSH stimulates follicle growth and estrogen secretion; the LH surge around day 14 triggers ovulation; the corpus luteum secretes progesterone, which maintains the endometrium; falling progesterone causes menstruation.

• Oogenesis yields one functional ovum plus polar bodies; meiosis II completes only after sperm entry. Fertilization normally occurs in the ampullary-isthmic junction of the fallopian tube.

• Embryonic development: cleavage (morula → blastocyst), implantation around day 6-7, gastrulation forming three germ layers, and placenta formation from chorionic villi and uterine tissue. hCG, hPL, estrogen and progesterone support pregnancy.

• Mendelian genetics: law of segregation (alleles separate in meiosis) and law of independent assortment (genes on different chromosomes assort independently). Test cross distinguishes homozygous dominant from heterozygous.

• Deviations from Mendel: incomplete dominance (e.g., Snapdragon flower color), codominance (ABO blood groups, multiple alleles), pleiotropy (single gene, many traits), polygenic inheritance (skin color), and sex-linked inheritance (hemophilia, color blindness on the X chromosome).

• Molecular basis: DNA is antiparallel double helix with A-T and G-C base pairing; semiconservative replication; transcription then translation via mRNA, tRNA and ribosomes following the genetic code (triplet, degenerate, non-overlapping).

• Evolution: Darwin's natural selection, Lamarck's discredited inheritance of acquired characters, Hardy-Weinberg equilibrium (p² + 2pq + q² = 1), and evidence from fossils, homologous and analogous organs, and molecular similarity.