NSEB cheat sheet

Biology (CBSE Class 12 syllabus)

• Sexual reproduction in angiosperms: microsporogenesis yields pollen grains from microspore mother cells by meiosis; the pollen tube enters the ovule through micropyle (porogamy), chalaza (chalazogamy), or integument (mesogamy), and double fertilization produces a diploid zygote plus triploid primary endosperm nucleus.

• Human reproduction: spermatogenesis produces four haploid sperm from one primary spermatocyte, while oogenesis yields one ovum and three polar bodies; fertilization occurs in the ampullary-isthmic junction of the fallopian tube, and implantation follows about 6-7 days after fertilization.

• Mendelian genetics: monohybrid F2 phenotypic ratio is 3:1 and genotypic ratio 1:2:1; dihybrid F2 phenotypic ratio is 9:3:3:1 only when genes assort independently (unlinked); test cross of a heterozygote gives 1:1.

• Molecular basis of inheritance: DNA replication is semiconservative (Meselson-Stahl); in a dsDNA sample A=T and G=C, so (A+G)=(T+C); the lac operon is inducible, switched on when lactose binds the repressor and RNA polymerase transcribes lacZ, lacY, lacA.

• Evolution: Hardy-Weinberg equilibrium requires no mutation, no migration, random mating, no selection and large population; p + q = 1 and p^2 + 2pq + q^2 = 1; the frequency of heterozygotes is 2pq and of recessive homozygotes q^2.

• Human health and disease: HIV is a retrovirus whose reverse transcriptase makes DNA from RNA; it infects helper T-lymphocytes, and AIDS is diagnosed by ELISA followed by Western blot; malaria is caused by Plasmodium species transmitted by female Anopheles.

• Biotechnology: restriction endonucleases cut at palindromic recognition sequences producing sticky ends; EcoRI recognizes GAATTC; Ti plasmid of Agrobacterium tumefaciens is a natural vector for plant transformation; PCR uses denaturation, annealing and extension cycles.

• Ecology: energy flow is unidirectional and follows the 10 percent law between trophic levels; gross primary productivity minus respiration equals net primary productivity; population growth is exponential (dN/dt = rN) or logistic (dN/dt = rN(K-N)/K).

• Human physiology: the nephron's counter-current mechanism in the loop of Henle maintains a medullary osmotic gradient; the SA node of the right atrium sets the heart's rhythm; oxygen dissociation from hemoglobin is favoured by low pH (Bohr effect) and high temperature.