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NSEB study materials
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NSEB: Biology: CBSE Class 11–12 Content Pushed to Olympiad Depth

NSEB · Work through your exam, one topic at a time.

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Biology: CBSE Class 11–12 Content Pushed to Olympiad Depth

The Biology domain of NSEB samples the entire CBSE Class 11–12 syllabus - diversity of organisms, plant and animal physiology, cell biology, genetics, molecular biology, biotechnology and ecology - but tests it a step beyond the textbook: numerical reasoning, experimental interpretation and links across chapters. The paper rewards a candidate who can take a familiar CBSE statement and push it one inference further.

Concepts that recur

  • Chemiosmosis unifies bioenergetics. Mitochondrial and chloroplast ATP synthases both run on a proton gradient; uncouplers such as DNP collapse the gradient, so electron transport continues while ATP synthesis stops.
  • Enzyme kinetics. Km is the substrate concentration giving half Vmax and is inversely related to affinity. A competitive inhibitor raises apparent Km and leaves Vmax unchanged; a non-competitive inhibitor lowers Vmax.
  • Mendelian ratios break. Incomplete dominance gives 1:2:1 phenotypes, codominance expresses both alleles, and a recessive lethal allele (Ay in mice) turns a 3:1 into 2:1.
  • Linkage overrides assortment. Recombination frequency = recombinants/total × 100, numerically equal to map distance in centimorgans, and never exceeds 50%.

Worked example

A testcross AaBb × aabb gives 1000 progeny: 400 AB, 400 ab, 100 Ab, 100 aB. Identify the parental classes first - the two most frequent, AB and ab - which shows the dominant alleles are in coupling (cis) in the heterozygote. Recombinants are the Ab and aB classes, 200 in total, so recombination frequency = 200/1000 × 100 = 20%, meaning the loci lie 20 cM apart. Had the parental classes been Ab and aB, the same 20 cM figure would hold but the arrangement would be repulsion (trans).

Traps that cost marks

  • Assuming 9:3:3:1 whenever two genes appear - check for linkage, lethality or epistasis first.
  • Confusing recombination frequency with the frequency of one recombinant gamete; RF adds both recombinant classes.
  • Reading a rightward shift of the oxygen dissociation curve as increased affinity - it means decreased affinity and easier unloading (Bohr effect).
  • Mixing gross and net primary productivity; net = gross minus respiration.