Q4. Using NCERT’s 20-minute generation model, what would be the proportions of hybrid and light DNA after 100 minutes in the N medium?
100 ÷ 20 = 5 generations, so 2 = 32 molecules, of which 2 are hybrid and 30 light: 1/16 (6.25 per cent) hybrid and 15/16 light (reasoned from NCERT pages 89–90).
Q5. Suppose E. coli grown for many generations in N medium is moved to N medium. What DNA would you find after one generation and after two?
Our reasoning from the semiconservative scheme (NCERT page 88): after one generation all the DNA is hybrid (one old N strand, one new N strand); after two generations, half is hybrid and half is heavy (both strands N).
Q6. Suppose instead that each parental double helix stayed whole and the copy were made entirely of new strands. What would Meselson and Stahl have seen one generation after the transfer to N, and what did they actually see?
Our reasoning: one heavy band (the old helices) and one light band (the new copies), with no hybrid DNA. They actually found only hybrid DNA after one generation (NCERT page 89), which this idea cannot explain and the semiconservative scheme predicts.
Q7. Why are the two strands of a long DNA molecule not separated along their whole length before replication?
Separating them along the entire length would need a very high amount of energy, so replication takes place within a small opening of the helix, the replication fork (NCERT page 90).
Q8. Why does a piece of DNA that must be propagated in recombinant DNA work need a vector?
DNA polymerases cannot initiate replication on their own, and replication starts only at an origin of replication; the vector provides that origin (NCERT pages 90–91).
Q9. What results if a cell fails to divide after its DNA has replicated?
Polyploidy, a chromosomal anomaly (NCERT page 91). Beyond the NCERT text: it arises when complete chromosome sets accumulate in a nucleus; a temporary G pause is not polyploidy, and failed cytokinesis may at first give a binucleate cell, so DNA content and chromosome-set number must be distinguished (reasoned from the cell cycle, NCERT Class 11 Biology, Chapter 10).