Q4. Why does the loss or gain of a segment of DNA alter a chromosome?
Because one DNA helix runs continuously from one end of a chromatid to the other, in a highly supercoiled form; losing or gaining a segment of it changes the chromosome, and the genes on it (NCERT page 72).
Q5. Using NCERT’s sentence RAM HAS RED CAP, what does inserting two letters, BI, after HAS give? Is the frame shifted?
RAM HAS BIR EDC AP; yes, every word after the insertion changes, so the frame is shifted (NCERT page 97).
Q6. Under what condition does sickle-cell haemoglobin change the shape of red blood cells, and to what shape?
Under low oxygen tension the mutant haemoglobin polymerises, and the cell changes from a biconcave disc to an elongated sickle-like structure (NCERT page 75).
Q7. Assertion: a deletion of three adjacent bases inside one codon leaves the codons after it unchanged. Reason: it removes one whole codon. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: deleting three bases removes one codon (one amino acid), and the reading frame is unaltered from that point onwards (NCERT pages 97–98).
Q8. A two-base insertion occurs in an untranslated region. Must it shift a protein’s reading frame?
No. The untranslated regions are not read as codons, so an insertion there shifts no protein’s reading frame; but the UTRs are needed for efficient translation, so it can still have other effects (reasoned from NCERT Chapter 5, pages 97–99).