Q4. Which amino acids have only one codon in NCERT Table 5.1, and which have six?
One codon: methionine (AUG) and tryptophan (UGG). Six codons: leucine, serine and arginine (NCERT Table 5.1, page 96).
Q5. How many codons in all code for leucine, and why does this make the code degenerate?
Six: UUA, UUG, CUU, CUC, CUA and CUG. Because one amino acid has more than one codon, the code is degenerate (NCERT page 96).
Q6. An mRNA reads AUG UUU GGC AAA UAA. Three bases, CCC, are inserted right after AUG. Write both polypeptides; is the frame shifted?
Before: Met-Phe-Gly-Lys. After: AUG CCC UUU GGC AAA UAA, Met-Pro-Phe-Gly-Lys: one codon and one amino acid (Pro) are added, and the frame after it is unaltered (NCERT Table 5.1, page 96; pages 97–98).
Q7. What did tRNA use to be called, and what shape does it really have?
sRNA, soluble RNA; its secondary structure is drawn as a clover-leaf, but the actual molecule is compact and looks like an inverted L (NCERT page 98).
Q8. Why did Crick propose an adapter molecule at all?
Because amino acids have no structural specialities that could read the code; something had to read a codon on one side and bind a specific amino acid on the other (NCERT page 98).