a. It is impossible. This is because; the sequence 5'-UAA-3' does not comprise the nucleotides C or G. So, transition to a GC pair in the deoxyribonucleic acid (DNA) cannot result in the sequence 5'-UAA-3'.
5'-UGA-3' has the deoxyribonucleic acid antisense-strand sequence of 3'-ACT-5'.
5'-UAG-3' has the deoxyribonucleic acid antisense-strand sequence of 3'-ATC-5'.
Transition can happen to both the stop codons UGA and UAG from the non-mutant sequence of 3'-ATT-5'.
But, a deoxyribonucleic acid sequence of 3'-ATT-5' results in a ribonucleic acid sequence of 5'-UAA-3'. In this case, UAA acts as stop codon.
b. Yes, because 5'-UGG-3' is a wild-type strain. Hydroxylamine causes nonsense mutations in this wild-type strain and changes into UAG. It codes for tryptophan which is 5'- UAG-3'.
c. No, HA treatment cannot revert nonsense mutations. In the three stop codons, namely UAG, UAA, and UGA the nucleotide that can be treated is G. If we take the example of UAG, replacing G with and A would give rise to 5'-UAA-3' which is a stop codon, So, HA treatment cannot cause reversion in the nonsense mutations.