Task 1. When nitrous acid acts on DNA molecules, cytosine is replaced by guanine. Determine the type of mutation. Explain the mechanism. What structure will a section of the synthesized protein have if a polypeptide with the following amino acid sequence was to be formed: ser - ile - tre - proser, but all cytosine nucleotides of the corresponding DNA region underwent the specified chemical transformation? Determine the type of mutation.
Nitrous acid reacts with bases that contain amino groups leading to deamination. Adenine is oxidatively deaminated to hypoxanthine, cytosine to uracil, and guanine to xanthine. As a result, nitrous acid leads to the substitution of one base pair for another causing mismatch pairing. The replacement of cytosine to guanine is a transversion mutation (the replacement of a pyrimidine by a purine).
As ser is encoded by the codon UCA, the mutation will lead to the formation of stop codon UGA. As a result, the polypeptide will not be synthesized and the protein will lack the required structure. The type of mutation is called a nonsense mutation.