Mutations can also be caused chemically, and nitrous acid is one of the most potent chemical mutagens. One explanation that has been suggested for the mutagenic effect of nitrous acid is the deamination reactions that it causes with purines and pyrimidines bearing amino groups. When, for example, an adenine-containing nucleotide is treated with nitrous acid, it is converted to a hypoxanthine derivative:

(a) Basing your answer on reactions you have seen before, what are likely intermediates in the adenine †’ hypoxanthine interconversion?
(b) Adenine normally pairs with thymine in DNA, but hypoxanthine pairs with cytosine. Show the hydrogen bonds of a hypoxanthine-cytosine base pair.
(c) Show what errors an adenine †’ hypoxanthine interconversion would generate in DNA through two replications.

NH2 N HNO2 Hypoxanthine nucleotide Adenine nucleotide