Question: Exposure to nitrous acid (see Section 19-16), sometimes found in cells, can convert cytosine to uracil.

1. Propose a mechanism for this conversion.
2. Explain how this conversion would be mutagenic upon replication.
3. DNA generally includes thymine, rather than uracil(found in RNA). Based on this fact, explain why the nitrous acid-induced mutation of cytosine to uracil is more easily repaired in DNA than it is in RNA.

Nitrous acid

As diazonium is unstable, thus nitrogen gas leaves quickly, driving the reaction forward. Then, an attack of water as nucleophile occurs at the carbon center. Further on proton abstraction by water, the enol tautomer of uracil forms, which gets converted into its more stable keto tautomer.

The mechanism for the conversion

Generally, in base pairing, cytosine pairs up with guanine. If cytosine is converted to uracil, each replication will not carry over the complement of cytosine, i.e., guanine. Instead, it will carry the complement of uracil, i.e., adenine. Thus, cytosine conversion to uracil will be mutagenic as the wrong base is inserted in a nucleic acid chain

In RNA, cytosine conversion to uracil is not detected as a problem because uracil, which is a base, is normally found in RNA and goes unrepaired. In DNA, thymine is found instead of uracil with an extra methyl group. DNA repair enzymes can detect cytosine conversion to uracil as mutation and correct it.