Practice question

Cytosine undergoes hydrolytic deamination at C4 position removing exocyclic amino group, producing uracil that normally resides exclusively in RNA. In DNA context, uracil pairs preferentially with adenine like thymine, so if replication proceeds before uracil-DNA glycosylase removes uracil via base excision repair, original G:C pair becomes A:U then A:T after second round, yielding C→T transition. Methylated 5-methylcytosine deaminates to thymine creating T:G mismatch that is less efficiently repaired, explaining CpG hypermutability. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.