Practice question

Purines and pyrimidines normally exist in keto-amino tautomers that form canonical Watson-Crick pairs G-C and A-T. Rare enol-imino tautomers relocate hydrogen bonding donors and acceptors altering pairing face. A transient tautomeric shift converts guanine to enol form presenting hydrogen bonding pattern similar to adenine, allowing stable pairing with thymine instead of cytosine. If uncorrected before next round of replication, replication past this mispair fixes G-C to A-T transition mutation. Phenomenon underlies spontaneous point mutations and explains mutagenesis by base analogs like bromouracil.