Practice question

Tautomeric shift is transient isomerization where hydrogen relocates within nitrogenous base converting keto to enol or amino to imino forms, changing hydrogen-bond donor-acceptor pattern. Rare tautomer can form non-Watson-Crick pair during replication, for example enol guanine pairs with thymine instead of cytosine. If proofreading fails to excise, next replication round incorporates adenine opposite thymine, fixing single nucleotide substitution. Thus tautomerization provides classic molecular basis for spontaneous point mutations, both transitions and occasional transversions, without altering backbone. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.