Practice question

Incorporated 5-bromouracil normally mimics thymine pairing with adenine via keto tautomer during first replication. However bromine substitution stabilizes rare enol tautomer that presents hydrogen bonding pattern complementary to guanine, allowing G:BrU pairing. During next round guanine templates cytosine, converting original A:T base pair where BrU replaced thymine into G:C pair. Net outcome is A:T→G:C transition opposite to spontaneous deamination direction, explaining bromouracil's specific mutational spectrum exploited to map gene fine structure in rII locus experiments. This principle illustrates essential molecular mechanisms governing replication fidelity and mutation fixation relevant for exam interpretation.