Direct Repair

Direct-repair mechanisms do not replace altered nu-cleotides but instead change them back into their original (correct) structures. One of the best-characterized direct-repair mechanisms is photoreactivation of UV-induced pyrimidine dimers. E. coli and some eukaryotic cells possess an enzyme called photolyase, which uses energy captured from light to break the covalent bonds that link the pyrim-idines in a dimer.

Direct repair also corrects 06-methylguanine, an alkyla-tion product of guanine that pairs with adenine, producing G-C:T-A transversions. An enzyme called 06-methyl-guanine-DNA methyltransferase removes the methyl group from 06-methylguanine, restoring the base to guanine (i Figure 17.28).

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