Mismatch repair

Despite the high fidelity of DNA replication and the backup proofreading system, it is still possible for the incorrect base to be opposite a template base.  To fix such potential mutations, the repair mechanism must have some way of distinguishing among the template strand and the daughter strand synthesized against it. In several bacteria a mature chromosomal DNA has methylated bases at the sequence GATC. Daughter DNA strands experience some delay  in methylation so newly  replicated DNA spends some time  in a hemimethylated form template strand methylated daughter strand not. The methyl-directed mismatch repair mechanism can identify correctly the nonmutated strand by means of methylation in Figure. The system can  only resolve single  base-pair mismatches and  requires the  participation of Dam  methylase, MutS,  MutH, MutL,  DNA helicase II single-stranded binding protein DNA exonuclease I, polymerase III,  exonuclease VII, RecJ nuclease, exonuclease X and  DNA ligase.