Methyl-mismatch DNA repair is a repair pathway activated when a mis-paired base is incorporated into the DNA strand.
Using the MUT system, MUT S binds to the mismatched site first, identifying the incorrect pairing. MUT L and MUT H then bind, being recruited by MUT S, onto the non-methylated nascent strand. The daughter strand is then dragged in to a loop by MUT L contracting by conformational change. This allows MUT H, and endonuclease, to nick the DNA, allowing the damaged section to be removed. UvrD, a helicase, then unwinds the DNA, allowing the removal of the damaged section from the nick formed by MUT H. DNA polymerase is then used to fill the gap, and DNA ligase completes the backbone.
This process, methyl-mismatch repair, is very similar to nucleotide excision repair. A protein complex is used to form a nick in the backbone, which then recruits UvrD helicase to unwind and remove the damaged section before the gap is refilled by DNA polymerase and ligase forming the final phosphodiester bonds in the sugar-phosphate backbone. Methyl-mismatch repair does have differences to nucleotide excision repair. Notably, the proteins identifying the site of damage are different. However, this is to be expected, as the damage sites form different structures. Additionally, nucleotide excision repair targets thymine dimers, while methyl-mismatch repair targets mis-matched nucleotides.