Many incorrectly inserted nucleotides that escape proofreading are corrected by mismatch repair

strand is not (< Figure 17.27a). In E. coli, the proteins MutS, MutL, and MutH are required for mismatch repair. MutS binds to the mismatched bases and forms a complex with MutL and MutH; this complex is thought to bring an unmethylated GATC sequence in close proximity to the mismatched bases. MutH nicks the unmethylated strand at the GATC site (< Figure 17.27b), and exonucleases degrade the unmethylated strand from the nick to the mismatched bases (< Figure 17.27c). DNA polymerase and DNA ligase fill in the gap on the unmethylated strand with correctly paired nucleotides (< Figure 17.27d).

Mismatch repair in eukaryotic cells is similar to that in E. coli, except that several proteins are related to MutS and several are related to MutL. These proteins function together in different combinations to detect different types of incorporation errors, such as mispaired bases and small unpaired loops. Eukaryotic cells do not have any proteins related to E. coli MutH. What enzyme makes the nick in eukaryotic cells is not clear. How the old and new strands are recognized in eukaryotic cells is not known, because in some eukaryotes, such as yeast and fruit flies, there is no detectable methylation of DNA.

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