Mismatch repair in Bacteria
In E. coli, methyl-directed mismatch repair is used when error remains in bases even after proofreading mechanism. Using this type of repair, mismatches are recognized, incorrect bases are excised, and repair synthesis is carried out. To identify erroneous base, you have to distinguish between the two strands. Usually the parental strand is the good strand. You can identify parental strand by finding the methylated A nucleotide at the beginning of the sequence. Nucleotides should be methylated but it takes sometime after replication for methylation to occur. Once unmethylated, erroneous nucleotide found, excision and repair can occur.