During mismatch repair, it is essential for the cell to be able to distinguish between the old template DNA strand and the newly synthesized DNA strand in order to make the appropriate correction of a mispairing created during DNA replication. How is this accomplished In E. coli?

The difference between the methylation states of the two DNA strands serves as a recognition point between the old and new strands.

Explanation:

DNA mismatch repair is a mechanism for recognizing and repairing errors during DNA synthesis such as insertion, deletion, and wrong base pairings that can arise during DNA replication and recombination.

Mismatch repair process begins as soon as new DNA has been made. First, a protein complex recognizes and binds to the mismatched base. A second complex cuts the DNA near the mismatch, and more enzymes cut out the incorrect nucleotide and some surrounding DNA. A DNA polymerase then replaces the excised DNA parts with correct nucleotides, and DNA ligase seals the gap.

In gram-negative bacteria, for example E. Coli, original and newly made strands of DNA differentiated by their methylation state. An old DNA strand will have methyl groups attached to some of its bases, while a newly made DNA strand will not yet have gotten its methyl group.