In E. coli, how does the mismatch repair system able to detect which of the two mismatched bases is "correct" and which is "incorrect?"
What will be an ideal response?
Ans: Both strands of E. coli DNA are methylated by a methylase coded by the dam gene. Specifically, the A in the sequence GATC is methylated. After replication, the template strand is methylated while the newly synthesized strand is not, so the repair system can recognize the methylated strand as the "correct" one. Eventually, however, Dam methylase will methylate the new strand as well.
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