Whenever a G -T mismatch is encountered in DNA, the cell repairs it using a specific mechanism which has a bias towards replacing T with C instead of replacing G with A. This helps in reducing mutations.
But the question is – how does the cell know that it has to selctively replace T with C in a G -T mismatch and how exactly it reduces mutations ?
Why does the cell replace only T and not G ?
Why is the cell always so confident about the correct presence of G at a G-T mismatch ?
Yeah, 5-methyl cytosine gets spontaneously converted into thymine and in every case where it happens the G is correctly placed and T is wrong.
But G-T mismatches can be formed due to some other reasons too ( e.g. replication errors) and there it is possible that the ‘G’ is incorrectly placed and ‘T’ is correct. Then replacing T will only create more mutations after each replication. How does the cell handle that?