G-T Mismatch Repair

[YASHRAJ]

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 ?

[pcrboy]

specific enzymes, probably something similar to glycosolases

[MrMistery]

hint: 5-methylcytosine + water.

[YASHRAJ]

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?

[MrMistery]

the cell just assumes the T came from methylcytosine. It's not 100%, and in some cases it does result in a mutation, but in most cases the initial base pair is restored.