ID #1824

How does sometimes a DNA strand with a mismatch gets methylated before the mistake is fixed ?

The fact that there are changes, genetic diseases, or just plain evolution to begin with is an indication that these repair systems don't always do their job. If proofreading doesn't catch an inappropriately-paired base, the mismatch repair system still might. If the mismatch repair system doesn't catch the mistake, the change becomes a permanent part of the genome, and would now be recognized as the "right" base to have there. Maybe the mismatch repair system doesn't physically encounter that section of DNA before the DNA gets methylated, or maybe it does and for whatever reason doesn't correct the mistake. It doesn't happen often, but it does happen.

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