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I was reading a section from my textbook about tautomeric shifts, and it seems to suggest that there are some mutagens that can be directly responsible for the phenomenon. The section is mainly describing spontaneous mutations as opposed to induced mutations, and examples of mutagens are mentioned. However, the author explicitly states that the sort of changes made to DNA due to spontaneous mutations also occur at a higher rate during induced mutagenesis.

If a tautomeric shift can occur due to mutagenic activity, is it possible for the mutagen to undo and "correct" the mutation by reversing the shift? If so, what kind of chemicals or physical mutagens (e.g. radiation) would be involved?

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It is possible but extremely unlikely.

When a base undergoes tautomeric shift the DNA does not contain a mutation yet, just an unmatched pair. The mutation will only becomes inscribed into the DNA permanently after the DNA is replicated or wrongly repaired.

In order to reverse the mutation you would need to provoke a chemical change to that specific base using the same or another mutagen and as most of them are unspecific the likelihood of obtaining both the correct mutation to reverse the effect AND at that specific location is extremely low. Just for the location itself (i.e. that base pair), in human you have ~0.000000033% chances (1/3 billions) to mutate that base. This is assuming equal probabilities across the genome and a single modification event which is of course not true but shouldn't modify much the conclusion. I am not even speaking about provoking the chemical modification that will reverse it which will even further reduce that probability.

Such induced or natural mutations will actually only rarely appear in the genome permanently as the cell has multiple DNA repair mechanisms (see this wiki). So in this case the modified base does not need to be reverse as the cell will make sure to repair it directly but using mechanisms not based on mutagenesis.

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    $\begingroup$ So near-zero probability aside, the mutation reversal can occur. Would this require the mutagen to be nonspecific (like UV or gamma rays)? $\endgroup$
    – user170231
    Commented Mar 30, 2015 at 2:10
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    $\begingroup$ The opposite. If you have an insanely specific mutagen that only mutates one base in particular it's about 1/8 or so probability to undo its own mutation, but a nonspecific mutagen has a much lower chance. Real specific mutagens don't really work like that, but the same principle applies. $\endgroup$
    – Resonating
    Commented Mar 30, 2015 at 2:17
  • $\begingroup$ I just finished reading a section describing photoreactivation as a means of repairing a mutation. In addition to this, the wiki page linked by @cagliari2005 mentions that the enzyme involved in the process can be activated by light in the blue/UV range. However, the page on photolyases says they require visible light for activation. Any ideas as to the discrepancy? Also, thanks for the replies, much appreciated! $\endgroup$
    – user170231
    Commented Mar 30, 2015 at 2:37
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    $\begingroup$ @user170231 Post a new question ;) $\endgroup$
    – cagliari2005
    Commented Mar 30, 2015 at 2:38
  • $\begingroup$ No need, I just answered my own question after a bit of reading :) Thanks again $\endgroup$
    – user170231
    Commented Mar 30, 2015 at 2:51

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