Mismatches & Mutations: Is a Mismatched Base a Mutation?

[bioquest]

please, could someone tell me, is a mismatched base considered a mutation, some or all of the time. somebody kind of confused me on this, they were answering my question through email but I am confused about this one thing
thanks

 

[Monique]

It is a mutation, but it can still be repaired by the mismatch machinery if the abnormal base pairing is still present (before DNA replication).

 

[bioquest]

I understand you can't see which cells have mutations without killing the cell but why can't you xray the cell or something and see if it has a mutation that way (without killing it) I mean you can xray through solid objects
?
is it because there would be too much movement or something?