Had Time for Fantastic Four

I watched fantastic four this week. After watching I was left confused as to if DNA can change in a humans in their life time? My perception was that DNA never changes. I came to this conclusion because I DNA is used to convict people. If DNA changes in a human being how could it be used to convict criminals? I also thought that DNA would be altered only if you are exposed to drastic environmental changes (as in the movie or a nuclear exposure) and this change too would be visible only in the off springs.
But after thinking for a while all that seemed to be wrong and what was shown in the movie seems to be correct. DNA does change naturally, but in very very small proportions. Therefore it can still be used to convict crime. It also makes logic that if an atom bomb can change DNA (causing effects like cancer etc), why can’t some other effect change our DNA for good. There also seems to be research going on in this field, i.e. to change the DNA in a controlled way. This would benefit lots of people as the bad part in their DNA can be removed to avoid decease.