Hi @mrstagg, brillinat question! (I think this might be a candidate for question of the week @modkatie!)
The answer is yes/no (not very clear I realise). DNA does change down generations due to mutations in the code, this is usually due to damage to the code, and usually considered a bad thing. If a mutation does not affect the organism in a way that might lead to its’ death before it reproduces and passes its genes on however, then the mutation will pass through many generations.
The other way that DNA may be modified (which I find absolutely fascinating!!) is by something called epigenetics. Now this needs a bit of explaining so bear with me.
When the human genome was first sequenced we were amazed at how few genes it encoded. As you have probably read on my profile, a cabbage has more genes than a human. So DNA must have another way of storing information that is not dependent on sequence of based pairs. A recent discovery is that DNA also holds information on how to construct a human being in its 3D structure. The structure of the molecule determines whether the cell can read the code. The 3D structure of DNA is changeable, and determined by lifestyle factors, so it is possible to re-code your DNA by the things you do.
Hope this helps, if not get back to me – I’m more than happy to talk about genetics all day!!