Re: How and what breaks down D.N.A.?

DNA gets broken down mainly in one of two ways chemically or
enzymatically.  Chemical damage to DNA can occur from certain chemicals
or UV light.  In living organisms this damage can be very dangerous as
it is completely random and can affect important bits of your DNA, a
common effect of this is that the cells grow uncontrollably (cancer).
Things that cause DNA damage of this type include strong sunlight,
radiaton and cigarette smoke.  This form of damage is not really natural
you can consider it as the DNA gets broken by physical damage.
The other main way that DNA gets broken down is by specific proteins.
These proteins are termed 'nucleases'. The term comes from NUCLEic acid
and the ending -ase which means an enzyme that work on ... therefore
they are enzymes which work on nucleic acid (nucleic acid is a more
general term for DNA and a similar molecule found in cells called RNA)
DNA is composed of lots of small molecules linked together to form one
big one, a bit like beads on a string, with the order of the beads
spelling something out.  The nuleases can either cut from the ends, they
nibble inwards from the end (called 'exonuleases' from the greek 'exo-'
= outside because they cut from the outside inwards) or they can cut in
the middle ('endonucleases' more greek, this time 'end-' meaning
inside).  These nucleases have several functions including generally
turning over the nucleic acids present so that the older stuff is broken
down and replacd with new.  They are also useful in protecting the cells
from attack by other organisms, if your own DNA can be protected then
you can have nucleases floating around to degrade any other DNA which
finds it's way in, such as an infection.

Hope this explanation is sufficient, I'm not sure what level you wanted
if there are any further points you want clarification or explanation of
then please reply and I'll try to help