Topoisomerases

[aaaddd...@hotmail.com]

We've been learning how they break both strands of DNA and rotate it
360 degrees then join them back together (for supercoiling). To me,
this sounds like a very risky thing to do. Does this process ever screw
up and cause mutations, or does it have some systems to ensure that
this doesn't happen?

Cheers

[William Knight]

It does seem rather risky on the face of it, doesn't it? But actually
living organisms possess very sophisticated sets of molecular tools for
maintaining the sequence and structure of DNA. There are multiple
enzymes for winding and unwinding the double-strands, repairing nicks
in single strands, sensing and removing damaged bases and replacing
them with new ones. Because of these numerous maintenance and repair
mechanisms, I believe that the normal mutation rate of DNA is quite
low. I don't remember the exact value, but I think that it can be in
the range of 10E-5 to 10E-6 per generation for some species. After a
few billion years of handling a critical substance, living cells have
acquired some powerful techniques for keeping it in good shape.