In a message dated 8/27/98 10:44:09 AM, asa@nada.kth.se wrote:
>Radiation or free radicals mess up single nucleotides, changing them
>into other molecules, but I think the major contribution is errors in
>replication. If a single base is damaged (or just unlucky), the enzyme
>doing the replication might make a wild guess and insert a random
>nucleotide in the copy.
>
>Just a guess from a computer scientists playing molecular biologist :-)
Bingo. It's not that the base is changed directly into another normal base; it's changed into something else which causes the replication or repair system to put the wrong thing in.
Incidentally, the main cause of point mutations is copying errors *unrelated* to DNA damage. DNA replication is superbly accurate, with fewer than 1 error in a million bases (that's a 6-sigma process, which is normally considered perfection in manufacturing). But when you copy a genome of 4 billion bases, you still get hundreds to thousands of errors each time.