Even though the long-term survival of species may be improved by genetic changes, the survival of individuals requires genetic stability. To achieve this, each living cell has a complement of enzymes whose function is to repair errors or damage in the DNA. Such lesions can arise spontaneously from normal cellular processes, for example errors in replication, or can be generated by external factors such as chemicals or radiation. The altered portion of the DNA is recognized, usually because the DNA becomes distorted, and the damaged bases of sequences removed by nucleases. The correct sequence is then re-synthesized by a polymerase and the ends joined to the original DNA by a ligase. There are a number of enzyme systems which repair DNA damage and nearly all rely on the existence of two copies of the genetic information, one on each strand of the DNA double helix. Thus...