Mutation Rate

The causes of most spontaneous mutations is not known, so the main factors affecting the spontaneous mutation rates are obscure. It is likely that the methylation of cytosine in the DNA is important for many spontaneous mutations, because many are found at sites in the genes where cytosine is methylated. Ionizing radiations, such as cosmic rays, probably account for less than 10 percent of spontaneous mutations. Other factors are errors made during replication of the DNA; exposure to mutagens produced by cells during their normal metabolic activity, with reactive oxygen species being the common suspect; spontaneous breakdown of DNA at body temperatures; and exposure to environmental agents. Many mutations are made when the mechanisms that repair DNA make mistakes, and many error-prone DNA repair enzymes are known.