Spontaneous base loss within a typical cell can occur to the tune of several thousand purines and several hundred pyrimidines per day. The inherent instability of nucleotides in DNA is sufficient that, in the absence of repair, all of the hereditary information in genomes would degenerate completely in just a few generations. Base pairing stabilizes, but does not eliminate such conversions. A wide variety of chemical agents can create numerous modifications to nucleic acid bases. These ROS can modify DNA bases, a common example of their work includes thymine oxidation to thymine glycol.
Many environmental chemicals can also modify DNA bases.
Alkylation can also occur through normal metabolism. Alkylation occurs most readily at nucleophilic positions such as those denoted with asterisks in the diagram below. Photodamage Ultraviolet light is absorbed by the nucleic acid bases, resulting in an influx of energy that can induce chemical changes. The most frequent result of this energy absorption is bond formation between adjacent pyrimidines within one strand, the most frequent being pyrimidine cyclobutane dimers. The extent of distortion of DNA chain structure caused by pyrimidine cyclobutane dimers is very extensive, as the two adjacent pyrimidines are pulled much closer together than in their normal state.