reactive oxygen species
Oxidation damage to DNA is caused by ionizing radiation (gamma rays, X-rays) or normal oxidative metabolism. Damage is caused by formation of free radicals, usually from water, which generates OH molecules with an unpaired electron (hydroxyl radical). Hydroxyl radicals cleave the deoxyribose sugar or the phosphate backbone, leaving a strand break, or they modify a base, resulting in a replication block or mis-incorporation during replication. The best studied oxidatively damaged base is 8-oxoguanine (8-oxoG). DNA polymerases misread oxoG as T and insert A instead of C.