DNA damage

Any modification of the chemical structure of DNA that changes its coding or replication properties. DNA damage occurs constantly as a result of both external and internal stressors. Environmental sources include UV radiation from the sun, ionizing radiation from space, various chemical agents, and pathogens. Internal factors include byproducts of normal cellular metabolism, such as free radicals, which are a continuous source of DNA damage. It's estimated that DNA damage occurs at a rate of 10,000 to 100,000 events per cell per day.