“It is estimated that a human cell repairs 10,000 – 20,000 DNA lesions per day” This is the opening sentence of Proc. Natl. Acad. Sci. vol. 112 pp. 3997 – 4002 ’15, but no source for this estimate is given. The lesions range from single and double strand breaks in the sugar phosphate backbone of the DNA helix, to hydrolytic losses of a DNA base from the backbone, to chemical modification of the DNA bases themselves — oxidation etc. etc.
What needs explaining then, is why we stay as well as we do. https://luysii.wordpress.com/2009/09/17/the-solace-of-molecular-biology/