Mutation

DNA replication is a highly accurate process, but mistakes occasionally happen — a polymerase inserting the wrong base, for instance. Cells have several layers of correction: the polymerase proofreads as it copies, mismatch repair cleans up after replication, and nucleotide excision repair removes bases damaged by outside factors such as UV.

A mutation is what gets through all of those layers. NHGRI puts it plainly: mutations are happening in our cells all the time, but almost none of them affect our health — partly because cells have very sophisticated machinery for repairing mutations very quickly, and partly because most mutations occur in somatic cells such as muscle or skin cells and can only affect the cell where the mutation occurred.

That is not the whole story, though, and the rest of it is on the same two source pages. NHGRI notes that mutations in germline cells — eggs and sperm — will be present in every cell that develops from that egg or sperm, an entire person, and can have larger effects; germline mutations can be passed on to offspring, while somatic mutations are not. And OpenStax reports that mutations in the repair genes themselves have been known to cause cancer: many mutated repair genes have been implicated in certain forms of pancreatic, colon and colorectal cancer.