Every cell in your body takes DNA damage constantly — from ultraviolet light, from reactive chemicals produced by normal metabolism, from simple copying mistakes every time a cell divides. Estimates put the number of DNA-damaging events per human cell at tens of thousands per day. The overwhelming majority never turn into a lasting mutation, not because the damage doesn't happen, but because a dedicated set of repair systems finds and fixes most of it before it can be passed on.
Different kinds of damage, different repair crews
Cells run several distinct DNA repair pathways, each specialised for a different category of damage. Base excision repair fixes small, localised chemical alterations to individual DNA bases — the kind of damage caused by normal metabolic byproducts. Nucleotide excision repair handles bulkier damage that distorts the DNA's shape, most notably the damage ultraviolet light causes by fusing adjacent bases together, which is exactly the repair pathway that malfunctions in people with the rare genetic condition xeroderma pigmentosum, leaving them acutely vulnerable to UV-induced skin cancer. Mismatch repair catches errors introduced during DNA replication itself — a wrong base paired in the wrong place — by recognising the newly copied strand and correcting it to match the original template.
Repair isn't optional — it's why cells can divide at all reliably
Without these repair pathways running continuously, the mutation rate in a growing organism would be far higher than it actually is, and cells have built additional safeguards on top of direct repair: checkpoints in the cell division cycle that pause division if damage is detected, giving repair machinery time to work before a cell copies its DNA and passes any unrepaired errors on to two new cells. When damage is too extensive to repair, cells have a further backup — triggering their own controlled death (apoptosis) rather than risk dividing with badly damaged DNA. Cancer generally requires damage to accumulate specifically in the genes that run this whole repair-and-checkpoint system, which is part of why cancer risk rises so sharply with age and cumulative exposure to damage over a lifetime.
What we're still unsure about
The core repair pathways are well characterised at the molecular level, thoroughly studied across many organisms, and this isn't a matter of ongoing dispute. What's harder to fully map is the cumulative, long-term picture: exactly how repair efficiency changes with age, how it varies between individuals based on genetic differences most people never get tested for, and how much of that variation explains differences in cancer risk between people with otherwise similar lifestyles and exposures. Translating detailed molecular knowledge of repair pathways into reliable, individual predictions of cancer risk remains a significant, only partially solved challenge in cancer research.
This sits inside Mutations & DNA Repair, one of eight topics in Genetics, one of six domains in Biology, one of seventeen subjects the app can quiz you on.