DNA repair

Imagine a library that not only stores books but constantly inspects them: librarians walk the shelves looking for smudged pages, torn corners, and copying errors, then carefully fix each one. DNA is under constant assault — from sunlight, chemicals, ordinary metabolism, and copying slip-ups — so cells run a whole suite of repair systems that patrol the genome, find damage, and mend it. Without this housekeeping, errors would pile up fast.

Two important repair systems show the general logic. Mismatch repair acts just after replication: if DNA polymerase and its proofreading both miss a wrongly paired base, mismatch repair detects the resulting bulge, identifies which strand is the new (and therefore wrong) one, cuts out the mistake, and lets polymerase refill it correctly. Excision repair handles damaged bases themselves — for example, bases warped by UV light or chemicals. The cell snips out the damaged stretch (in base-excision repair, a single bad base; in nucleotide-excision repair, a short patch around a bulky lesion), then a polymerase fills the gap from the undamaged opposite strand and ligase seals it. The undamaged complementary strand is the key: it always provides a correct template to copy the right answer from.

DNA repair matters because it is what keeps the genome stable across a lifetime of copying and damage; it lowers the mutation rate by orders of magnitude beyond proofreading alone. Its failure is serious: inherited defects in repair genes cause disorders with extreme sun sensitivity or sharply raised cancer risk, and crippled repair is a hallmark of many tumors. A common misconception is that DNA, once damaged, is simply lost or always mutated; in fact most damage is caught and fixed, and a permanent mutation results only when repair fails or is overwhelmed.