Biology › Section G · Chromosomes and DNA › Topic G.6

DNA repair and mutation

Every time a cell copies its roughly six billion bases, a few slip through wrong — and every day, sunlight and chemistry batter the copies that already exist. Life survives because three repair systems patrol the DNA: proofreading during replication, mismatch repair after it, and nucleotide excision for bulky damage. When the patrols fail — or when the repair genes themselves are broken — the result is mutation, sometimes cancer, and occasionally a disease like xeroderma pigmentosum, where sunlight itself becomes dangerous.