---
title: "DNA Damage and the Repair Pathways That Fix It"
description: "Bases are damaged constantly by chemistry inside the cell and by radiation outside it, and each kind of damage has a repair route matched to it. Proofreading and mismatch repair together cut the error"
canonical: https://lightmysky.com/learn/science/dna-damage-and-the-repair-pathways-that-fix-it-mt_r0qipp0YRR
source: https://lightmysky.com/learn/science/dna-damage-and-the-repair-pathways-that-fix-it-mt_r0qipp0YRR.md
retrieved: 2026-09-12
---

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# DNA Damage and the Repair Pathways That Fix It

Bases are damaged constantly by chemistry inside the cell and by radiation outside it, and each kind of damage has a repair route matched to it. Proofreading and mismatch repair together cut the error rate of copying by orders of magnitude.

Subject: Science · Area: Biochemistry & Molecular Biology · Ages 19 to 20
Page: https://lightmysky.com/learn/science/dna-damage-and-the-repair-pathways-that-fix-it-mt_r0qipp0YRR

## Ready when they can

- Matches a damage type to a repair pathway and says what the pathway uses as its template.
- Explains how mismatch repair knows which of the two strands carries the error.
- Links a repair pathway failure to a raised mutation rate rather than to an immediate phenotype.

## Lesson: Two checkpoints before mutation

Damage never stops: one cell takes tens of thousands of hits per day from inner chemistry and outside radiation. Copying defends itself twice. First, polymerase proofreads as it goes, reading each newly added base before adding the next, and its exonuclease clip removes a wrong base on the spot.

Survivors face mismatch repair behind the fork. The crew finds the bulge of a mispair and cuts a patch out of the new strand, then rebuilds it to match the old one. Telling old from new is the clever part: the new strand still carries nicks and lacks the methyl marks the old strand has, so the crew always cuts the right one.

**Example.** The two checkpoints multiply their power. Raw pairing errs about once per ten million bases, and proofreading plus mismatch repair push that below one in a billion. Copying stays fast, with bacterial polymerase laying down over 700 pairs per second, while staying astonishingly exact.

**Tip.** When a repair pathway fails, mistakes stop being rare events and become inherited mutations. That is why repair defects raise cancer risk instead of causing instant sickness. Each damage kind has its matched route, and every route rebuilds from an intact template.

**Recap.** Clip mistakes during copying, recut the new strand after, and keep the template sacred.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Gene Mutations and Their Consequences](https://lightmysky.com/learn/science/gene-mutations-and-their-consequences-mt_CDcaL6-iVp)
- [Replication Machinery: Origins, Forks and Telomeres](https://lightmysky.com/learn/science/replication-machinery-origins-forks-and-telomeres-mt_v95MJWeV_B)

## Opens up

- [Homologous Recombination and Double-Strand Break Repair](https://lightmysky.com/learn/science/homologous-recombination-and-double-strand-break-repair-mt_yWlwqAsXEf)