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.
What a learner can do afterwards
- 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.
1 · Read
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.
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.
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.
Clip mistakes during copying, recut the new strand after, and keep the template sacred.
More to read
DNA Repair: Proofreading and Mismatch Repair
Polymerase copies fast and proofreads as it goes: when a wrong base slips in, its exonuclease clip removes it on the spot. Survivors face a second checkpoint behind the fork. Mismatch repair scans the fresh duplex, finds the bulge of a mispair, and cuts a patch out of the new strand. The old strand stays untouched as the template, and polymerase rebuilds the patch to match it. 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. When this pathway fails, mistakes stop being rare events and become inherited mutations, which is why repair defects raise cancer risk instead of causing instant sickness.
How Fast and Accurate Is DNA Copying?
Copying runs at startling speed: E. coli polymerase lays down more than 700 base pairs per second, with helicase unwinding ahead and topoisomerase relieving the strain. Raw pairing makes about one mistake per 10 million bases, and proofreading plus mismatch repair push that below one in a billion. Repair matters because damage never stops: the DNA in one cell takes tens of thousands of hits per day from internal chemistry and outside radiation. Cells fix most of it most of the time, which keeps genomes stable across generations while still letting a rare mutation through on occasion.
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2 · Watch
Take it off screen
Where it sits
Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.