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.
2 · Watch
Take it off screen
Where it sits
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.