DNA Damage and the Repair Pathways That Fix It · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Two checkpoints before mutation

Science · Biochemistry & Molecular Biology · ages 19-20
Name ______________________   Date ____________

  1. Mismatch repair cuts the old template strand.

    Circle one:   True   False

  2. How does DNA polymerase correct its own mistakes?

    • It slows copying to let damage decay
    • It skips damaged bases and fills gaps later
    • Its exonuclease clip removes the just-added wrong base

  3. How does mismatch repair tell the new strand from the template?

    • The new strand still carries nicks and lacks the old methyl marks
    • The new strand is longer than the old one
    • The old strand glows under repair enzymes

  4. A repair pathway fails. What follows?

    • Instant sickness the same day
    • A raised mutation rate, hence higher cancer risk
    • Total immunity to future damage

  5. Raw pairing errs about once per ten million bases, and both checkpoints push errors below one per billion. By about what factor do they cut mistakes? Type the number.

    Answer: ______________

  6. Mismatch repair rebuilds the cut patch. Which strand serves as the template?

    • The new strand, since it is fresher
    • Both strands equally, to be safe
    • The old strand, since it was correct before

  7. A tumour cell line lacks mismatch repair. What do you predict about its mutation rate?

    • Its mutation rate falls below normal
    • Its mutation rate climbs far above normal
    • Its copying speed doubles to compensate

  8. Alex says proofreading happens after replication finishes, scanning the finished duplex. What is wrong?

    • Proofreading checks each base during copying; the after-copy scan is mismatch repair
    • Proofreading never removes bases at all
    • Mismatch repair works before polymerase arrives
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Two checkpoints before mutation W1-mt_r0qipp0YRR-s1

  1. False · The old strand stays untouched as the template; the new one is cut.
  2. Its exonuclease clip removes the just-added wrong base · Proofreading checks each new base before the next is added.
  3. The new strand still carries nicks and lacks the old methyl marks · Nicks plus missing marks always point at the fresh strand.
  4. A raised mutation rate, hence higher cancer risk · Unfixed mistakes become inherited mutations rather than instant symptoms.
  5. 100 · From 1 in ten million to below 1 in a billion is about a hundred times fewer.
  6. The old strand, since it was correct before · Rebuilding must copy the strand that predates the mistake.
  7. Its mutation rate climbs far above normal · Without the second checkpoint, survivors stop being rare.
  8. Proofreading checks each base during copying; the after-copy scan is mismatch repair · Timing separates the two checkpoints: during versus after.
Worksheet · LightMySky