Homologous Recombination and Double-Strand Break Repair · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

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Science · Genetics & Evolution · ages 19-21
Name ______________________   Date ____________

  1. Homologous recombination can repair a break accurately with no partner sequence.

    Circle one:   True   False

  2. Why is a double-strand break the most dangerous DNA lesion?

    • Only one backbone is nicked
    • Both backbones are severed with no intact template in the same duplex
    • The bases are merely mispaired

  3. What does the resected end do in homologous recombination?

    • It invades the partner and copies the missing sequence
    • It trims both ends and glues them directly
    • It degrades the whole chromosome arm

  4. Why does homologous recombination dominate in S and G2?

    • The enzymes only wake up in those phases
    • Sisters are present then to serve as partners
    • Breaks never happen outside S and G2

  5. How do the two repair routes compare in accuracy?

    • End joining is accurate and recombination is sloppy
    • Both routes always leave large deletions
    • Recombination is accurate while end joining often leaves small indels

  6. A linkage map shows 1 percent recombinants between two markers. What does that mean?

    • One crossover per hundred meioses in that interval
    • One gene per hundred base pairs
    • One hundred crossovers per single meiosis

  7. A break happens in G1 with no sister chromatid nearby. Which route repairs it, and at what cost?

    • Recombination, with perfect restoration guaranteed
    • End joining, quick but possibly scarred
    • Neither route; the cell must divide instead

  8. Jo claims end joining is the accurate route because it is faster. What is wrong?

    • She is right; faster enzymes make fewer mistakes
    • Recombination is slower and therefore sloppier
    • Speed is not accuracy; joining has no template, so indels remain
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Answer key

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  1. False · Accuracy costs a partner: sisters in S and G2 supply it.
  2. Both backbones are severed with no intact template in the same duplex · With both strands cut, the duplex cannot guide its own repair.
  3. It invades the partner and copies the missing sequence · Strand invasion plus copying restores the missing stretch.
  4. Sisters are present then to serve as partners · Dependence on a partner restricts recombination to when sisters exist.
  5. Recombination is accurate while end joining often leaves small indels · A template means fidelity; no template means small scars.
  6. One crossover per hundred meioses in that interval · Map distance and crossover frequency speak the same language.
  7. End joining, quick but possibly scarred · Context picks the route: no partner means direct ligation.
  8. Speed is not accuracy; joining has no template, so indels remain · Fidelity comes from copying a partner, not from working quickly.
Worksheet · LightMySky