DNA Structure and Replication · USMLE Step 1

What is the role of topoisomerase (or DNA gyrase in prokaryotes) during DNA replication?

  1. To synthesize the RNA primer for replication
  2. To unwind the duplex DNA at replication origins
  3. To relieve torsional strain ahead of the fork
  4. To ligate adjacent Okazaki fragments together
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Correct answer: To relieve torsional strain ahead of the fork

As helicase unwinds the double helix, overwinding ahead of the fork causes torsional strain; topoisomerase relieves this by introducing transient breaks and resealing DNA, allowing replication to proceed smoothly.

Difficulty: Medium Question 7 of 20

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