DNA Structure and Replication · USMLE Step 1

What distinguishes leading-strand synthesis from lagging-strand synthesis during DNA replication?

  1. Leading strand is synthesized 3′→5′; lagging 5′→3′
  2. Leading strand is continuous; lagging strand is discontinuous in Okazaki fragments
  3. Lagging strand uses RNA; leading uses DNA directly
  4. Lagging strand does not require ligase
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Correct answer: Leading strand is continuous; lagging strand is discontinuous in Okazaki fragments

The leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized discontinuously in short Okazaki fragments because DNA polymerase can only synthesize in the 5′→3′ direction.

Difficulty: Medium Question 4 of 20

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