DNA Structure and Replication · USMLE Step 1

Why are Okazaki fragments necessary on the lagging strand during DNA replication?

  1. Because the lagging strand template is read in the 3′→5′ direction by polymerase
  2. Because DNA polymerase cannot synthesize continuously on the lagging template
  3. Because the lagging strand lacks a primer for polymerase to extend
  4. Because topoisomerase is unable to act on the lagging strand template
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Correct answer: Because DNA polymerase cannot synthesize continuously on the lagging template

Because DNA polymerase synthesizes only in 5′→3′ direction, the antiparallel orientation of the lagging template means replication must proceed in short discontinuous fragments (Okazaki fragments) rather than continuously.

Difficulty: Medium Question 10 of 20

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