Explain how the antiparallel nature of DNA strands presents a challenge during replication, and outline how this challenge is resolved.

IB DP Biology Higher Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

DNA polymerase can only synthesise new DNA in the 5′→3′ direction. Because the two template strands are antiparallel, only one template strand (the leading‑strand template) runs 3′→5′ toward the replication fork, allowing continuous synthesis. The other template strand (the lagging‑strand template) runs 5′→3′ toward the fork, so synthesis must occur discontinuously, away from the fork. Short RNA primers are repeatedly laid down on the lagging strand and DNA polymerase extends each primer, producing Okazaki fragments that are later joined by DNA ligase.

Examiner tips

  • Show the 5′→3′ restriction of DNA polymerase first
  • Explain which strand is leading and which is lagging
  • Describe Okazaki fragment formation and ligase joining
  • Use correct terminology (template, primer, ligase)

Common mistakes

  • Confusing the direction of the template strands
  • Forgetting that the lagging strand is synthesised discontinuously
  • Not mentioning RNA primers or DNA ligase

Mark scheme (4 marks)

  1. DNA polymerase can only synthesise new DNA in the 5′ to 3′ direction
  2. Because the two template strands are antiparallel, only one (the leading strand template) runs 3′ to 5′ continuously toward the replication fork, allowing continuous synthesis
  3. The other template strand (lagging strand template) runs 5′ to 3′ toward the fork, so synthesis must occur discontinuously / away from the fork
  4. Short RNA primers are laid down repeatedly and DNA polymerase extends each primer, producing Okazaki fragments that are later joined by DNA ligase

Key terms in this question

antiparallel

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