Explain how the antiparallel orientation of the two strands in a DNA double helix is essential for the process of semi-conservative replication.

IB DP Biology Higher Level (2023 syllabus) — A1.2 Nucleic acids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The two strands of DNA run in opposite (5'→3') directions, so one strand is 5'→3' and its complement 3'→5'. DNA polymerase III can only add nucleotides in the 5'→3' direction, therefore it must read the template strand in the 3'→5' direction. Because the template strands are antiparallel, one strand (the leading strand) can be synthesised continuously towards the replication fork, while the other strand (the lagging strand) must be synthesised discontinuously away from the fork as Okazaki fragments. Semi‑conservative replication is maintained because each new double helix contains one original parental strand and one newly synthesised strand; the antiparallel arrangement allows both parental strands to serve as templates at each fork.

Examiner tips

  • Show the 5'→3' vs 3'→5' orientation of the strands
  • Explain polymerase directionality and its consequence for leading/lagging strands
  • Mention that each new helix keeps one parental strand
  • Use the term ‘semi‑conservative’ to link to the question

Common mistakes

  • Confusing the direction of the template strand with the direction of synthesis
  • Forgetting that the lagging strand is synthesised in Okazaki fragments
  • Not linking antiparallel orientation to the ability of both strands to act as templates

Mark scheme (4 marks)

  1. The two strands of DNA run in opposite (5'→3') directions, meaning one strand runs 5'→3' and the complementary strand runs 3'→5'.
  2. DNA polymerase III can only add nucleotides in the 5'→3' direction, so it can only synthesise a new strand by reading the template in the 3'→5' direction.
  3. Because the two template strands are antiparallel, one strand (the leading strand) is synthesised continuously towards the replication fork, while the other (the lagging strand) must be synthesised discontinuously away from the fork as Okazaki fragments.
  4. Semi-conservative replication is maintained because each resulting double helix retains one original (parental) strand and one newly synthesised strand; the antiparallel arrangement ensures both parental strands can act as templates simultaneously at each replication fork.

Key terms in this question

antiparallel · semi-conservative replication

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