Explain how complementary base pairing in DNA contributes to the accurate replication of genetic information.

IB DP Biology Standard 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)

1. Each base on the template strand pairs with only one specific complementary base (A with T, C with G), ensuring the correct nucleotide is inserted opposite each template base.
2. The two strands of the parent DNA are separated (e.g. by helicase breaking hydrogen bonds), each strand acting as a template for the synthesis of a new complementary strand.
3. Free deoxyribonucleotides align opposite each template base via hydrogen bonds, and DNA polymerase joins them by forming phosphodiester bonds, producing a new complementary strand.
4. The result is two identical DNA molecules each consisting of one original (parental) strand and one newly synthesised strand, preserving the base sequence – this is semi‑conservative replication.

Examiner tips

  • Use the exact base‑pairing terms (A‑T, C‑G) and mention helicase and DNA polymerase. Show the sequence of events: separation → base‑pairing → phosphodiester bond formation. Remember to state the semi‑conservative outcome.
  • common_mistakes
  • :
  • Confusing complementary pairing with antiparallel orientation. Omitting the role of helicase or DNA polymerase. Failing to mention that each new strand is complementary to the template.

Mark scheme (4 marks)

  1. Each base on the template strand pairs with only one specific complementary base (A with T, C with G), ensuring the correct nucleotide is inserted opposite each template base.
  2. The two strands of the parent DNA are separated (e.g. by helicase breaking hydrogen bonds), each strand acting as a template for the synthesis of a new complementary strand.
  3. Free deoxyribonucleotides align opposite each template base via hydrogen bonds, and DNA polymerase joins them by forming phosphodiester bonds, producing a new complementary strand.
  4. The result is two identical DNA molecules each consisting of one original (parental) strand and one newly synthesised strand, preserving the base sequence — this is semi-conservative replication.

Key terms in this question

complementary base pairing

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