Explain how hydrogen bonding between nitrogenous bases contributes to the stability of the DNA double helix.

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)

Hydrogen bonds form between complementary base pairs on opposite strands. Adenine pairs with thymine via two hydrogen bonds, and cytosine pairs with guanine via three hydrogen bonds. Although individual hydrogen bonds are weak, the large number along the length of the molecule collectively provides considerable stability. The relative weakness of individual hydrogen bonds allows the two strands to be separated during replication and transcription.

Examiner tips

Use the exact terminology – ‘hydrogen bonds’, ‘complementary base pairs’, ‘adenine–thymine’, ‘cytosine–guanine’.
Show the number of bonds for each pair to hit all marking points.
Explain both stability and the functional consequence of bond weakness.

Common mistakes

Confusing base‑pairing rules (e.g., saying A pairs with C).
Omitting the number of hydrogen bonds for each pair.
Failing to mention that weak bonds allow strand separation.

Mark scheme (4 marks)

Hydrogen bonds form between complementary base pairs on opposite strands
Adenine pairs with thymine via two hydrogen bonds, and cytosine pairs with guanine via three hydrogen bonds
Although individual hydrogen bonds are weak, the large number along the length of the molecule collectively provides considerable stability
The relative weakness of individual hydrogen bonds allows the two strands to be separated during replication and transcription

Key terms in this question

hydrogen bond · nitrogenous bases · double helix · stability