Explain how the structure of the DNA double helix allows it to be both stable under normal cellular conditions and yet accessible for transcription and replication.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The DNA double helix is stabilised by hydrogen bonds between complementary bases (A–T and G–C). Although each hydrogen bond is weak, the large number of them along the length of the helix gives strong overall stability. The sugar–phosphate backbone, formed by covalent phosphodiester bonds, provides a rigid, covalently bonded framework that keeps each strand intact during and after strand separation. Because individual hydrogen bonds are weak, enzymes such as helicase can break them locally, allowing the strands to separate for transcription or replication without permanently disrupting the whole molecule.
Examiner tips
- Mention both base‑pair hydrogen bonds and phosphodiester backbone
- Explain that many weak bonds give overall stability
- Show that enzymes can break individual bonds locally
- Use correct terminology (hydrogen bond, phosphodiester, helicase)
Common mistakes
- Confusing hydrogen bonds with covalent bonds
- Forgetting the role of the backbone
- Saying the helix is stable only because of covalent bonds
Mark scheme (4 marks)
- Hydrogen bonds between complementary base pairs (A–T and G–C) contribute to the stability of the double helix
- The large number of hydrogen bonds along the entire length of the molecule collectively provides strong overall stability, even though each individual hydrogen bond is weak
- Because hydrogen bonds are individually weak, enzymes such as helicase can break them locally, allowing strand separation for transcription or replication without permanently disrupting the entire molecule
- The sugar–phosphate backbone (phosphodiester bonds) provides a stable, covalently bonded framework that maintains the integrity of each strand during and after strand separation
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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