Explain how the antiparallel orientation of the two strands in a DNA double helix is related to its structural stability.
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 directions (5'→3' and 3'→5'), so they are antiparallel.
Because they are antiparallel, complementary bases (A–T and C–G) can pair across the strands, forming hydrogen bonds that hold the strands together.
The antiparallel arrangement lets the bases of both strands face inward, giving maximum hydrophobic stacking interactions between adjacent base pairs and stabilising the helix.
The sugar–phosphate backbones lie on the outside of the helix; the antiparallel orientation keeps the negatively charged phosphates on the exterior, preserving a consistent helical geometry.
Because they are antiparallel, complementary bases (A–T and C–G) can pair across the strands, forming hydrogen bonds that hold the strands together.
The antiparallel arrangement lets the bases of both strands face inward, giving maximum hydrophobic stacking interactions between adjacent base pairs and stabilising the helix.
The sugar–phosphate backbones lie on the outside of the helix; the antiparallel orientation keeps the negatively charged phosphates on the exterior, preserving a consistent helical geometry.
Examiner tips
- Show the 5'→3' and 3'→5' directions first; link this to base‑pairing and then to stacking and backbone placement.
- Use the exact terms ‘hydrogen bonds’, ‘hydrophobic stacking’, and ‘sugar‑phosphate backbone’ as the mark scheme recognises them.
Common mistakes
- Confusing parallel with antiparallel strands; students write both strands 5'→3'. Mis‑identifying the direction of base‑pairing or omitting the role of hydrogen bonds. Failing to mention the placement of the phosphate groups on the outside of the helix.
Mark scheme (4 marks)
- The two strands run in opposite (5'→3' and 3'→5') directions, making them antiparallel.
- Complementary base pairing (A–T and C–G) occurs between the two antiparallel strands, forming hydrogen bonds that hold the strands together.
- The antiparallel arrangement allows the bases of both strands to face inward, maximising hydrophobic stacking interactions between adjacent base pairs along the helix.
- The sugar–phosphate backbones of both strands run along the exterior of the helix; the antiparallel orientation allows the negatively charged phosphate groups to be positioned on the outside, maintaining a consistent helical geometry.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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