Explain how the chemical properties of the nitrogenous bases in DNA result in the specificity of base pairing between complementary strands.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hydrogen bonds form between bases on opposite strands, providing the inter‑strand attraction.
Adenine pairs only with thymine via two hydrogen bonds, and cytosine pairs only with guanine via three hydrogen bonds.
Specificity arises because each base has a precise shape and arrangement of hydrogen bond donors and acceptors that is complementary only to its partner base.
Purines (adenine, guanine) always pair with pyrimidines (thymine, cytosine), maintaining a constant width of the double helix because a purine–pyrimidine pair has the correct combined geometry to span the distance between the two sugar–phosphate backbones.
Adenine pairs only with thymine via two hydrogen bonds, and cytosine pairs only with guanine via three hydrogen bonds.
Specificity arises because each base has a precise shape and arrangement of hydrogen bond donors and acceptors that is complementary only to its partner base.
Purines (adenine, guanine) always pair with pyrimidines (thymine, cytosine), maintaining a constant width of the double helix because a purine–pyrimidine pair has the correct combined geometry to span the distance between the two sugar–phosphate backbones.
Examiner tips
- Use the term ‘hydrogen bonds’ and specify the number of bonds for each pair. Mention purine–pyrimidine rule to show understanding of geometry. Keep sentences concise and use correct terminology (e.g., ‘donors and acceptors’).
Common mistakes
- Confusing the number of hydrogen bonds (e.g., saying 3 for AT). Forgetting to state the purine–pyrimidine rule or the role of shape/geometry.
Mark scheme (4 marks)
- Hydrogen bonds form between bases on opposite strands, providing the inter-strand attraction.
- Adenine pairs only with thymine via two hydrogen bonds, and cytosine pairs only with guanine via three hydrogen bonds.
- Specificity arises because each base has a precise shape and arrangement of hydrogen bond donors and acceptors that is complementary only to its partner base.
- Purines (adenine, guanine) always pair with pyrimidines (thymine, cytosine), maintaining a constant width of the double helix because a purine–pyrimidine pair has the correct combined geometry to span the distance between the two sugar–phosphate backbones.
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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