Explain how the structure of a nucleotide monomer enables DNA to store genetic information and to be replicated accurately.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A nucleotide is made of a deoxyribose sugar, a phosphate group and a nitrogenous base (A, T, G or C). The order of the bases along a strand is the genetic code, because different sequences encode different proteins. During replication each strand is a template: A pairs with T and G pairs with C through hydrogen bonds, so the new strand receives the correct complementary bases. The phosphate groups link the sugars via phosphodiester bonds, forming a stable sugar‑phosphate backbone that keeps the strand intact and preserves the base sequence for accurate copying.
Examiner tips
- Use the exact terms: deoxyribose, phosphate, nitrogenous base; A–T, G–C; phosphodiester bonds.
- Show the logical flow: structure → information storage → base pairing → accurate replication.
- Keep it concise – 4 marks, one sentence per point.
Common mistakes
- Confusing ribose with deoxyribose; writing RNA bases (U) instead of DNA bases.
- Lacking the mention of phosphodiester bonds or the backbone’s role in stability.
Mark scheme (4 marks)
- A nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine).
- The sequence of bases along a polynucleotide strand constitutes the genetic information, because the variable base sequence can encode instructions for protein synthesis.
- Complementary base pairing (A–T and G–C via hydrogen bonds) between specific bases on opposite strands ensures each strand acts as a template, so the base sequence is copied accurately during replication.
- The phosphate groups link adjacent deoxyribose sugars via phosphodiester bonds to form a stable sugar–phosphate backbone, maintaining the integrity of the polynucleotide strand so the base sequence is preserved between replications.
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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