Explain why DNA is described as a polymer and outline the role of complementary base pairing in holding the two strands of DNA together.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA is a polymer because it is made up of monomers called nucleotides.
Each nucleotide contains a sugar, a phosphate group and one of four bases (A, T, C or G).
Complementary base pairing means A pairs with T and C pairs with G.
Weak hydrogen bonds form between the complementary base pairs, linking the two strands together.
Each nucleotide contains a sugar, a phosphate group and one of four bases (A, T, C or G).
Complementary base pairing means A pairs with T and C pairs with G.
Weak hydrogen bonds form between the complementary base pairs, linking the two strands together.
Examiner tips
- Use the exact terms "polymer", "monomers", "nucleotides", "sugar", "phosphate", "bases", "complementary base pairing", "hydrogen bonds".
- Show the A‑T and C‑G pairing explicitly.
- Mention that the bonds are weak hydrogen bonds, not covalent.
- Keep the answer concise and to the point.
Common mistakes
- Confusing hydrogen bonds with covalent bonds.
- Omitting the sugar and phosphate components of a nucleotide.
- Not specifying that A pairs with T and C pairs with G.
Mark scheme (4 marks)
- DNA is a polymer because it is made up of monomers called nucleotides
- Each nucleotide contains a sugar, a phosphate group and one of four bases (A, T, C or G)
- Complementary base pairing means A pairs with T and C pairs with G
- Weak hydrogen bonds form between the complementary base pairs, linking the two strands together
Key terms in this question
polymer · complementary base pairing
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →