Explain why DNA is described as a polymer, and describe how the two strands of a DNA molecule are held together.

AQA A-Level Biology (7402) — 3.1.5 Nucleic Acids · Explain and Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

DNA is a polymer because it consists of many repeating units joined together. The repeating units are nucleotides, each containing a phosphate, a pentose sugar and a nitrogenous base. The two strands of a DNA molecule are held together by hydrogen bonds between complementary bases – adenine pairs with thymine and cytosine pairs with guanine. The strands run in opposite directions (antiparallel), giving the double‑helix structure.

Examiner tips

Use the term ‘polymer’ and ‘monomers’ – examiners look for these words.
Show the complementary base pairs explicitly (A‑T, C‑G).
Mention antiparallel orientation to hit the extra point.
Keep the answer concise – 5 marks, so one or two sentences per point.

Common mistakes

Confusing RNA bases (U instead of T).
Forgetting to state that strands are antiparallel.
Writing the bases in the wrong order (e.g., C‑T).

Mark scheme (5 marks)

DNA is a polymer because it is made of many repeating units / monomers joined together
The monomers / repeating units of DNA are nucleotides
The two strands are held together by bonds / base pairing between the bases
The base pairing is complementary / specific — A pairs with T, and C pairs with G
The two strands run in opposite directions / are antiparallel, forming a double helix

Key terms in this question

polymer