Describe the structure of DNA and explain why the base-pairing rules are important for storing genetic information.

Eduqas GCSE Biology — 7.1 The genome and gene expression · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

DNA consists of two polynucleotide strands wound into a right‑handed double helix.
The strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine and cytosine pairs with guanine.
The order of these bases along a strand constitutes a genetic code that directs protein synthesis.
Because base‑pairing is specific, each strand can serve as a template for synthesising an identical complementary strand, so the genetic information is faithfully copied.

Examiner tips

  • Use the phrase "right‑handed double helix" to show structure. Mention the specific base pairs (A‑T, C‑G). Explain that the sequence is the code for proteins. Show that complementary pairing allows accurate replication.

Common mistakes

  • Confusing single‑stranded DNA with double helix. Omitting the specific base‑pairing rule. Failing to link base sequence to protein synthesis. Not stating that complementary strands enable faithful copying.

Mark scheme (4 marks)

  1. DNA is made up of two strands wound into a double helix
  2. The strands are held together by bases, which are paired in a specific way (A with T, and C with G)
  3. The sequence of bases along a strand forms a genetic code / acts as a code for making proteins
  4. Specific base pairing ensures that when DNA is copied, both new strands carry the same genetic information as the original

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