A student investigating DNA structure notices that the two strands of a DNA molecule are held together by base pairs. Describe the structure of DNA and explain the importance of complementary base pairing during DNA replication.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
DNA is a polymer found in the nucleus of eukaryotic cells. It carries all the genetic information needed for an organism to function.
Model answer (5 marks)
DNA is a polymer of nucleotides linked by phosphodiester bonds, forming two strands that twist into a right‑handed double helix. The strands are antiparallel and held together by complementary base pairs: adenine pairs with thymine via two hydrogen bonds and cytosine pairs with guanine via three.
During replication the double helix unzips, and each original strand serves as a template. DNA polymerase adds complementary nucleotides to each template strand, producing two new strands. Thus two identical DNA molecules are formed, each containing one original and one newly synthesised strand.
During replication the double helix unzips, and each original strand serves as a template. DNA polymerase adds complementary nucleotides to each template strand, producing two new strands. Thus two identical DNA molecules are formed, each containing one original and one newly synthesised strand.
Examiner tips
- Use the term ‘double helix’ and ‘antiparallel strands’ to gain structure points. Explain that complementary base pairing allows each strand to act as a template, ensuring accurate copying. Mention the result: two identical molecules – one original + one new strand.
- common_mistakes
- :
- Saying the strands are parallel instead of antiparallel. Forgetting to state that each new strand is complementary to its template. Using ‘single‑stranded DNA’ instead of specifying the double‑helix structure.
Mark scheme (5 marks)
- DNA is a polymer made of two strands twisted into a double helix
- The two strands are held together by complementary base pairs
- During replication, the two strands of the double helix separate / unzip
- Each original strand acts as a template for a new complementary strand to be built
- Two identical DNA molecules are produced, each containing one original strand and one new strand
Key terms in this question
DNA · complementary base pairing · DNA replication
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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