Explain why scientists describe DNA as a 'double helix'.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA is a double helix because it is made of two polynucleotide strands that are linked by hydrogen bonds between complementary bases. The strands run in opposite (antiparallel) directions, and the paired bases twist the two strands into a spiral shape.
Examiner tips
- Use the term ‘double helix’ and explain the two strands, hydrogen bonds, antiparallel orientation and spiral twist.
- Show the link between base pairing and the helical structure to hit all four points.
- Keep the answer concise – one sentence per point is sufficient for full marks.
Common mistakes
- Confusing ‘parallel’ with ‘antiparallel’ orientation.
- Omitting the hydrogen bonds or the spiral twist.
- Using vague wording such as ‘two strands’ without mentioning base pairing or directionality.
Mark scheme (4 marks)
- DNA consists of two strands (polynucleotide chains)
- The two strands are held together by hydrogen bonds between complementary bases
- The two strands run antiparallel / are orientated in opposite directions to each other
- The double-stranded molecule is twisted / coiled into a spiral / helix shape
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →