Topic 4.1 · Supplement
DNA Structure and Base Pairing
Chapter 4 introduces DNA at a deliberately simple level, and it is worth resisting the urge to reach for anything more detailed than what is actually asked for here. You need the shape of the molecule, what runs along it, and one fixed pairing rule — not the chemistry of the bonds, and not what any of it does once you get to genes.
A DNA molecule consists of two strands, coiled around each other into a double helix. Each strand carries a sequence of bases, and the two strands are held together by bonds between pairs of bases — those base pairs work like the rungs of a ladder, cross-linking the two sides and holding the whole structure together. The four bases are represented by the letters A, T, C and G, and Cambridge does not require you to know or give their full chemical names at this stage — the letters alone are the required answer.
Complementary base pairing
DNA bases do not pair randomly. They pair in exactly one fixed way: A always pairs with T, and C always pairs with G. A never pairs with C or G, and T never pairs with C or G — the two pairs never mix. This is called complementary base pairing, and because the rule is fixed, knowing the base sequence on one strand is enough to work out the sequence on the strand opposite it.
Work through an unfamiliar strand one base at a time rather than trying to hold the whole sequence in your head at once. Given the strand
A – T – C – G – G – A
the complementary strand is found by swapping each base for its fixed partner in turn: A needs T, T needs A, C needs G, G needs C, G needs C, A needs T — giving
T – A – G – C – C – T
That base-by-base method is worth more than memorising the finished pair of sequences, because it is the method that still works when the exam gives you a strand you have never seen before.
Where this deliberately stops
This syllabus point is kept narrow on purpose. What you need for Chapter 4 is the double helix, the two strands, the bases along them, the bonds between paired bases holding the strands together, and the A–T / C–G pairing rule — nothing beyond that. Nucleotide chemistry, the sugar-phosphate backbone and the nature of the hydrogen bonds between bases are not required here, and importing that level of detail into a Chapter 4 answer does not earn extra credit. Genes, chromosomes and how DNA actually controls protein production are a separate, later topic — you can find that fuller treatment in chromosomes, genes and proteins and protein synthesisonce you reach Topic 17. Keep this page’s answers to structure only, and save that functional detail for when it is actually asked for.
Expect this to be tested through recall, description, and — distinctively for this topic — labelling or identifying features on a DNA diagram, so it is worth being able to recognise the two strands, the helix shape and the paired bases in a drawing, not only in written description. This content is Supplement only and is not assessed on the practical paper.