Describe the salient features of the double helix model of DNA proposed by Watson and Crick. Draw a labelled diagram.

1. 1 · Open with the model. Watson and Crick (1953) proposed the double helix, using X-ray data from Wilkins and Franklin.
2. 2 · Describe the chains. Two polynucleotide chains, antiparallel (5′→3′ and 3′→5′), with sugar-phosphate backbones outside.
3. 3 · Explain base pairing. A pairs with T by two hydrogen bonds, G with C by three; a purine always faces a pyrimidine (Chargaff's rule).
4. 4 · Give the dimensions. Right-handed coil; pitch 3.4 nm; about 10 bp per turn; 0.34 nm between base pairs.
5. 5 · Explain stability and copying. Stacking of base pairs and hydrogen bonds stabilise the helix; complementary strands allow copying.
6. 6 · Draw the helix. [Diagram: two strands, base pairs, 5′ and 3′ ends, pitch 3.4 nm, 0.34 nm]
7. 7 · Close in one line. So the double helix explains how DNA stores information and how it can be copied exactly.

Scoring tip: Label 5′ and 3′ on both strands in opposite directions. 'Antiparallel' is a key marking word.