The double helix

The two strands wind round a common axis with the base pairs stacked between them like the treads of a spiral stair. The same chemistry can take more than one shape: the familiar B form, the squat A form, and the left-handed Z form.

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Form

Base pairs

A T G C

The three forms

Form	Hand	bp per turn	Rise per bp (nm)	Twist per bp	Pitch (nm)	Diameter (nm)	Base-pair tilt	Grooves	Found in
A	right	11	0.256	+32.7°	2.82	≈ 2.3	+20°	major deep and narrow; minor wide and shallow	dehydrated DNA; RNA double helices and DNA–RNA hybrids take this shape
B	right	10	0.338	+36.0°	3.38	≈ 2.0	-6°	major wide; minor narrow	the usual form in water and in cells; in solution 10.4 ± 0.1 bp per turn
Z	left	12	0.372	-30.0°	4.46	≈ 1.8	-7°	no real major groove; minor narrow and deep	alternating purine–pyrimidine runs such as (CG)n, at high salt or under negative supercoiling

A and B: fibre models of Arnott & Hukins 1972; Z: the crystal structure of Wang et al. 1979. Diameters and tilts are rounded, as given in Saenger 1984. In solution B-DNA turns a little less tightly than in fibres: Wang 1979 measured 10.4 ± 0.1 bp per turn. The build checks that the twist is 360° divided by the base pairs per turn, and that the pitch is the rise times the base pairs per turn. The 3D model is drawn from these numbers. It is a diagram, not an atomic model.

Then and now

The 1953 model had 10 base pairs per turn and a pitch of 3.4 nm, read from Rosalind Franklin and Raymond Gosling’s X-ray photographs of the B form. The left-handed Z form was not seen until 1979, when a short (CG)₃ crystal was solved at atomic resolution. In it the backbone zigzags because the repeating unit is two base pairs, not one.