The A form is a double helix, dextrorse like the B form, but wider. The fictitious line joining the phosphorus
atoms defines the trajectory of the backbone; note its helicity.
Note also the inclination of the base pairs,
stronger than in B-DNA.
Note that the base pairs are positioned on one side of the helix axis; this is the reason why the helix becomes wider.
Spacefilling model, made of
(bases and backbone).
What was the major groove in B-DNA becomes in the A form a narrow and deep groove, while the minor groove is now wide and shallow.
The Z form is a sinistrorse double helix,
what makes it different from the A and B forms, and it is thinner. Note how the backbone (fictitious line joining the phosphorus atoms)
progresses turning left and in addition makes a zig-zag line. This feature gives the Z form its name.
Spacefilling model, made of
(bases and backbone).
What was the major groove in B-DNA becomes in the Z fom a shallow groove, while the minor groove is here narrow and deep.