Variants of the Double-Helix Structure

N-glycosidic linkage, lead to considerable variation in the conformations adopted by double-helix structures. 

The most striking conformational variant observed for a DNA double helix with Watson-Crick base pairing is referred to as the Z form. In Z DNA the backbone is twisted in the left-handed (counterclockwise) direction. This structure was first detected by Alex Rich and his co-workers (fig. 25.8). The Z form is a considerably slimmer helix than the B form and contains 12 bp/turn rather than 10. In the Z form, the planes of the base pairs are rotated approximately 180° with respect to the helix axis from their orientation in the B form (fig. 25.9). 

Because of the different orientations of the bases in Z DNA, this DNA conformation requires that the sequence of purine and pyrimidine bases in each chain strictly alternate. An alternating sequence of G and C, or T and G or A and C residues can adopt a Z conformation. Of course, in all cases the opposing strand must contain a sequence of bases that is complementary as in all DNA duplex structures. An alternating A-T DNA sequence cannot adopt the Z confer- 

The biologic significance of Z DNA is currently unclear. However, several cellular proteins that bind specifically to Z DNA have been isolated from the nuclei of Drosophila fruit flies. The mere existence of such proteins suggests that they may function in some specific role when they encounter stretches of DNA that can adopt a Z conformation. 

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