Figure 23.3.1.4 

Infinite Z-DNA helix, generated as before from the central four base pairs of the hexamer C-G-C-G-C-G (Z1). G and C bases alternate along each chain. The sugar–phosphate backbone adopts a pronounced zigzag pathway, rising vertically past each guanine, but travelling horizontally across the helix at cytosines. Hence, the formal helix repeat is two base pairs, G followed by C, rather than a single base pair, as in the A and B helices. Note that the structures of Z-DNA and A-DNA are in many ways the inverse of one another. The Z helix is left-handed, tall and slim, with a deep minor groove, a flattened major groove and small propeller twist. The A helix is right handed, short and broad, with a deep major groove, a shallow minor groove and large propeller twist. (From Dickerson, 1983.) Reprinted courtesy of the estate of Irving Geis. Rights owned by Howard Hughes Medical Institute.