What is the handedness of Z-DNA?

Z-DNA is left-handed. Unlike the familiar B-DNA helix, which twists to the right, Z-DNA winds counterclockwise and has a distinctive zigzag-shaped backbone, giving it its name.

Z-DNA is favored by certain alternating purine–pyrimidine sequences, especially repeating guanine–cytosine patterns. Negative supercoiling, high salt concentrations, and some cations can also encourage DNA to shift from the usual B form into the Z form. At the boundaries, the molecule forms B–Z junctions where the two helical geometries meet.

The structure was confirmed by X-ray crystallography in 1979, when Alexander Rich, Andrew Wang, and colleagues solved a crystal structure of a short DNA fragment. Z-DNA is not simply a permanently separate kind of DNA; it can form temporarily under particular chemical or mechanical conditions. A common mix-up is assuming that every DNA helix is right-handed, when A-DNA and B-DNA are right-handed but Z-DNA is left-handed.