Which form of DNA has a left-handed helical twist?

Z-DNA is the form of DNA with a left-handed helical twist.

Unlike the familiar B-DNA helix, which winds to the right, Z-DNA winds to the left and has a distinctive zigzag-shaped phosphate backbone. Its repeating structural unit contains two base pairs rather than one, helping produce its unusual geometry. Z-DNA is narrower than B-DNA, with a diameter of approximately 1.8 nanometers.

Z-DNA is not usually the dominant shape of DNA in cells. It is favored by certain alternating purine–pyrimidine sequences, especially repeating cytosine–guanine patterns, and by conditions such as negative supercoiling or high salt concentrations. Segments can switch from B-DNA into Z-DNA and form junctions where the two conformations meet.

The structure’s history is notable: the first atomic-resolution crystal structure of a DNA fragment was reported in 1979 by Andrew Wang, Alexander Rich, and colleagues. Z-DNA is also biologically interesting because proteins including ADAR1 can recognize its left-handed conformation.