A left-handed helix (molecular structure) of DNA, in contrast to A-DNA and B-DNA which are right-handed helix structures. The difference is in the direction of the double-helix twist. Z-DNA has the most base pairs per turn (in the helix), and so has the least twisted structure; it is very “skinny” and its name is taken from the zigzag path that the sugar-phosphate “backbone” follows along the helix. This is quite different from the smoothly curving path of the backbone of B-DNA. The Z-form of DNA has been found in polymers that have an alternating purine-pyrimidine sequence. The possible biological importance of Z-DNA is that it is much more stable at lower salt concentrations.