Z-DNA is a left-handed form of DNA, produced by introducing a double-helical 'twist' in the molecule. Examine the structure from the left-hand end: the top strand at first spirals away to the "right" (Clockwise). At the started of the first kinked Z-DNA region, the same strand suddently begins to spiral away to the "left" (counterclockwise). At the end of the region a second twist restores the right-hand spiralling and the conventional B-DNA structure. The B-DNA region between the two Z-DNA regions is subject to torsional (twisting) stress, and the typical periodicity is extended.

    Note that the same phenomenon is apparent if the DNA is examined from the right-hand end: directionality of the helix is an intrinsic property of the molecule and not the manner of looking at it.
