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", at the started of the first kinked Z-DNA region, the same strand suddently begins to spiral away to the "left". At the end of the region the right-hand spiralling is resumed and the conventional B-DNA structure is restored. The B-DNA region between the two Z-DNA regions is subject to torsional (twisting) stress, and the normal periodicity is extended. Note that the same phenomenon is apparent if the DNA is examined from the right-hand end: directionality is a property of the molecule and not the manner of looking at it.

Figure 2012 TA Brown, Introduction to Genetics (1st ed.); additional text 2013 by Steven M. Carr