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
Note that the same phenomenon is apparent
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.