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.
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.
Figure © 2012 TA Brown, Introduction to Genetics (1st
ed.); additional text © 2014 by Steven M. Carr