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