A and B forms of DNA
[Corrected: the labels are reversed in the textbook]
Both the B
and A forms are right-handed. The B form [left] is the standard form
in biological systems. The A form
[right] occurs in crystalline structures where the water
concentration is reduced: the structure is distorted such
that the bases are no longer co-planar, although the double
helical-form is still apparent. [The crystalline form of DNA
was the first studied by X-ray
crystallography, and hence was designated the A
form].
Figure © 2012 TA Brown, Introduction
to Genetics (1st ed.); additional
text © 2013 by Steven M. Carr