B and A 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 condensed 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 © 2014 by Steven M. Carr