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