Characteristics of the DNA Double-Helix

DNA will adopt two different forms of helices under different conditions--the B- and A-forms. These two forms differ in their helical twist, rise, pitch and number of base pairs per turn. The twist of a helix refers to the number of degrees of angular rotation needed to get from one base unit to another. In the B-form of helix, this is 36 degrees while in the A-form it is 33 degrees. Rise refers to the height change from one base pair to the next and is 3.4 angstroms in the B-form and 2.6 angstroms in the A-form. The pitch is the height change to get one full rotation (360 degrees) of the helix. This value is 34 angstroms in the B-form since there are ten base pairs per turn. In the A-form, this value is 28 angstroms since there are eleven base pairs per full turn.

Of the two forms, the B-form is far more common, existing under most physiological conditions. The A-form is only adopted by DNA under conditions of low humidity. RNA, however, generally adopts the A-form in situations where the major and minor grooves are closer to the same size and the base pairs are a bit tilted with respect to the helical axis.