A-DNA is one of the possible double helical structures which DNA can adopt. A-DNA is thought to be one of three biologically active double helical structures along with B-DNA and Z-DNA. It is a right-handed double helix fairly similar to the more common B-DNA form, but with a shorter, more compact helical structure whose base pairs are not perpendicular to the helix-axis as in B-DNA. It was discovered by Rosalind Franklin, who also named the A and B forms. She showed that DNA is driven into the A form when under dehydrating conditions. Such conditions are commonly used to form crystals, and many DNA crystal structures are in the A form. The same helical conformation occurs in double-stranded RNAs, and in DNA-RNA hybrid double helices.
A-DNA is fairly similar to B-DNA given that it is a right-handed double helix with major and minor grooves. However, as shown in the comparison table below, there is a slight increase in the number of base pairs (bp) per turn (resulting in a smaller twist angle), and smaller rise per base pair (making A-DNA 20-25% shorter than B-DNA). The major groove of A-DNA is deep and narrow, while the minor groove is wide and shallow.
Dehydration of DNA drives it into the A form, and this apparently protects DNA under conditions such as the extreme desiccation of bacteria. Protein binding can also strip solvent off of DNA and convert it to the A form, as revealed by the structure of a rod-shaped virus.
It has been proposed that the motors that package double-stranded DNA in bacteriophages exploit the fact that A-DNA is shorter than B-DNA, and that conformational changes in the DNA itself are the source of the large forces generated by these motors. In this model, ATP hydrolysis is used to drive protein conformational changes that alternatively dehydrate and rehydrate the DNA, and the DNA shortening/lengthening cycle is coupled to a protein-DNA grip/release cycle to generate the forward motion that moves DNA into the capsid.