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 and well-known 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 appears likely that it occurs only in dehydrated samples of DNA, such as those used in crystallographic experiments, and possibly it's also assumed by DNA-RNA hybrid helices. The same helical conformation is the most commonly seen one in double-stranded RNA's.
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 per rotation (resulting in a tighter rotation angle), and smaller rise per turn. This results in a deepening of the major groove and a shallowing of the minor.
Comparison Geometries of the Most Common DNA Forms
|Repeating unit||1 bp||1 bp||2 bp|
|Inclination of bp to axis||+19°||−1.2°||−9°|
|Rise/bp along axis||2.4 Å (0.24 nm)||3.4 Å (0.34 nm)||3.7 Å (0.37 nm)|
|Rise/turn of helix||24.6 Å (2.46 nm)||33.2 Å (3.32 nm)||45.6 Å (4.56 nm)|
|Mean propeller twist||+18°||+16°||0°|
|Glycosyl angle||anti||anti||pyrimidine: anti,
|Nucleotide phosphate to phosphate distance||5.9 Å||7.0 Å||C: 5.7 Å,
G: 6.1 Å
|Sugar pucker||C3'-endo||C2'-endo||C: C2'-endo,
|Diameter||23 Å (2.3 nm)||20 Å (2.0 nm)||18 Å (1.8 nm)|