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. 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 and the same geometrical conformation is commonly seen 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/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,
|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)|