A-DNA

The A-DNA structure.

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 is also assumed by DNA-RNA hybrid helices. The same helical conformation is the most commonly seen one in double-stranded RNA's.

Structure

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

Side and top view of A-, B-, and Z-DNA conformations.

Geometry attribute:	A-form	B-form	Z-form
Helix sense	right-handed	right-handed	left-handed
Repeating unit	1 base pair	1 base pair	2 base pairs
Rotation/bp	32.7°	34.3°	60°/2
Mean bp/turn	11	10.5	12
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, purine: syn
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, G: C3'-endo
Diameter	23 Å (2.3 nm)	20 Å (2.0 nm)	18 Å (1.8 nm)

External links

Structure

Comparison Geometries of the Most Common DNA Forms

See also

External links