A-DNA

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

Structure[edit]

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[edit]

Side and top view of A-, B-, and Z-DNA conformations.
Yellow dots represent the location of the helical axis of A-, B-, and Z-DNA with respect to a Guanine-Cytosine base-pair.
Geometry attribute: A-form B-form Z-form
Helix sense right-handed right-handed left-handed
Repeating unit 1 bp 1 bp 2 bp
Rotation/bp 33.6° 35.9° 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°
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)

See also[edit]

External links[edit]