A-DNA
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 (bp) 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
| 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 | 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) |
See also
External links
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