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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 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 was discovered by Rosalind Franklin, who also named the A and B forms. She showed that DNA is driven into the A form when under dehydrating conditions. Such conditions are commonly used in to form crystals, and many DNA crystal structures are in the A form. The same helical conformation occurs in double-stranded RNAs, and in DNA-RNA hybrid double helices.


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 turn (resulting in a smaller twist angle), and smaller rise per per base pair (making A-DNA 20-25% shorter than B-DNA). The major groove of A-DNA is deep and narrow, while the minor groove is wide and shallow.

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 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 2.6 Å (0.26 nm) 3.4 Å (0.34 nm) 3.7 Å (0.37 nm)
Rise/turn of helix 28.6 Å (2.86 nm) 35.7 Å (3.57 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)

Biological Functions[edit]

Dehydration of DNA drives it into the A form, and this apparently protects DNA under conditions such as the extreme desiccation of bacteria.[1] Protein binding can also strip solvent off of DNA and convert it to the A form, as revealed by the structure of a rod-shaped virus.[2]

It has been proposed that the motors that package double-stranded DNA in bacteriophages exploit the fact that A-DNA is shorter than B-DNA, and that conformational changes in the DNA itself are the source of the large forces generated by these motors.[3] In this model, ATP hydrolysis is used to drive protein conformational changes that alternatively dehydrate and rehydrate the DNA, and the DNA shortening/lengthening cycle is coupled to a protein-DNA grip/release cycle to generate the forward motion that moves DNA into the capsid.

See also[edit]


  1. ^ Whelan DR, et al. (2014). "Detection of an en masse and reversible B- to A-DNA conformational transition in prokaryotes in response to desiccation". J R Soc Interface 11: 20140454. doi:10.1098/rsif.2014.0454. PMID 24898023. 
  2. ^ Di Maio F, Egelman EH, et al. (2015). "A virus that infects a hyperthermophile encapsidates A-form DNA". Science 348: 914–917. doi:10.1126/science.aaa4181. PMID 25999507. 
  3. ^ Harvey, SC (2015). "The scrunchworm hypothesis: Transitions between A-DNA and B-DNA provide the driving force for genome packaging in double-stranded DNA bacteriophages". Journal of Structural Biology 189: 1–8. doi:10.1016/j.jsb.2014.11.012. PMID 25486612. 

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