DNA is one of the possible double helical structures which
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 to form crystals, and many
structures are in the A form. The same helical conformation occurs
in double-stranded RNAs, and in DNA-
RNA hybrid double helices.
2 Comparison geometries of the most common
3 Biological function
4 See also
6 External links
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 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. A-
DNA is broader and apparently more compressed
along its axis than B-DNA.
Comparison geometries of the most common
Side and top view of A-, B-, and Z-
Yellow dots represent the location of the helical axis of A-, B-, and
DNA with respect to a Guanine-Cytosine base pair.
Inclination of bp to axis
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
Nucleotide phosphate to phosphate distance
C: 5.7 Å,
G: 6.1 Å
23 Å (2.3 nm)
20 Å (2.0 nm)
18 Å (1.8 nm)
DNA drives it into the A form, and this apparently
DNA under conditions such as the extreme desiccation of
bacteria. Protein binding can also strip solvent off of
convert it to the A form, as revealed by the structure of a rod-shaped
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. Experimental evidence
DNA as an intermediate in viral biomotor packing comes from
Förster resonance energy transfer
Förster resonance energy transfer measurements showing
DNA is shortened by 24% in a stalled ("crunched") A-form
intermediate. 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
DNA into the capsid.
Mechanical properties of DNA
^ Rosalind, Franklin (1953). "The Structure of Sodium Thymonucleate
Fibres. I. The Influence of Water Content" (PDF). Acta
Crystallographica. 6: 673–677. doi:10.1107/s0365110x53001939.
^ Dickerson, Richard E. (1992). "
DNA Structure From A to Z" (PDF).
Methods in Enzymology. 211: 67–111 – via Elsevier Science
^ 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. PMC 4208382 .
^ 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. PMC 5512286 .
^ Harvey, SC (2015). "The scrunchworm hypothesis: Transitions between
DNA and B-
DNA provide the driving force for genome packaging in
DNA bacteriophages". Journal of Structural Biology.
189: 1–8. doi:10.1016/j.jsb.2014.11.012. PMC 4357361 .
^ Oram, M (2008). "Modulation of the packaging reaction of
bacteriophage t4 terminase by
DNA structure". J Mol Biol. 381:
^ Ray, K (2010). "
DNA crunching by a viral packaging motor:
Compression of a procapsid-portal stalled Y-
DNA substrate". Virology.
398: 224–232. doi:10.1016/j.virol.2009.11.047.
Cornell Comparison of
Nucleic Acid Nomenclature
Types of nucleic acids
precursor, heterogenous nuclear
Small Cajal Body RNAs
Trans-acting small interfering