The Sanderson Group Webpages
Department of Chemistry
Durham University, Durham, UK
DNA Structure - An Overview
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| Structural Features |
The Double Helix
The double helical structure of DNA as first elucidated by James
Watson and Francis Crick, with the help of Rosalind Franklin, consists of
two antiparallel DNA strands which wind around each other in a complementary
fashion. The four principal nucleosides found in DNA are cytidine (C),
thymidine (T) , guanosine (G) and adenosine (A). The former two are
purinic and the latter pyrimidinic. A:T and G:C form hydrogen bonded
pairs in the DNA helix, with G:C being slightly more stable than A:T due to
the presence of an extra hydrogen bond. G:C rich DNA therefore melts
at a higher temperature than A:T rich DNA. The polymeric 2-deoxyribose/phosphate
backbone of each of the strands lies at the outside of the structure, defining
two grooves which run along the length of the helix - the major and minor
grooves, which have diameters of 1.2 and 0.6 nm respectively. The
major groove in particular, contains accessible hydrogen bonding sites, and
is important for the binding of proteins and other modulators of DNA activity.
The external diameter of the helix measures 2 nm, and is constant along the
length of the molecule. This occurs because the geometries of the A:T
and G:C base pairs are very similar, allowing them to be superposed.
The base pairs are arranged in a stacked arrangement, with each pair being
separated by a distance of approximately 0.34 nm. Some chemical species,
such as planar aromatic compounds with a suitable electrostatic potential,
are able to insert themselves (intercalate) between the stacked bases.
This forms the basis for some DNA detection methods and some novel ways of
mimicing the action of restriction enzymes. The helical structure repeats
at 10 base pair (3.4 nm) intervals.