DNA Structure - An Overview

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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.