The structure of DNA is a double helix, composed of two
polynucleotide strands that are coiled about one another in a spiral
(3,4). Each polynucleotide strand is held together by phosphodiester
bonds linking adjacent deoxyribose moieties. The two
polynucleotide strands are held together by a variety of noncovalent
interactions, including lipophilic interactions between adjacent
bases and hydrogen-bonding between the bases on opposite
strands. The sugar-phosphate backbones of the two complementary
strands are antiparallel; that is, they possess opposite chemical
polarity. As one moves along the DNA double helix in one
direction, the phosphodiester bonds in one strand will be oriented
5′–3′, whereas in the complementary strand, the phosphodiester
bonds will be oriented 3′–5′. This configuration results in basepairs
being stacked between the two chains perpendicular to the
axis of the molecule. The base-pairing is always specific: Adenine
is always paired to thymidine, and guanine is always paired to
cytosine.