DNA is a complex, double-stranded polysaccharide composed of a backbone containing phosphate groups bonded to deoxyribose sugar.
Two phosphate groups are covalently bonded to each ring-shaped sugar. One is linked to the third carbon (3') on the molecule, while the other is linked to the fifth (5') carbon, forming a 3'-5' glycosidic bond. This confers directionality to each nucleoside, since one is constructed (biosynthesized) from 3' to 5'and the other from 5' to 3'. The physical torsion on the molecule owing to this structure causes the DNA molecule to twist, taking on a shape called a double helix.
Each sugar is in turn bonded one of four nitrogen-containing bases. Each combined phosphate-sugar-nitrogenous base forms a unit called a nucleotide. Nucleotides linked together compose a nucleoside.
Two of these bases, adenine and guanine, are called purines and have a double-carbon ring structure.
The other two bases, thymine and cytosine, are called pyrimidines and have a single-carbon ring structure.
Adenine on one nucleoside of the DNA molecule can form two hydrogen bonds with thymine on the other nucleoside.
Guanine can form three hydrogen bonds with cytosine in a similar fashion. This complementary base pairing insures that during replication of DNA, the exact code for the genetic makeup of an organism is retained in each new cell.
The length of a DNA molecule varies by species and is generally not expressed in metric units. Instead, length is expressed as the number of base pairs (bp, i.e. A-T, G-C) of nucleotides each molecule contains.
Sequences of DNA base pairs that carry information (code) for the production of a specific product or regulate the activities of other DNA sequences that do this are called genes.