DNA is a long chain, or polymer, composed of similar units repeated over and over. The units in DNA are composed of a sugar (2-deoxy-D-ribose), phosphoric acid, and one of four types of basic (ammonia-like) molecules called purines and pyrimidines. The backbone of DNA is composed of alternate sugar and phosphoric acid groups, connected together. Attached to each sugar molecule is also a purine or a pyrimidine base. The base, its sugar, and the attached phosphoric acid group, are called a nucleotide.
There are two polymer chains in a DNA molecule. They run in an antiparallel direction, and are held together by many very weak hydrogen bonds.
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Last revised on 5 January 2004
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