The building-blocks of DNA and RNA

Abstract

Nucleic acid is composed of individual acid units termed nucleotides. Each repeating unit in a nucleic acid polymer comprises three units linked together—a phosphate group, a sugar, and one of the four bases. The bases are planar aromatic heterocyclic molecules and are divided into two groups—the pyrimidine bases thymine and cytosine and the purine bases adenine and guanine. Thymine is replaced by uracil in ribonucleic acids, which also have an extra hydroxyl group at the 2' position of their (ribose) sugar groups. Individual nucleoside units are joined together in a nucleic acid in a linear manner, through phosphate groups attached to the 3' and 5'positions of the sugars. Hence the full repeating unit in a nucleic acid is a 3', 5'-nucleotide. Nucleic acid and oligonucleotide sequences use single-letter codes for the fiveunit nucleotides—A, T, G, C, and U. The two classes of bases can be abbreviated as Y (pyrimidine) and R (purine). Phosphate groups are usually designated as p.