A nucleotide is composed of a nitrogenous base, a five-carbon sugar (either ribose or 2'-deoxyribose), and one to three phosphate groups. Together, the nitrogenous base and sugar comprise a nucleoside. The phosphate groups form bonds with either the 2, 3, or 5-carbon of the sugar, with the 5-carbon site most common. A nucleotide is one of the building blocks of ribonucleic acids (RNA) and deoxyribonucleic acid (DNA). Nucleotides are linked by enzymes in order to make long, chainlike polynucleotides of defined sequence. The order or sequence of the nucleotide units along a polynucleotide chain plays an important role in the storage and transfer of genetic information.
A nucleotide molecule contains three functional groups: a base, a sugar, and a phosphate. It may seem puzzling that a nucleic acid should contain a base. While the base portion does have weakly basic properties, the nucleotide as a whole acts as an acid, due to the phosphate group.