DNA and RNA are together referred to as nucleic acid. Nucleotides are the structural units of nucleic acid. Each nucleotide is composed of three parts a nitrogen containing base, a pentose sugar called deoxyribose or ribose and a phosphate group. The nitrogenous base are purine (A and G) and pyrimidine (T, C, U). The term nucleosides refers to the combination of a purine or pyrimidine plus a pentose sugar.