The deoxyribose nucleic acid (DNA) consists of four types of basic units called nucleotides. Each nucleotide is made up of a pentose sugar (deoxyribose type), a phosphate group and a nitrogenous base. A subunit
composed of only sugar and the nitrogen base is known as nucleoside. The four nucleosides differ from each other in the type of the base, which could be adenine (A), guanine (G), thymine (T), or cytosine (C). The adenine and guanine are the purines and thymine and cytosine are the pyrimidines.