Nucleic acids are polymers made up of sub-units called nucleotides. Each nucleotide is composed of three smaller units ; a nitrogenous base, a pentose (5 carbon) sugar and a phosphate. The nitrogenous base is a cyclic compound that can be anyone of the two forms - purines (two rings) and pyrimidines (one ring). There are two types of purines - adenine and guanine (G), and three types of pyrimidines - thymine (T), cytosine (C) and uracil (U).