DNA and RNA, the principal genetic materials of living organisms are chemically called nucleic acids. These are polymers of nucleotides. Each nucleotide is composed of phosphoric acid, pentose sugar and nitrogenous base. The nitrogenous bases are of two type, ie, purine and pyrimidines. Purines are heterocyclic and two rings compound, cg, adenine, guanine. Pyrimidines are single ring compound, eg, thymine, cytosine, uracil.