The nitrogenous bases are purines and pyrimidines. A nucleic acid sequence is the order of nucleotides within a DNA (GACT) or RNA (GACU) molecule that is determined by a series of letters. The pyrimidines are thymine, uracil and cytosine. Nucleic acids can be defined as organic molecules present in living cells. Nucleic acids are formed when nucleotides come together through phosphodiester linkages between the 5' and 3' carbon atoms. Nucleic acids consists nucleotides made of of nitrogenous base, sugar and phosphate. Purine bases found in nucleic acids and are heterocyclic compounds consisting of a pyrimidine ring and an imidazole ring fused together. The elements found in nucleic acids are: carbon, hydrogen, oxygen, nitrogen and phosphorus. The purines are adenine and guanine. The two purine bases are- Adenine (6-Amino Purine): (C5H5N5), found in both RNA and DNA, is a white crystalline purine base, with Molecular weight 135.15 daltons and melting point 360 to 365 C. The purines are double ring structure while the pyrimidines are single ring made up of carbon hydrogen oxygen and nitrogen. A nucleic acid is a complex organic substance which is present in all living cells, such as DNA or RNA. The “backbone” of a nucleic acid molecule is formed by the repeating sequence of pentose and phosphate groups, and this is the same in all molecules. Nucleic acids are long-chain polymeric molecules. The monomer or the repeating unit is known as the nucleotides and hence sometimes nucleic acids are referred to as polynucleotides. It plays a key factor in transferring genetic information from one generation to the next.