The heterocyclic compounds in nucleic acids are the nitrogenous bases named adenine, guanine, uracil, cytosine, and thymine. Adenine and guanine are substituted purines while the rest are substituted pyrimidines. The skeletal heterocyclic ring is composed of purine and pyrimidine. The sugar found in polynucleotides is either ribose (a monosaccharide pentose) or 2’ deoxyribose. A nucleic acid containing deoxyribose is called deoxyribonucleic acid (DNA), while that which contains ribose is called ribonucleic acid (RNA). Another difference between DNA and RNA is that DNA has thymine (a pyrimidine) while the RNA has uracil (a pyrimidine).