NH2 Nitrogenous bases N Bicyclic purines Adenine(A) Guanine (G) Monocyclic pyrimidine Cytosine(C) Uracil (U;R=H) Thymine(T;R=CHa) Thymine(T)is 5-methyluracil(U) Nitrogenous bases Bicyclic purines: Monocyclic pyrimidine: Thymine (T) is 5-methyluracil (U)