Nitrogenous bases are of two types-purines and pyrimidines. Purines are 9-membered double ring nitrogen bases, which possess nitrogen at 1,3,7 and 9 position, eg, adenine , guanine (G). Pyrimidines are 6-membered nitrogen bases that contain nitrogen at 1 and 3 positions, eg, cytosine , thymine (T), uracil (U).