Purines and pyrimidines are the nitrogenous bases found in the genetic material. Purines include Adenine (A) and Guanine (G) while pyrimidines include Thymine (T), Cytosine (C) and Uracil (U). Purines always pair with pyrimdine and keeps the strands of DNA at the same distance. THIS IS BECAUSE, PURINES ARE DOUBLE RINGED STRUCTURES AND PYRIMIDINES ARE SINGLE RINGED STRUCTURES. PAIRING OF PURINES WITH PYRIMIDINES MAINTAIN THE STRUCTURE OF THE DNA.