Why purine is opposite to pyrimidine or vice versa in a nucleic acid?

Purines and pyrimidines are complementary to each other.

In DNA base pairing takes place, it is called complementary base pairing because purines (adenine, guanine) are complementary to pyrimidines (thymine, cytosine, uracil) and thus a purine always pairs with a pyrimidine. Adenine pairs with thymine and cytosine pairs with guanine always in a DNA molecule.  

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