Why must a puritine pair with a pyradine?

Pairing of a specific purine to a specific pyrimidine is due to the structure and properties of each ring and the three-dimensional fit between complements. Purine and pyrimidine are organic, aromatic ring compounds that act as the building blocks of nucleic acid. In DNA, purines occur as adenine and guanine, while pyrimidines occur as thymine and cytosine. In DNA base pairing, adenine pairs with thymine, and guanine pairs with cytosine