According to the double-helical structure of DNA, the nitrogenous bases of the two polynucleotide chains are paired through hydrogen bonds. Adenine (a purine) always pairs with Thymine (a pyrimidine) via two hydrogen bonds, and Guanine (a purine) always pairs with Cytosine (a pyrimidine) via three hydrogen bonds . Uracil is present in RNA instead of thymine and is not found in DNA . Therefore, the complementary base pairs in DNA are adenine with thymine, and guanine with cytosine.