Explain how we know base pairing takes place between a purine and pyrimidine. Purine and pyrimidine a Only the pairing between a purine and pyrimidine can explain the uniform diameter of the DNA double helix. Adenine and thymine form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. b Only the pairing between a purine and pyrimidine can explain the non-uniform diameter of the DNA double helix. Adenine and thymine form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. c Only the pairing between a purine and pyrimidine can explain the uniform diameter of the DNA double helix. Adenine and thymine form three hydrogen bonds, whereas cytosine and guanine form two hydrogen bonds. d Only the pairing between a purine and pyrimidine can explain the uniform diameter of the DNA double helix. Adenine and thymine form two disulfide bonds, whereas cytosine and guanine form three hydrogen bonds.