Why can’t a purine hydrogen bond with another purine within the sugar phosphate backbone of DNA? State your reason with all the details that are required to answer the question for full credit.

Adenine and guanine are purines (two-carbon nitrogen ring bases) that pair with thymine and cytosine pyrimidine nitrogen bases (one-carbon nitrogen ring bases), respectively. These specific base-pair rules are due to their hydrogen bonds (i.e., van der Waals forces), which stabilize the structure of the DNA helix. Cytosine and guanine have three sites to form hydrogen bonds to each other, while adenine and thymine have only two sites. The specific interactions between purines and pyrimidines are critical for proper bonding and attachment to the sugar-phosphate DNA backbone (i.e., for the formation of the DNA double-helical arrangement).