The four bases in DNA are Adenine, Guanine, Cytosine and Thymine. Adenine and Guanine are Purines (2 ring structure). Cytosine and Thymine are Pyrimidines (single ring structure). Adenine pairs with Thymine through 2 hydrogen bonds. Guanine pairs with Cytosine through 3 hydrogen bonds. Since all base pairs contain a Purine and a Pyrimidine, they have the same molecular dimensions. Solution The four bases in DNA are Adenine, Guanine, Cytosine and Thymine. Adenine and Guanine are Purines (2 ring structure). Cytosine and Thymine are Pyrimidines (single ring structure). Adenine pairs with Thymine through 2 hydrogen bonds. Guanine pairs with Cytosine through 3 hydrogen bonds. Since all base pairs contain a Purine and a Pyrimidine, they have the same molecular dimensions..