AP BIOLOGY

Detailed explanation-1: -All purines are nitrogen bases present in DNA and RNA that posses double ring structures. The purines present are adenine and guanine. So, the correct option is ‘Adenine’.

Detailed explanation-2: -The purines (adenine and guanine) have a two-ringed structure consisting of a nine-membered molecule with four nitrogen atoms, as you can see in the two figures below.

Detailed explanation-3: -As you can see, cytosine and thymine have a single-ring structure, while adenine and guanine have a double-ring structure. Cytosine and thymine are known as pyrimidines, while adenine and guanine are known as purines. Therefore, the two categories of these bases are called purines and pyrimidines.

Detailed explanation-4: -The two-carbon nitrogen ring bases (adenine and guanine) are purines, while the one-carbon nitrogen ring bases (thymine and cytosine) are pyrimidines.

Detailed explanation-5: -Note that the purine bases (adenine and guanine) have a double ring structure while the pyrimidine bases (thymine and cytosine) have only a single ring.