Which of the following are the purines?

The purines are classified as a group of nitrogenous bases that are characterized by a two-ring structure. In the context of nucleic acids, adenine and guanine are the two purines found in both DNA and RNA. They play a vital role in the storage and transfer of genetic information.

Adenine is essential for the formation of adenosine triphosphate (ATP), which serves as the energy currency of the cell, while guanine is involved in various processes, including metabolism and signal transduction. The structural complexity of purines allows them to pair with pyrimidines (such as cytosine, thymine, and uracil) in the formation of nucleic acids.

The other answer choices contain combinations of nitrogenous bases that do not include both members of the purine group. For example, choices containing only pyrimidines or a mixture where neither is a purine do not fit the definition of purines. Hence, adenine and guanine remain the correct answer as they are recognized as the purines in biological systems.