Which of the following bases are classified as purines?

Purines are one of the two classes of nitrogenous bases found in nucleic acids, the other being pyrimidines. The purines are characterized by their two-ring structure, which distinguishes them from pyrimidines, which have a single-ring structure.

In this context, adenine and guanine are the only two nitrogenous bases that are classified as purines. Adenine is a fundamental component of DNA and RNA and plays a critical role in energy transfer through ATP (adenosine triphosphate). Guanine, similarly, is a key component of both DNA and RNA, contributing to the coding of genetic information and the regulation of cellular metabolism.

Cytosine and thymine, as well as cytosine and uracil, are classified as pyrimidines. These bases have a single-ring structure, differing from purines in their chemical makeup and properties. This structural distinction helps to categorize them properly within the framework of molecular biology and genetics.

Thus, the identification of adenine and guanine as purines is correct because they share the necessary two-ring structure characteristic of purines, making option B the correct choice.