Which of the following bases are classified as purines?

The classification of bases into purines and pyrimidines is an essential concept in molecular biology. Purines are characterized by their double-ring structure, while pyrimidines have a single-ring structure.

In the context of nucleic acids, the purines are adenine (A) and guanine (G). When looking at the options provided, the choice indicating both G and A aligns correctly with this classification. These two bases play crucial roles in the structure of DNA and RNA, with adenine pairing with thymine in DNA and uracil in RNA, and guanine pairing with cytosine in both types of nucleic acids.

Other options contain bases that do not fall within the purine category. For example, cytosine (C), thymine (T), and uracil (U) are classified as pyrimidines due to their simpler single-ring structure. Therefore, the correct identification of G and A as purines is fundamental to understanding the structure of nucleic acids and their function in genetic coding and expression.