Advanced Placement (AP) Biology Practice Exam

Which nitrogen bases are classified as purines?

1. Cytosine and thymine

2. Adenine and guanine

3. Uracil and guanine

4. Adenine and uracil

The correct answer is: Adenine and guanine

Purines are one of the two classes of nitrogenous bases found in nucleic acids, the other class being pyrimidines. Purines have a two-ring structure, which distinguishes them from pyrimidines that have a single-ring structure. The two purine bases in DNA and RNA are adenine and guanine. Adenine and guanine play crucial roles in cellular processes, including the formation of ATP (adenosine triphosphate), which is vital for energy transfer, and the construction of nucleic acids during DNA and RNA synthesis. Recognizing adenine and guanine as purines is fundamental in understanding the molecular structure of DNA and RNA, which directly relates to how genetic information is stored and transmitted within cells.