Advanced Placement (AP) Biology Practice Exam

Pyrimidines are one of the two categories of nitrogenous bases found in nucleic acids, the other being purines. The pyrimidines include cytosine, thymine, and uracil. These bases are characterized by their single-ring structure, which distinguishes them from purines, which have a double-ring structure.

Cytosine is found in both DNA and RNA, thymine is specific to DNA, and uracil replaces thymine in RNA. Understanding the classification of these bases is essential for comprehending the structure of nucleic acids and their roles in genetic information storage and transfer.

Other options presented do not correctly identify the pyrimidines; for instance, adenine and guanine are purines, while the reference to the categories of bases doesn't specify any classification as pyrimidines. Therefore, identifying cytosine, thymine, and uracil as pyrimidines is accurate and reinforces the basic organization of nucleotides within DNA and RNA.