Science Olympiad Designer Genes Practice Exam

Pyrimidines are a class of nitrogenous bases that have a single six-membered ring structure. In the context of nucleic acids, the pyrimidines are cytosine, thymine, and uracil.

Cytosine and thymine are specifically recognized as pyrimidines in DNA. Their structure is characterized by a single ring that contains two nitrogen atoms. This is significant because it differentiates them from purines, which have a fused double-ring structure and include adenine and guanine.

In the selection of bases described, the other options include combinations of both purines and pyrimidines, which does not align with the definition of pyrimidines. Therefore, the choice that includes only cytosine and thymine accurately represents pyrimidines, making it the correct answer.