A Level Biology Practice Exam

What distinguishes pyrimidines from purines as nitrogenous bases?

1. Pyrimidines have a double-ring structure

2. Pyrimidines include adenine and guanine

3. Pyrimidines have a single-ring structure

4. Pyrimidines are larger than purines

The correct answer is: Pyrimidines have a single-ring structure

Pyrimidines are distinguished from purines by their structure, specifically because they have a single-ring structure. This characteristic sets them apart from purines, which are characterized by their double-ring structure. The single-ring structure of pyrimidines consists of a six-membered ring containing carbon and nitrogen atoms, while purines contain a fused two-ring system made up of one six-membered ring and one five-membered ring. Understanding the structural differences is crucial, as this influences how these bases pair with each other in DNA and RNA. For instance, pyrimidines include the bases cytosine, thymine, and uracil, and they pair with purines, such as adenine and guanine, during the formation of nucleic acids.