Which statement about pyrimidine bases is true?

Which statement about pyrimidine bases is true?

• A. They are composed of two ring structures.
• B. They pair with purines through three hydrogen bonds.
• C. They are found exclusively in RNA.
• D. They have a single-ring structure.

Answer: (D)

Pyrimidine bases in nucleic acids are defined by having a single-ring structure, which distinguishes them from purines that have two rings. This single-ring characteristic is why the statement about their structure is correct. The idea that they pair with purines through three hydrogen bonds isn’t universal— cytosine pairs with guanine using three hydrogen bonds, but thymine and uracil pair with adenine using only two. Also, pyrimidines aren’t found exclusively in RNA, since thymine is present in DNA.

Pyrimidine bases in nucleic acids are defined by having a single-ring structure, which distinguishes them from purines that have two rings. This single-ring characteristic is why the statement about their structure is correct. The idea that they pair with purines through three hydrogen bonds isn’t universal— cytosine pairs with guanine using three hydrogen bonds, but thymine and uracil pair with adenine using only two. Also, pyrimidines aren’t found exclusively in RNA, since thymine is present in DNA.