Identify the group which contains only pyrimidines.
A. Adenine, cytosine
B. Uracil, cytosine
C. Guanine, uracil and cytosine
D. Adenine, guanine

Hint: The purines and pyrimidines function as the main genetic material. DNA and RNA are completely made of these nitrogenous bases. Besides being a part of genetics, pyrimidines have other uses as well. Pyrimidines along with their derivatives can be found in vitamin supplements, antibiotics, and some toxins. Some derivatives are used in medicines such as anti-epilepsy, anti-malaria, and blood pressure medication.

Complete answer:
Pyrimidines are aromatic compounds that form a ring structure. The three pyrimidines are – cytosine, thymine, and uracil. They are a part of the basic functional and structural blocks of nucleic acids like DNA and RNA.
As we already know, the two strands of DNA are bound by hydrogen bonding. A purine binds to its corresponding pyrimidine. This happens in the following manner:
- Adenine binds to thymine with two hydrogen bonds
- Guanine binds to cytosine with three hydrogen bonds

When DNA turns into RNA, the thymine residues get replaced by uracil.
So, an RNA can have the bases – Adenine, Uracil, Guanine, and Cytosine.

Pyrimidines are made of two nitrogen atoms and four carbon atoms and each of the carbon atoms is bound to a hydrogen atom. The nitrogenous bases in DNA and RNA are actually modified forms of pyrimidines. They have a carbonyl group on the 2’ carbon atom. This results in two double bonds and four single bonds in the ring structure. Depending on the particular base, a functional group is also attached to one of the carbons.

Hence, the correct answer is option (B).

Note: Adenine, guanine, cytosine, thymine, uracil are all nitrogenous bases that are found in DNA and RNA. However, in RNA, uracil is present at the place of thymine. Two of them are purines and three are pyrimidines.
Purines and pyrimidines are different in structures. Purines are made of one carbon-hydrogen ring, whereas pyrimidines are made of two carbon-hydrogen rings.