Animated Solution for Chemistry - Biomolecules: Which one of the following is correct structure for cytosine?

Cytosine is a fundamental building block of life, serving as one of the four main chemical bases found in both DNA and RNA. To identify its correct structure among various options, we need to understand its chemical composition.

At its core, cytosine is a pyrimidine derivative. This means its foundational structure is a single, six-membered heterocyclic aromatic ring that contains two nitrogen atoms. This ring forms the backbone upon which specific functional groups are attached.

What distinguishes cytosine from other pyrimidine bases (like thymine and uracil) are the specific substituents attached to this ring:

1. Amino Group: Cytosine features an amino group ($-N{H}_2$) attached at position 4 of the pyrimidine ring. 2. Keto Group: It also has a keto group ($=O$) attached at position 2.

When we examine the given options, we are looking for a structure that combines these three elements: a pyrimidine ring, an amino group at position 4, and a keto group at position 2.

By carefully analyzing the provided chemical structures, we can see that option (c) perfectly matches this description. It displays the six-membered ring with the two nitrogen atoms, the $=O$ at the bottom (position 2), and the $-N{H}_2$ at the top (position 4). Therefore, option (c) represents the correct structure of cytosine.