Animated Solution for Chemistry - Organic Chemistry: Among the following compounds, which one is found in RNA?

DNA vs RNA

To solve this problem, we must first embark on a journey into the molecular core of life itself. Both DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are polymers made up of repeating units called nucleotides. Each nucleotide consists of three parts: a phosphate group, a sugar molecule, and a nitrogenous base.

While the sugar backbone differs slightly (deoxyribose in DNA versus ribose in RNA), the most critical functional difference lies in the nitrogenous bases they employ to encode genetic information.

Both DNA and RNA share three common bases: - Adenine (A) - Guanine (G) - Cytosine (C)

However, they diverge on the fourth base. DNA utilizes Thymine (T), whereas RNA utilizes Uracil (U). This subtle substitution is a hallmark of molecular biology and is exactly what this question is testing.

The bases we are dealing with in this question belong to a family of molecules called pyrimidines. A pyrimidine is a simple aromatic ring containing four carbon atoms and two nitrogen atoms. Let's analyze the suspects provided in the options one by one.

Option (a): Uracil Look closely at the first structure. It features the classic six-membered pyrimidine ring with two carbonyl ($C=O$) groups located at positions 2 and 4. There are no other substituents attached to the ring carbons. This pristine, unadorned structure is Uracil. As we established, Uracil is the signature base found exclusively in RNA, where it pairs with Adenine during the transcription process.

Option (b): Thymine Now, examine the second structure. At first glance, it looks almost identical to Uracil. It has the same pyrimidine ring and the same two carbonyl groups. However, there is a crucial addition: a methyl ($-C{H}_3$) group attached to the 5th carbon position. Because of this, Thymine is chemically known as 5-methyluracil. This extra methyl group is nature's way of tagging the base for use exclusively in DNA. Therefore, Thymine is not found in RNA.

Option (c): Cytosine The third structure is quite different. Instead of two carbonyl groups, it has an amino ($-N{H}_2$) group at the 4th position and a single carbonyl group at the 2nd position. This is Cytosine. While it is true that Cytosine is found in RNA, it is also a fundamental component of DNA. In the context of multiple-choice questions asking "which one is found in RNA," the examiner is invariably looking for the base that uniquely distinguishes RNA from DNA. That unique identifier is Uracil.

Option (d): N-methyluracil The final structure shows a methyl group attached directly to the nitrogen atom at the bottom of the ring (position 1 or 3 depending on numbering conventions, but here it's an N-methylated derivative). This is N-methyluracil. This molecule is a synthetic derivative and does not occur naturally as a standard base in either DNA or RNA.

By systematically decoding the chemical structures, we have identified Uracil, Thymine, Cytosine, and a synthetic derivative. Because Uracil is the characteristic and exclusive pyrimidine base of RNA, it is the definitive answer to the question.