Question

Identify which of the following heterocyclic nitrogenous bases is found exclusively within the macromolecular structure of RNA, and is completely absent in standard double-stranded DNA structures.

• A. Thymine
• B. Adenine
• C. Uracil
• D. Cytosine

Hint:

Chemically, Thymine is also known as 5-Methyluracil. This means you can think of DNA's base as a methylated version of RNA's uracil, which provides greater structural stability for long-term genetic storage.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Nucleic acids, the genetic material of all living organisms, are polymers called polynucleotides.
The two main types are DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).
Each monomer (nucleotide) is made of a pentose sugar, a phosphate group, and a nitrogenous base.
There are five standard nitrogenous bases, classified into Purines (double rings) and Pyrimidines (single rings).
Step 2: Detailed Explanation:
To solve this, we compare the base composition of DNA and RNA:

DNA Base Composition:
- Purines: Adenine (A) and Guanine (G).
- Pyrimidines: Cytosine (C) and Thymine (T).

RNA Base Composition:
- Purines: Adenine (A) and Guanine (G).
- Pyrimidines: Cytosine (C) and Uracil (U).

Analysis:
Adenine, Guanine, and Cytosine are common to both types of nucleic acids.
Thymine is found only in DNA.
Uracil is found only in RNA.

Chemically, Thymine is very similar to Uracil; it is actually 5-methyluracil. The presence of the methyl group in DNA's thymine provides greater structural stability and protection against certain types of chemical damage, which is essential for long-term genetic storage. RNA, being a more temporary messenger molecule, utilizes the simpler uracil.
Step 3: Final Answer:
Since the question asks for a base found exclusively in RNA, the answer is Uracil.
The correct option is (C).