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

Concept: Nucleic acids are biopolymers constructed from nucleotide monomers. Each nucleotide contains a pentose sugar, a phosphate group, and a nitrogenous base. These nitrogenous bases are divided into purines (double-ring structures) and pyrimidines (single-ring structures).

Step 1: Compare the nitrogenous bases used in DNA versus RNA.

• DNA Base Composition: Uses the purines Adenine (A) and Guanine (G), and the pyrimidines Cytosine (C) and Thymine (T).
• RNA Base Composition: Uses the exact same purines, Adenine (A) and Guanine (G), and the pyrimidine Cytosine (C). However, it replaces thymine with a different pyrimidine base called Uracil (U).

Step 2: Identify the base unique to RNA.
Uracil is found exclusively in RNA molecules, where it forms two hydrogen bonds with adenine during transcription. DNA structures utilize thymine instead.