Nitrogenous base not present in DNA?

Appeared in: NVS- 2025

Explanation

Uracil is a pyrimidine base that is found almost exclusively in RNA (Ribonucleic Acid), where it replaces Thymine.
In DNA, the four bases are Adenine, Guanine, Cytosine, and Thymine.
The structural difference between Uracil and Thymine is a methyl group present on Thymine, which is absent in Uracil.
In RNA, Uracil pairs with Adenine, just as Thymine pairs with Adenine in DNA.

Option A: Cytosine is a pyrimidine base that is a fundamental component of both DNA and RNA.
Option C: Adenine is a purine base that is a fundamental component of both DNA and RNA.
Option D: Guanine is a purine base that is a fundamental component of both DNA and RNA.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Composition of nitrogenous bases in DNA and RNA as background academic context rather than a clinical decision trigger.
Understanding the difference between DNA and RNA bases is fundamental to genetics and molecular biology, which underpins many diagnostic tests (like PCR for viral DNA or RT-PCR for viral RNA) and therapies (like mRNA vaccines).
Many antiviral drugs, such as Acyclovir or Zidovudine (AZT), are nucleoside analogues. They work by mimicking natural bases and interfering with viral DNA or RNA synthesis. Knowledge of base composition is key to understanding their mechanism.
What if? - If the question asked which base is NOT present in RNA, the correct answer would be Thymine. Thymine is unique to DNA and is replaced by Uracil in RNA molecules.

How to Approach the Question

First, identify the core concept of the question, which is the composition of DNA.
The keyword 'NOT' is critical; you are looking for the component that is absent from DNA.
Recall the four nitrogenous bases that make up DNA: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). A helpful mnemonic is 'All Teachers Go Crazy'.
Recall the four bases in RNA: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
Compare the options provided with the known composition of DNA.
Identify the base from the options that is not in the list of DNA bases. In this case, Uracil is present in RNA but not DNA.

Concept Tested & Keywords

QdxqoiQB0JbIfw1FtnejCa

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart A (pp 1-239 of 478) p. 216-218

E6 Biochemistry U Satyanarayana— Part 1 (pp 26-420 of 840) p. 61-63

E6 Physiology Ganong 27e Vol 1 Part 1 p. 21-23

Practise the full NVS- 2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.