Question

Which of the following is NOT a nucleotide base in DNA?

• A. A
• B. T
• C. G
• D. U

Hint:

DNA = A, T, G, C.
RNA = A, U, G, C.
Think: "RNA is Unstable," so it has Uracil.
Also remember: Thymine is chemically \(5\)-methyluracil.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the nitrogenous base that is absent in Deoxyribonucleic acid (DNA). DNA and RNA share most bases but differ by one specific pyrimidine.
Detailed Explanation:

• Nucleotide Bases in DNA:
DNA is composed of four nitrogenous bases:
Adenine (A) and Guanine (G), which are double-ringed Purines.
Cytosine (C) and Thymine (T), which are single-ringed Pyrimidines.
These bases pair specifically: \(A\) with \(T\) (two hydrogen bonds) and \(G\) with \(C\) (three hydrogen bonds).

• Difference in RNA:
Ribonucleic acid (RNA) also contains Adenine, Guanine, and Cytosine.
However, instead of Thymine, RNA contains Uracil (U).
In RNA, Uracil pairs with Adenine.

• Why Uracil (U) is not in DNA:
DNA uses Thymine because it is more stable. Cytosine can spontaneously deaminate to form Uracil.
If DNA naturally contained Uracil, the repair enzymes wouldn't be able to distinguish between a "natural" Uracil and one formed by damage to Cytosine.
By having Thymine (\(5\)-methyluracil) as a standard base, any Uracil found in DNA is immediately recognized as damage and repaired.

Step 3: Final Answer:
Uracil (U) is found in RNA, not DNA.