Which nitrogenous base is found only in RNA and not in DNA?

Introduction / Context:
Nucleic acids such as DNA and RNA are made up of nitrogenous bases that carry genetic information. This question checks your ability to distinguish which base is unique to RNA and not present in DNA. This is a basic but essential fact in genetics and molecular biology.

Given Data / Assumptions:
• DNA contains the bases adenine (A), thymine (T), cytosine (C), and guanine (G). • RNA contains the bases adenine (A), uracil (U), cytosine (C), and guanine (G). • Only one base in the list is present in RNA but absent in DNA.

Concept / Approach:
The main difference in nitrogenous base composition between DNA and RNA is that DNA uses thymine, whereas RNA uses uracil in its place. Both molecules share adenine, cytosine, and guanine. Therefore, the base that is unique to RNA and not found in DNA is uracil. This difference is important for base pairing during processes like transcription and translation.

Step-by-Step Solution:
Step 1: List the DNA bases: A, T, C, and G. Step 2: List the RNA bases: A, U, C, and G. Step 3: Compare the two sets and identify the base that appears only in RNA. Step 4: Notice that thymine appears only in DNA and uracil only in RNA. Step 5: Choose uracil as the base found only in RNA.

Verification / Alternative check:
You can confirm this by looking at standard diagrams of nucleotide structures in biology textbooks. RNA nucleotides are drawn with uracil, while DNA nucleotides show thymine. In many practice problems, questions about DNA RNA differences emphasize the thymine uracil substitution. This consistent emphasis across sources verifies that uracil is unique to RNA in this context.

Why Other Options Are Wrong:
Option A (Thymine): Thymine is found in DNA but is not present in RNA, so it is unique to DNA, not RNA. Option B (Adenine): Adenine is present in both DNA and RNA, so it is not unique to RNA. Option D (Cytosine): Cytosine is also found in both DNA and RNA. Option E (Guanine): Guanine is shared by both DNA and RNA as well.

Common Pitfalls:
A common confusion is to mix up which molecule has thymine and which has uracil. Some students remember that there is a difference but cannot recall which base goes with which nucleic acid. A helpful trick is to associate the letter T with DNA and the letter U with RNA, since T and D are both straight letters and U and R both have a curve, making a small mental pattern. This can help you quickly recall that uracil is found only in RNA.

Final Answer:
The nitrogenous base found only in RNA and not in DNA is uracil.