Nucleotide composition — Uracil (U) occurs naturally in which nucleic acid type under standard cellular conditions?

Introduction / Context:
Distinguishing DNA from RNA by their constituent bases is a basic competency in molecular biology. The presence of uracil instead of thymine differentiates RNA from DNA and has consequences for stability, repair pathways, and chemical reactivity of nucleic acids.

Given Data / Assumptions:

• Standard cellular nucleic acids are considered (not synthetic or damaged DNA).
• DNA uses thymine (T) where RNA uses uracil (U).
• Exceptions such as deaminated cytosine or specialized DNA modifications are not the norm.

Concept / Approach:
RNA contains adenine, guanine, cytosine, and uracil; DNA contains adenine, guanine, cytosine, and thymine. Uracil in DNA is typically recognized as damage (e.g., from cytosine deamination) and is removed by base-excision repair. Therefore, the correct natural home for uracil is RNA.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays for RNA often exploit U-specific labeling or enzymes, reflecting the ubiquity of uracil in RNA.

Why Other Options Are Wrong:

• DNA: normally uses thymine; uracil presence triggers repair.
• Both: incorrect in the canonical sense.
• None: incorrect because RNA is correct.

Common Pitfalls:
Confusing rare DNA modifications or damage with standard composition; remember the canonical distinction U (RNA) vs T (DNA).

Final Answer:
RNA