Which nucleobase is found in RNA but not in DNA?

• A. Uracil
• B. Thymine
• C. Adenine
• D. Cytosine

Answer: (A)

Uracil is used in RNA instead of thymine, so RNA contains adenine, cytosine, guanine, and uracil, while DNA contains adenine, cytosine, guanine, and thymine. During transcription, RNA polymerase pairs uracil with adenine, reading the DNA template and producing an RNA strand that can be quickly used or degraded. The presence of thymine in DNA (instead of uracil) helps protect the genome and aids repair processes by clearly distinguishing true DNA bases from chemically altered cytosine (which can deaminate to uracil). Since only RNA uses uracil, this base stands out as the one found in RNA but not in DNA. The other bases—adenine, cytosine, and guanine—are common to both molecules.