Which nucleotide base is found only in RNA molecules (and replaces thymine in DNA)?

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

Answer: (A)

Verified step by step guidance

1. Understand the basic structure of nucleic acids: DNA and RNA are both made up of nucleotide bases, but they differ slightly in their components.
2. Recall that DNA contains four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
3. Recall that RNA also contains four bases, but instead of thymine, it has uracil (U) paired with adenine.
4. Recognize that uracil is unique to RNA and replaces thymine, which is found only in DNA.
5. Therefore, the nucleotide base found only in RNA molecules (and replacing thymine in DNA) is uracil.