Which base is found in DNA but not in RNA?

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

Answer: (A)

This question tests the difference in base composition between DNA and RNA. DNA uses thymine to pair with adenine, while RNA uses uracil instead of thymine. Thymine has a methyl group that uracil lacks, and this chemical difference helps stabilize DNA and helps repair systems recognize true bases versus damaged or misread ones (for example, distinguishing deaminated cytosine, which would produce uracil, from normal thymine in DNA). Because RNA does not use thymine and instead uses uracil, thymine is the base found in DNA but not in RNA. The other bases—adenine and cytosine—appear in both DNA and RNA, while uracil is specific to RNA.