Which nitrogenous base is found in DNA but not in RNA?

The nitrogenous base found in DNA but not in RNA is thymine. Thymine, abbreviated T, pairs with adenine in DNA and helps form the information-carrying sequence of the double helix.

RNA usually uses uracil, abbreviated U, in place of thymine. Uracil also pairs with adenine, so the substitution does not change the basic pairing logic. The other options—cytosine and guanine—are found in both DNA and RNA.

Chemically, thymine is 5-methyluracil, meaning it has an additional methyl group compared with uracil. This difference contributes to DNA’s long-term stability and helps cells detect some types of damage. Cytosine can spontaneously lose an amino group and become uracil; because normal DNA uses thymine, repair systems can recognize an unexpected uracil as damage.

The rule is a useful simplification: RNA molecules containing thymine do exist in unusual biological contexts, but standard introductory biology treats thymine as DNA’s base and uracil as RNA’s corresponding base.