Chemistry - Biomolecules Question with Solution | TestHub

The structures of the given nitrogenous bases are as follows:

Adenine is 6-aminopurine and contains no keto group.

Guanine is 2-amino-6-oxopurine and contains one keto group.

Cytosine is 4-amino-2-oxopyrimidine and contains one keto group.

Thymine is 5-methyl-2,4-dioxopyrimidine and contains two keto groups.

Therefore, Thymine is the nitrogen base that contains two keto groups.

Answer: Thymine