Which base contains a methyl group as part of its structure?

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

Answer: (B)

A methyl group attached to a DNA base is a distinguishing feature that helps identify the molecule as thymine. In thymine, the methyl group sits at the 5-position of the pyrimidine ring. This CH3 substituent is not involved in forming the standard base-pairing hydrogen bonds—thymine still pairs with adenine using two hydrogen bonds, just like the other pyrimidines. What matters is that thymine uniquely carries that 5-position methyl group, whereas the other bases listed do not. Uracil is like thymine structurally but lacks the 5-methyl group, making it the RNA counterpart. Cytosine and adenine have their own distinct substituents and ring structures, but neither carries a 5-position methyl group. This methyl helps distinguish DNA from RNA and contributes to DNA's stability and recognition by cellular machinery.