Question

What would be the correct structure of thymine base?

• A. figA
• B. figB
• C. figC
• D. figD

Hint:

A simple memory trick is: Thymine = Uracil + Methyl group at the 5-position.

Answer 1

The Correct Option is B

Concept: Thymine is one of the pyrimidine nitrogenous bases present in DNA. The pyrimidine bases are: \[ \text{Cytosine},\quad \text{Thymine},\quad \text{Uracil} \] Thymine differs from uracil by the presence of a methyl group at the 5-position of the pyrimidine ring.

Step 1: Recall the structures of important pyrimidine bases.
Cytosine: \[ 4\text{-amino-1H-pyrimidin-2-one} \] Uracil: \[ 1\text{H-pyrimidine-2,4-dione} \] Thymine: \[ 5\text{-methyl-1H-pyrimidine-2,4-dione} \]

Step 2: Compare the given options.
Option (A) represents cytosine. Option (C) represents uracil. Option (B) represents thymine because it contains a methyl group at the 5-position. Option (D) is not the naturally occurring thymine structure.

Step 3: Select the correct structure.
Since thymine is 5-methyl uracil, its correct IUPAC description is \[ 5\text{-methyl-1H-pyrimidine-2,4-dione} \]