Q.30 Methylation of Uracil forms which of the following?

(1) Adenine

(2) Cytosine

(3) Guanine

(4) Thymine

What is Methylation of Uracil?

Uracil (U) is a pyrimidine base in RNA, while thymine (T) replaces it in DNA. Methylation adds a methyl group (-CH₃) to uracil’s structure, converting it to thymine. This enzymatic reaction, catalyzed by thymidylate synthase, is crucial for DNA synthesis and stability, preventing mutations from cytosine deamination.

Reaction: dUMP + CH₂-THF → dTMP + DHF
(Where dUMP is deoxyuridine monophosphate.)

Correct Answer: Methylation of Uracil Forms Thymine (Option 4)

The correct answer is (4) Thymine.

Thymine is 5-methyluracil. The methyl group attaches at the 5-position of uracil’s ring, distinguishing DNA from RNA bases. This modification enhances DNA fidelity, as thymine pairs specifically with adenine.

Biochemical pathway: In de novo synthesis, uracil derivative dUMP gains a methylene group from 5,10-methylene tetrahydrofolate.

Explanation of All Options

• (1) Adenine: Purine base (6-aminopurine). Synthesized from PRPP via IMP pathway; no relation to uracil methylation.

• (2) Cytosine: Pyrimidine base. Formed from UMP via CTP synthetase (adds amino group); uracil precursor, but methylation doesn’t produce it.

• (3) Guanine: Purine base (2-amino-6-oxopurine). Derived from IMP with amination and oxidation; unrelated to pyrimidines.

• (4) Thymine: Pyrimidine (5-methyluracil). Direct product of uracil methylation in DNA synthesis.

Structure comparison:

• Uracil: \chemfig∗∗6(=−=−=−O)−NH−O\chemfig∗∗6(=−=−=−O)−NH−O

• Thymine: \chemfig∗∗6(=−=−C(−CH3)−O)−NH−O\chemfig∗∗6(=−=−C(−CH3)−O)−NH−O (methyl at C5).

Importance in DNA vs. RNA

Thymine’s methylation protects DNA from uracil-DNA glycosylase repair (which removes uracil as damage). Folate deficiency impairs this, causing issues like neural tube defects.

Master pyrimidine synthesis or folate metabolism in our biochemistry guides.