5Methyluracil

5-Methyluracil, commonly known as thymine, is one of the four nucleobases found in DNA. It is a pyrimidine base with a carbonyl group at position 2 and 4 and a methyl group at position 5, giving the formula C5H6N2O2. Thymine is distinguished from uracil by this methyl substituent and is not used in RNA, where uracil serves as the corresponding base.

In double-stranded DNA, thymine pairs with adenine through two hydrogen bonds, contributing to the stability and fidelity of the genetic code. The exclusion of thymine from RNA and its presence in DNA helps the cellular machinery differentiate DNA from RNA and aids in recognizing certain forms of DNA damage.

Biosynthesis and metabolism: Thymine is supplied to DNA synthesis in the form of deoxythymidine nucleotides, primarily produced from deoxyuridine monophosphate (dUMP) via the enzyme thymidylate synthase. This reaction uses methylenetetrahydrofolate as the methyl donor and is essential for DNA replication and repair. Thymidylate synthase is a target for anticancer and antiparasitic drugs, such as fluorouracil, which inhibits dTMP production.

Genomic significance: The presence of thymine in DNA, rather than uracil, helps distinguish genomic DNA from RNA and from transient cytosine deamination products. Spontaneous deamination of 5-methylcytosine in DNA yields thymine, which can lead to C-to-T transitions; cells rely on repair systems to correct these mismatches. This interplay between thymine, cytosine methylation, and repair processes is a recognized factor in mutation patterns and epigenetic considerations.

See also: Uracil, Cytosine, Thymine (nucleobase).