5-Methylcytosine

A modified form of the DNA base cytosine

5-Methylcytosine (5mC) is a methylated form of the nucleotide cytosine that plays a significant role in the epigenetic regulation of gene expression in eukaryotic organisms. It is a key component of the DNA methylation process, which is crucial for normal development and cellular differentiation.

Structure and Formation

5-Methylcytosine is formed by the addition of a methyl group to the 5th carbon of the cytosine pyrimidine ring. This methylation is catalyzed by a family of enzymes known as DNA methyltransferases (DNMTs). The reaction mechanism involves the transfer of a methyl group from S-adenosyl methionine (SAM) to the cytosine base, resulting in the formation of 5-methylcytosine.

File:DNMT reaction mechanism.tif

Mechanism of DNA methylation by DNA methyltransferases.

Biological Function

5-Methylcytosine is primarily found in the CpG dinucleotide context in vertebrate genomes. It is involved in the regulation of gene expression, genomic imprinting, X-chromosome inactivation, and suppression of transposable elements. Methylation of cytosine residues in promoter regions is generally associated with transcriptional repression.

Role in Epigenetics

As an epigenetic marker, 5-methylcytosine is heritable through cell division but can be dynamically regulated in response to environmental factors. Changes in DNA methylation patterns are associated with various diseases, including cancer, where abnormal methylation can lead to the silencing of tumor suppressor genes.

Deamination and Mutation

5-Methylcytosine can undergo spontaneous deamination to form thymine, which can lead to point mutations if not repaired. This process is a common source of C to T transition mutations in the genome.

Deamination of 5-methylcytosine to thymine.

Related pages

• DNA methylation
• Epigenetics
• DNA methyltransferase
• Cytosine