5-Methylcytosine is a methylated form of cytosine in which a methyl group is attached to carbon 5, altering its structure without altering its base-pairing properties.

5-Methylcytosine is an epigenetic modification formed by the action of DNA methyltransferases. In bacteria, 5-methylcytosine can be found at a variety of sites, and is often used as a marker to protect DNA from being cut by native methylation-sensitive restriction enzymes. In plants, 5-methylcytosine occurs at both CpG and CpNpG sequences. In fungi and animals, 5-methylcytosine predominately occurs at CpG dinucleotides. Although most eukaryotes methylate only a small percentage of these sites, in vertebrates 70-80% of CpG cytosines are methylated.

While spontaneous deamination of cytosine forms uracil, which is recognized and removed by DNA repair enzymes, deamination of 5-methylcytosine forms thymine. This conversion of a DNA base from cytosine (C) to thymine (T) can result in a transition mutation.

5-Methylcytosine can be deaminated to form thymine with use of reagents such as nitrous acid; cytosine deaminates to uracil under similar conditions. 5-Methylcytosine is resistant to deamination by bisulfite treatment, which deaminates cytosine residues; this property is often exploited to analyze DNA cytosine methylation patterns with bisulfite sequencing.

References

Clark, Susan J., Janet Harrison, Cheryl L. Paul, and Marianne Frommer. (1994). High sensitivity mapping of methylated cytosines Nucleic Acids Research, Vol. 22, No. 15, 2990-2997.

Colot, Vincent and Jean-Luc Rossignol. (1999). Eukaryotic DNA methylation as an evolutionary device BioEssays, Vol. 21, 402-411.

Griffiths, Anthony J.F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; & Gelbart, William M. (2000). Chapter 15: Gene Mutation in An Introduction to Genetic Analysis (7th Ed.). New York: W. H. FREEMAN. ISBN 0-7167-3520-2. (available online at the United States National Center for Biotechnology Information)

Pyrimidines