5-Methylcytosine

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.

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In vivo

5-Methylcytosine is an epigenetic modification formed by the action of DNA methyltransferases. Its function varies significantly among species:^[1]

• 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 CpG, CpNpG and CpNpN sequences (where N = A, C or T).
• In fungi and animals, 5-methylcytosine predominantly 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.

In vitro

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.^[2]

Additional images

References

1. ^ Colot V, Rossignol JL (1999). "Eukaryotic DNA methylation as an evolutionary device". Bioessays 21 (5): 402-11. PMID 10376011.
2. ^ Clark SJ, Harrison J, Paul CL, Frommer M (1994). "High sensitivity mapping of methylated cytosines". Nucleic Acids Res. 22 (15): 2990-7. PMID 8065911.

• Griffiths, Anthony J. F. (1999). An Introduction to genetic analysis. San Francisco: W.H. Freeman, Chapter 15: Gene Mutation. ISBN 0-7167-3520-2.  (available online at the United States National Center for Biotechnology Information)