Medical Definition of Methylation

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Reviewed on 12/21/2018

Methylation: 1. In chemistry, the addition of a methyl group to a molecule. 2. On a protein level, the addition of a methyl group or groups to the amino acid arginine or lysine in a protein. 3. On a DNA level, the addition of a methyl group to a cytosine residue to convert it to 5-methylcytosine. Methylation of DNA occurs at CpG sites, where cytosine (C) lies next to guanine (G). The CpG sites are in regions near the promoters of a genes. These regions are known as CpG islands. The state of methylation of CpG islands is critical to both gene activity and gene expression.

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Reviewed on 12/21/2018