Methylation is a chemical process that involves adding a methyl group (-CH3) to a molecule. In the case of DNA methylation, a methyl group is added to a cytosine base that is followed by a guanine base, creating a CpG site. CpG sites are found in high frequency in regions called CpG islands, which are often located near gene promoters.

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The addition of a methyl group to a CpG site can have various effects on gene expression. Typically, DNA methylation at gene promoters is associated with decreased gene expression, while methylation at gene bodies can have the opposite effect.

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Methylation is catalyzed by a family of enzymes called DNA methyltransferases (DNMTs). These enzymes use a methyl donor molecule called S-adenosyl methionine (SAM) to add a methyl group to cytosine. During DNA replication, DNMTs can "read" the existing methylation pattern and use it as a template to maintain the methylation pattern in the newly synthesized DNA strand.

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In addition to DNA methylation, methylation can also occur on other molecules in cells, such as RNA and proteins. Methylation on RNA molecules (called m6A methylation) can also have an impact on gene expression and cellular function.

Overall, methylation is an important epigenetic mechanism that helps regulate gene expression and cellular function in various biological processes, including development, differentiation, and disease.