metylation

Metylation, often spelled methylation, is the process of adding a methyl group to a molecule. The methyl donor is usually S-adenosylmethionine (SAM), and enzymes called methyltransferases catalyze the transfer. In DNA, methylation typically occurs at cytosines within CpG dinucleotides and is a major epigenetic mechanism influencing gene expression and genome stability. DNA methylation patterns are established during development and can change in response to signals; they can be preserved through cell divisions or erased through demethylation pathways.

In mammals, DNA methyltransferases such as DNMT1, DNMT3A, and DNMT3B establish and maintain methylation. Demethylation involves TET enzymes and base-excision repair processes. These dynamics contribute to processes like development, genomic imprinting, and X chromosome inactivation, while helping to suppress transposable elements and regulate gene activity.

Histone methylation is another key layer of epigenetic regulation. Lysine and arginine residues on histone tails can be mono-, di-, or tri-methylated by histone methyltransferases, and removed by demethylases. These marks influence chromatin structure and recruit effector proteins, thereby modulating gene expression. Methylation also affects nonhistone proteins, altering their function, interactions, or localization.

Methylation also occurs beyond DNA and histones. RNA methylation, such as N6-methyladenosine (m6A), modulates RNA stability, splicing, and translation. Lipid and protein methylation extend regulatory circuits in signaling and metabolism. Methods to study methylation include bisulfite sequencing for DNA, chromatin immunoprecipitation for histone marks, and various high-throughput approaches. Aberrant methylation patterns are linked to diseases such as cancer, imprinting disorders, and neurological conditions, and methylation profiling is increasingly used in research and clinical diagnostics.