DNA methylation reduces gene expression. While some enzymes methylate the tails of histone proteins, other enzymes methylate certain bases in DNA itself. ○ The DNA of most plants, animals, and fungi has methylated bases, usually cytosine. Inactive DNA is generally highly methylated compared to DNA that is actively transcribed. ○ For example, the inactivated mammalian X chromosome in females is heavily methylated. Genes are usually more heavily methylated in cells where they are not expressed. Demethylating certain inactive genes turns them on. DNA methylation proteins recruit histone deacetylation enzymes, providing a mechanism by which DNA methylation and histone deacetylation cooperate to repress transcription. In some species, DNA methylation is responsible for the long-term inactivation of genes during cellular differentiation.
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