What term describes the reversible addition of a methyl group (-CH3) to chromatin?

Methylation refers to the process in which a methyl group (-CH3) is added to chromatin, which can involve DNA and histone proteins. This process is reversible and plays a significant role in the regulation of gene expression. The addition of methyl groups can lead to the condensation of chromatin structure, making the DNA less accessible for transcription, thereby silencing certain genes.

In the context of chromatin, enzymes known as methyltransferases facilitate this addition, and the impact of methylation can depend on the specific cytosine residues in DNA being methylated or the lysine and arginine residues in histones. Understanding methylation is important in epigenetics, as it helps explain how environmental factors can affect gene expression without altering the underlying DNA sequence.

Acetylation, while also an epigenetic modification, involves the addition of acetyl groups and is linked to gene activation. Phosphorylation involves the addition of phosphate groups, often affecting protein function and signaling pathways, and hydroxylation refers to the addition of hydroxyl groups, which does not pertain to the addition of methyl groups in chromatin structure. Hence, the focus on the reversible addition of methyl groups makes methylation the most appropriate term in this context.