methylation
Meaning of methylation: Adding a single small carbon tag to a molecule, a change tiny in size but large in what it switches off.
Definition of methylation
Methylation is a chemical reaction that attaches a methyl group, one carbon atom carrying three hydrogen atoms, onto another molecule. In cells it acts as a switch: tags added to DNA usually quiet a gene, and tags added to proteins change how those proteins behave. The change is reversible, so cells add and remove these tags constantly.
What gets methylated?
DNA is the best known target. In humans the tag is placed on cytosine, one of the four bases, and almost always where a cytosine sits directly beside a guanine. Proteins are the second major target, particularly the histones that DNA winds around. Small molecules are methylated too, since several hormones and drugs are cleared from the body by having a methyl group added, which alters their solubility and their activity.
How does a tag switch a gene off?
A gene has a control region at its start where the machinery that reads it must attach. Heavy methylation of that region blocks the attachment directly, and it also draws in proteins that pack the surrounding DNA more tightly. Packed DNA cannot be read. The gene itself is present and undamaged; it is simply out of reach. This is how a liver cell keeps nerve genes silent while carrying an identical genome, and the pattern is copied to daughter cells at every division, so cell identity is inherited without any change to the sequence.
How does it compare with other chemical tags?
| Tag | Where it is added | Usual effect |
|---|---|---|
| Methyl group | DNA and histone proteins | Generally silences |
| Acetyl group | Histone proteins | Generally loosens and activates |
| Phosphate group | Proteins of many kinds | Switches activity on or off |
The methyl group is the smallest of the three, and the acetyl group works largely in the opposite direction on the same histone proteins. Together they make up much of what is called epigenetic control: changes in how genes are used rather than in what those genes say.
Why does it matter in disease?
Patterns of methylation shift with age and are disturbed in cancer. Tumor cells commonly lose methylation across the genome as a whole while gaining heavy methylation at the control regions of particular genes, including ones whose job is to restrain growth. Silencing those has much the same effect as deleting them. Because the change is chemical rather than structural it can in principle be reversed, and drugs that block the enzymes adding these tags are used in some blood disorders and leukemias.
Used in a sentence
Testing showed methylation of the gene promoter, indicating that the gene had been silenced.
Good to know
A methyl group is one carbon atom with three hydrogen atoms, and methylation changes whether a gene is read rather than altering the genetic sequence itself.
Word origin
Where “methylation” comes from
Methyl comes through methylene, coined from the Greek methy, meaning wine, and hyle, meaning wood, after the wood alcohol from which it was first obtained.