epigenetic mark

An epigenetic mark is one of the chemical sticky notes that a cell attaches to its DNA or to the proteins around it, signaling whether a nearby gene should be read or kept quiet. The note changes how the instruction is used without rewriting the instruction itself.

In practice, the most common marks are methyl groups added to DNA and an assortment of chemical groups added to histone tails, such as acetyl or methyl groups. Each mark is placed by specific writer enzymes, recognized by reader proteins, and can be taken off by eraser enzymes, so the marks form a reversible signaling system.

Marks rarely act alone. A gene's activity reflects the combined pattern of marks across its DNA and surrounding chromatin, which together define whether that region is open and active or closed and silent.

Crucially, many marks can be copied onto daughter cells when DNA replicates, which is what gives some epigenetic states their heritability. That copying is imperfect and regulated, allowing cells to remember a state yet still change it when needed.