histone modification

DNA does not float freely; it is wound around spool-like proteins called histones. Histone modification is like decorating those spools with small chemical flags that tell the cell whether to loosen the thread and read the genes nearby, or to wind them tighter and keep them quiet.

These flags are chemical groups added to the tails of histone proteins, including acetyl, methyl, and phosphate groups. Specific combinations are read by other proteins as instructions: acetylation generally loosens chromatin and favors transcription, while certain methylation marks can either activate or repress depending on which amino acid is tagged.

Enzymes that add these marks are sometimes called writers, those that remove them erasers, and the proteins that recognize them readers. The pattern of marks across a region is part of what some researchers loosely call a histone code, though the marks act in combination rather than as a strict one-to-one cipher.

Unlike DNA methylation, many histone marks turn over quickly, making them well suited to responsive, short-term regulation. They work together with methylation and chromatin remodeling to set a gene's activity.