Chromatin

Chromatin is the material chromosomes are made of: DNA bound together with proteins, chiefly histones, along with other non-histone proteins and some RNA. A human cell holds roughly two meters of DNA, and chromatin is how that thread fits inside a nucleus a few thousandths of a millimeter across while staying readable.

The packing happens in levels. First, the DNA wraps around a core of eight histone proteins to form a nucleosome; a chain of nucleosomes linked by short stretches of DNA looks like beads on a string under the electron microscope. This chain coils into a thicker fiber, which folds further into loops and domains. As a cell prepares to divide, its chromatin condenses into the compact chromosomes visible during mitosis, then loosens again afterward.

Chromatin is more than storage. Loosely packed euchromatin usually holds active genes, while densely packed heterochromatin, common near centromeres and telomeres, is mostly silent. Cells change how tightly a region is packed by adding or removing chemical tags, such as acetyl and methyl groups on histone tails or methyl groups on DNA. This chromatin remodeling decides which genes the transcription machinery can reach and is a central topic in epigenetics. The name refers to color, because chromatin takes up stains so readily.