Chromatin is the complex of DNA, histones and various other proteins and RNAs that makes up chromosomes in the nuclei of eukaryotic cells. The DNA is wrapped around histone units in a bead-like structure called a nucleosome, and multiple nucleosomes are packaged together to form chromatin.

Chromatin serves to package DNA into a smaller volume so that large amounts of genetic information can be stored within the small space of the nucleus. Chromatin also regulates the replication and transcriptional expression of genes by packaging DNA in various ways; for instance, heterochromatin packages DNA in a way that renders it inaccessible to the proteins required for replication and transcription and this causes those regions of DNA to be silenced, while euchromatin allows the proteins to access DNA for replication and gene expression, causing those regions of DNA to be active. Modifications to both DNA and histone proteins, especially methylation and acetylation, also serve to regulate the accessibility of DNA.