Chromatin forms chromosomes during cell division, packages DNA, reduces DNA volume, protects DNA from damage, and controls gene expression and DNA replication. Chromatin is a substance located in the nucleus of plant and animal cells consisting of RNA, DNA and other proteins.
Chromatin is only present in cells containing a nucleus. The exact structure and function of chromatin mainly depends upon the phase of the cell cycle. Chromatin is structured in three main ways. In interphase, DNA coding genes that are "turned on," or actively transcribed, wrap around histone proteins forming nucleosomes. This is called euchromatin, and in this structure, chromatin remains loose to allow for RNA and DNA transcription and replication.
DNA coding genes that are inactive, or "turned off," are more tightly packed together. Chromatin in this structure is called heterochromatin. In this form, multiple histones are wrapped into a fiber of multiple nucleosomes. In mitosis or meiosis, chromatin forms as an even more tightly packaged structure to facilitate segregation of chromosomes. In this organization, higher-level DNA is packaged to form what are called metaphase chromosomes.
The main proteins of chromatin are histones, which compact DNA. Histones package and arrange chromatin, modifying its form through chemical modifications of histone proteins via the processes of methylation and acetylation.