Chromatin is basically a combination of proteins and DNA that make up the contents of the cells’ nucleus. The main chromatin function is to package DNA (deoxyribonucleic acid) into smaller volume that fit in the cell; to control gene expression as well as DNA replication; and to strengthen the DNA to promote mitosis and meiosis while preventing DNA damage. The primary protein components of chromatin are histones that compact the DNA. Chromatin is a complex of the DNA, RNA and protein that is mainly histones called H1, H2A, H2B, H3, and H4—which makes up chromosomes. It can only be found in eukaryotic cells as prokaryotic cells have a very different DNA organization, referred to as genophore (not chromatin).
Chromatin contains twice as much protein as DNA and when stained, it produces a colored material. It is mainly found in the nucleus and exists in two forms which are euchromatin or the extended form; and the heterochromatin, the condensed form. The variation of histones such as acetylation and methylation; as well as the non-histone DNA binding proteins can greatly affect the overall structure of chromatin. Methylated histones can hold DNA more tightly, therefore restricting access and blocking transcription. The acetylation of histones can promote loosening the chromatin; and facilitating transcription and replication. The methylated histones tightly hold the DNA, restricting access and impeding transcription. Methylation of lysine-27 and lysine-4 on histone-3 could be involved in development.
Chromatin can be put into seven various structures, with the DNA strand as the simplest form. Addition of core histones to the DNA creates the nucleosome and the nucleosome-DNA complex forms what is known as "beads-on-a-string", wherein genes are under a more active transcription under this structure. In 1974, Roger Kornberg was first to described nucleosome. The presence of the linker histone H1 results in the formation of 30nm fibers where genes are transcribed less-actively. The addition of further scaffold proteins creates the active chromosome which is present during interphase. Chromatin is randomly organized in the cell nucleus. However, there are certain regions that are bound to the nuclear membrane and other regions are being bound together by protein complexes.