Chromatin

The DNA double helix in the cell is packaged by special proteins called histones to form a protein/DNA complex called chromatin. The structural unit of chromatin is the nucleosome. It consists of a central protein complex, the histone octamer, and two turns of DNA, about 146 base pairs, which are wrapped around the histone octamer complex. There are four different types of core histones which form the octamer containing two copies each of H2A, H2B, H3 and H4.

There is a linker histone, H1, which contacts the exit/entry of the DNA strand on the nucleosome (yellow). The nucleosome together with histone H1 is called a chromatosome. Chromatosomes are held together by the continuous DNA strand, thus forming linker DNA of 30-50 base pairs in length.

The higher order structure of the chromatin fiber in the cell is not known in detail. EM studies have demonstrated a fiber with a diameter of 30 nm which is highly dynamic such that it seems to unfold into a 10 nm fiber when transversed by an RNA polymerase engaged in transcription.