Nucleosome
Nucleosomes are the unit of packaging within eukaryotic cells which allows double stranded DNA to be packaged into the compact space inside the cell's nucleus. Nucleosomes are made up of double stranded DNA circling chromatin, with linker histone molecules joining individual nucleosomes together in a tightly compact structure.
Chromatin
Chromatin is maded up of core histone molecules which are highly conserved in nature throughout most species. The main core histones are:
These core histones come together to form octamers that make up chromatin. H3 and H4 form a dimer and then interact with another H3/H4 dimer to form a H3/H4 tetramer. H2A and H2B also interact to form dimers. The H3/H4 tetramer interacts with 2 H2A/H2B dimers to form an octamer which forms chromatin.
DNA chromatin interaction
Double stranded DNA interacts with the molecule of chromatin by wrapping around the chromatin molecule twice, forming a single nucleosome. The DNA passes between individual nucleosomes, forming the 10nm fibre. The linker Histone H1 binds to the DNA inbetween individual nucleosomes and compacts the nucleosomes into a fibre approx 30nm in diameter.
This method can compact a a genome of 2m in length (human genome) into the nucleus of a single cell.
Modification of chromatin structure
But the action of nucleosome packaging means that transcription is inhibited as most of the DNA bound to the chromatin is inaccessible. This therefore requires nucleosome structure to be modulated in order to make certain strands of DNA accessible to transcription molecules such as RNA polymerase. Such methods of chromatin structure modulation include:
These are post-translation modifications which are applied to the N terminals of the histone molecules that make up the chromatin. These N terminals protrude from the nucleosome structure and are easily accessible.
These modifications are thought to act as beacons for other proteins with bromodomains to interact with the histones to make the DNA accessible to the required proteins within the cell.