Chromatin, composed of DNA and proteins, is the physiological template of the cellular processes that occur in the nucleus of the cell. The nucleosome is the principle unit within chromatin in which 147bp of DNA is wrapped around 2 pair of each of histone H2A, H2B, H3 and H4. In addition, there is a linker histone, histone H1, that binds to where DNA enters and exits the nucleosome to allow greater compaction. This is the principle role that chromatin fulfills - the compaction and organization of ~ 2 meters of DNA within the nucleus of each cell.
Chromatin structure and organization is important for several fundamental nuclear processes, including DNA replication and repair, gene transcription, and cell division and growth. This organization and structure is dynamic and subject to change. Changes are largely induced by covalent modifcations to the nucleosome. The acetylation, methylation, phosphorylation, ubiquitination, sumoylation, and ribosylation of histone subunits initiate or propogate cellular signals or recruit proteins that further modify the chromatin landscape and regulate DNA accessibility. Different chromatin landscapes determine the state or phenotype of a cell at any given time by controling the relative expression and silencing of specific genes. Thus, a cell's phenotype is determined both by information stored in its DNA, or genome, and in its chromatin landscape, or "epigenome".
Above: Left to Right;
- Hierarchical representation of chromatin structure and organization.
- Visualization of DNA and nucleosomes or "beads on a string" by electron microscopy. Nucleosome arrays stack and condense to form organized chromatin. (Image by Olins and Olins, Published in Science, 1974)
Because of their ciritical roles in development and cellular differention, gene expression, the cell cycle, and genome stability and DNA repair, chromatin and its modifiers are extensively researched. We currently study the maintainenance of the telomere's specific chromatin environment as well as the histone supply chain pathway that provides the subunits for chromatin assembly.