Sunday 13 July 2003
Definition: Chromatin is the DNA/protein/RNA complex extracted from eukaryotic lysed interphase nuclei. Just which of the multitudinous substances present in a nucleus will constitute a part of the extracted material will depend in part on the technique each researcher uses. Furthermore, the composition and properties of chromatin vary from one cell type to the another, during development of a specific cell type, and at different stages in the cell cycle.
Chromatin is found inside the nucleus of a cell. Chromatin is the structural building block of a chromosome and consists of a complex of DNA and protein in eukaryotic cells. It can be made visible by staining, hence its name, which literally means coloured material.
The nucleic acids are generally in the form of double-stranded DNA - i.e. the famous DNA-double helix. The major proteins involved in chromatin are histone proteins but other non-histone chromosomal proteins are prominent too.
DNA is packaged into chromatin thereby constraining the size of the molecule and allowing the cell to control expression of the chromatin packaged genes.
Dynamic regulation of chromatin structure is thought to be a prerequisite for nuclear functions that require accessibility to DNA such as replication, transcription and DNA repair.
Simplistically, there are three major levels of chromatin organization:
nucleosome - "beads on a string"
30 nm condensed chromatin fiber consisting of nucleosome arrays in their most compact form
The hierarchy continues with increasing DNA-packaging density until the metaphase chromosome is attained.
The human genome contains 23 000 genes that must be expressed in specific cells at precise times. Cells manage gene expression by wrapping DNA around clusters (octamers) of globular histone proteins to form nucleosomes.
These nucleosomes of DNA and histones are organized into chromatin. Changes to the structure of chromatin influence gene expression: genes are inactivated (switched off) when the chromatin is condensed (silent), and they are expressed (switched on) when chromatin is open (active).
Alterations in these normal epigenetic patterns can deregulate patterns of gene expression, which results in profound and diverse clinical outcomes.
Sperm cell chromatin
Sperm cell chromatin is an exception to the above. During spermiogenesis, the spermatid’s chromatin is remodelled into a more tightly packaged, compact, almost crystal-like structure. This process is associated with the cessation of transcription and involves nuclear protein exchange. The histones are mostly displaced, and replaced by protamines, small, arginine-rich proteins.
- euchromatin is a type of chromatin that is rich in gene concentration (contrast this to heterochromatin). This type of chromatin generally appears as light-colored bands when stained in GTG banding and observed under an optical microscope. In contrast to heterochromatin, euchromatin is not tightly wrapped around histones meaning that genes within the region are frequently transcribed to messenger RNA (mRNA) in the cell.
Pathology (chromatin diseases)
chromatin remodeling disorders
- Williams-Beuren syndrome
- rhabdoid tumor syndrome
pathology of chromatin assembly
- Rett syndrome
chromatin remodelling and chromatin remodeling complexes
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