Chromatin

Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are: to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis, prevent DNA damage, and to control gene expression and DNA replication. The primary protein components of chromatin are histones that compact the DNA. Chromatin is only found in eukaryotic cells: prokaryotic cells have a very different organization of their DNA which is referred to as a genophore (a chromosome without chromatin).

The structure of chromatin depends on several factors. The overall structure depends on the stage of the cell cycle: during interphase the chromatin is structurally loose to allow access to RNA and DNA polymerases that transcribe and replicate the DNA. The local structure of chromatin during interphase depends on the genes present on the DNA: DNA coding genes that are actively transcribed ("turned on") are more loosely packaged and are found associated with RNA polymerases (referred to as euchromatin) while DNA coding inactive genes ("turned off") are found associated with structural proteins and are more tightly packaged (heterochromatin). Epigenetic chemical modification of the structural proteins in chromatin also alter the local chromatin structure, in particular chemical modifications of histone proteins by methylation and acetylation. As the cell prepares to divide, i.e. enters mitosis or meiosis, the chromatin packages more tightly to facilitate segregation of the chromosomes during anaphase. During this stage of the cell cycle this makes the individual chromosomes in many cells visible by optical microscope.

In general terms, there are three levels of chromatin organization:

  1. DNA wraps around histone proteins forming nucleosomes; the "beads on a string" structure (euchromatin).
  2. Multiple histones wrap into a 30 nm fibre consisting of nucleosome arrays in their most compact form (heterochromatin).
  3. Higher-level DNA packaging of the 30 nm fibre into the metaphase chromosome (during mitosis and meiosis).

There are, however, many cells which do not follow this organisation. For example, spermatozoa and avian red blood cells have more tightly packed chromatin than most eukaryotic cells, and trypanosomatid protazoa do not condense their chromatin into visible chromosomes for mitosis.

Read more about Chromatin:  During Interphase, Change in Structure, Chromatin and Bursts of Transcription, Metaphase Chromatin, Chromatin: Alternative Definitions, Alternative Chromatin Organizations, Nobel Prizes

Other articles related to "chromatin":

Chromatin - Nobel Prizes
... scientists were recognized for their contributions to chromatin research with Nobel Prizes Year Who Award 1910 Albrecht Kossel (University of Heidelberg) Nobel Prize in Physiology ...
Salt-and-pepper Chromatin
... In pathology, salt-and-pepper chromatin, also salt-and-pepper nuclei and stippled chromatin, refers to cell nuclei that demonstrate granular chromatin (on light microscopy) ... Salt-and-pepper chromatin is typically seen in endocrine tumours such as medullary thyroid carcinoma, neuroendocrine tumours and pheochromocytoma ...
Salt-and-pepper Chromatin - Additional Images
... A pheochromocytoma showing finely granular chromatin ... H E stain A pheochromocytoma showing finely granular chromatin ... H E stain Salt-and-pepper chromatin (pheochromocytoma) ...
Long Non-coding RNA - Long NcRNA Functions - Long NcRNAs in Epigenetic Regulation
... biology, primarily including regulation of large numbers of genes by remodeling broad chromatin domains (Kiefer 2007 Mikkelsen 2007) ... some time that RNA is an integral component of chromatin (Nickerson 1989 Rodriguez-Campos 2007), it is only recently that we are beginning to appreciate the means by which RNA is involved in ... expression of the homeotic gene, Ubx, by recruiting and directing the chromatin modifying functions of the trithorax protein Ash1 to Hox regulatory elements (Sanchez-Elsner 2006) ...
SMARCD3
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