Related topics: protein · genes · gene expression · dna · cell nucleus

New mechanisms describe how the genome regulates itself

An organism's genome contains all of the information necessary for each of its cells and tissues to develop and function properly. Written in DNA, each individual gene encodes for something, whether it is a structural protein ...

Chromatin organizes itself into 3-D 'forests' in single cells

A single cell contains the genetic instructions for an entire organism. This genomic information is managed and processed by the complex machinery of chromatin—a mix of DNA and protein within chromosomes whose function ...

Scientists detail how chromosomes reorganize after cell division

Researchers have discovered key mechanisms and structural details of a fundamental biological process—how a cell nucleus and its chromosomal material reorganizes itself after cell division. The new findings in chromosomal ...

Highly sensitive epigenomic technology combats disease

Much remains unknown about diseases and the way our bodies respond to them, in part because the human genome is the complete DNA assembly that makes each person unique. A Virginia Tech professor and his team of researchers ...

Unraveling gene expression

The DNA of a single cell is two to three meters long end-to-end. To fit in the nucleus and function correctly, DNA is packaged around specialized proteins. These DNA-protein complexes are called nucleosomes, and they are ...

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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 and meiosis and 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:

There are, however, many of 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.

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