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

New method to analyze nucleosomes

Northwestern Medicine scientists have developed a new method to analyze the protein composition of intact nucleosomes without losing combinatorial information present in chromatin. The technique, called Nuc-MS, could help ...

Protein can release trapped histones in the cell

In the cell nucleus, histones play a crucial role packaging DNA into chromatin. Histones are however very sticky to both DNA and RNA, so to ensure they are transported to the cell nucleus after synthesis and bind to the right ...

The eukaryotic cell nucleus resembles the layout of a superstore

The headquarter of a eukaryotic cell is the nucleus, and most of the cell's information and instructions are stored there in the form of DNA (Deoxyribonucleic acid). The DNA, which is twisted, rolled and bundled in a two-meter-long ...

Chromatin remodelers never rest to keep our genome open

Chromatin remodelers are needed to take nucleosomes away from DNA in order to make room for transcription factors to bind, and regulate the activity of our genes. It has been unclear how dynamic this process is. Researchers ...

How epigenetic switches control gene expression

Scientists at Tokyo Institute of Technology have deciphered how to quantitatively assess the effects of specific epigenetic changes on the rate of transcription by developing a mathematical model. Using their method, they ...

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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 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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