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

Recipe for making a fruitfly

Researchers have used mass spectroscopy to determine the absolute copy numbers of nuclear proteins and histone modifications in the Drosophila embryo. The results provide new insights into the mechanisms of animal development.

Scientists unwind mystery behind DNA replication

The molecules of life are twisted. But how those familiar strands in DNA's double helix manage to replicate without being tangled up has been hard to decipher. A new perspective from Cornell physicists is helping unravel ...

A catalog of DNA replication proteins

Maintenance of genome integrity—and prevention of diseases such as cancer—requires complete and faithful replication of the genome every cell division cycle.

Mathematical modeling shows why animals see at night

Nocturnal and diurnal mammals see the same—but only for a brief time. When mice are born, the chromatin in the cells of their eyes has a diurnal structure. Day by day, the layout of this chromatin slowly inverts, allowing ...

Does rearranging chromosomes affect their function?

Molecular biologists have long thought that domains in the genome's 3-D organization control how genes are expressed. After studying highly rearranged chromosomes in fruit flies, EMBL researchers now reveal that while this ...

A model to decipher the complexity of gene regulation

How, where and when genes are expressed determine individual phenotypes. If gene expression is controlled by many regulatory elements, what, ultimately, controls them? And how does genetic variation affect them? The SysGenetiX ...

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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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