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Jai Ganesh

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Registered: 2005-06-28

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Chromatin Network

Chromatin Network

Gist

The chromatin is the network of the cell nucleus, which contains all the DNA of the nucleus of the cell. The DNA in the nucleus is packaged by the histone proteins histones. The protein and DNA complex is called chromatin. Chromosomes, consisting of chromatin, are found within the nucleus.

Summary

Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in reinforcing the DNA during cell division, preventing DNA damage, and regulating gene expression and DNA replication. During mitosis and meiosis, chromatin facilitates proper segregation of the chromosomes in anaphase; the characteristic shapes of chromosomes visible during this stage are the result of DNA being coiled into highly condensed chromatin.

The primary protein components of chromatin are histones. An octamer of two sets of four histone cores (Histone H2A, Histone H2B, Histone H3, and Histone H4) bind to DNA and function as "anchors" around which the strands are wound. In general, there are three levels of chromatin organization:

* DNA wraps around histone proteins, forming nucleosomes and the so-called beads on a string structure (euchromatin).
* Multiple histones wrap into a 30-nanometer fiber consisting of nucleosome arrays in their most compact form (heterochromatin).
* Higher-level DNA supercoiling of the 30 nm fiber produces the metaphase chromosome (during mitosis and meiosis).

Many organisms, however, do not follow this organization scheme. For example, spermatozoa and avian red blood cells have more tightly packed chromatin than most eukaryotic cells, and trypanosomatid protozoa do not condense their chromatin into visible chromosomes at all. Prokaryotic cells have entirely different structures for organizing their DNA (the prokaryotic chromosome equivalent is called a genophore and is localized within the nucleoid region).

The overall structure of the chromatin network further 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 specific genes present in the DNA. Regions of DNA containing genes which are actively transcribed ("turned on") are less tightly compacted and closely associated with RNA polymerases in a structure known as euchromatin, while regions containing inactive genes ("turned off") are generally more condensed and associated with structural proteins in heterochromatin. Epigenetic modification of the structural proteins in chromatin via methylation and acetylation also alters local chromatin structure and therefore gene expression. There is limited understanding of chromatin structure and it is active area of research in molecular biology.

Details

Chromosomes are made up of a complex mixture of DNA and proteins called chromatin.

Chromatin is a DNA and protein complex that shapes chromosomes in eukaryotic cells’ nuclei. Chromatin appears as beads on a string under the microscope in its expanded form. Nucleosomes are the name for the beads.

DNA is wrapped around eight proteins called histones in each nucleosome. It’s located within eukaryotic cells’ nuclei. Heterochromatin (condensed) and euchromatin (extended) are two types of chromatin.

Heterochromatin is a form of DNA that is tightly packed or condensed and comes in a variety of shapes and sizes. Between the two extremes of constitutive heterochromatin and facultative heterochromatin, both varieties exist on a spectrum. Gene expression is influenced by both of these factors.

Euchromatin is a form of chromatin that is densely packed with genes and frequently under active transcription. Within the cell nucleus, euchromatin is the most active component of the genome. The human genome is euchromatic in 90% of cases.

Within cells, chromatin folds into distinct structures known as chromosomes. During interphase, chromatin is at its least concentrated and tends to be strewn around the nucleus. During prophase, chromatin condensation starts, and chromosomes become noticeable.

The nucleus houses the cell's DNA and controls ribosome and protein synthesis. The nucleolus is a condensed region of chromatin found within the nucleoplasm where ribosome synthesis takes place. The nucleoplasm stores chromatin, which is made up of DNA wrapped around histone proteins.

Note:
1) When DNA binds with histone proteins the structure is called chromatin.
2) The chromatin is of two types.
3) Out of the two types of chromatin, one is functional and the other is not.

Additional Information

Chromatid is one of a pair of daughter strands of a replicated chromosome. Chromatids serve an essential role in cell division, ensuring the accurate division and distribution of chromosomes to new daughter cells.

Chromatids are formed during chromosome duplication, which occurs prior to cell division via the processes of mitosis and meiosis. The two “sister” chromatids in a pair are identical and are joined by a centromere. The centromere is the point of attachment of the kinetochore, a protein structure that is connected to the spindle fibres. The spindle fibres pull the chromatids to opposite ends of the cell, causing each chromatid pair to separate; each chromatid becomes a separate chromosome at this point. In mitosis, the cell then divides, forming two daughter cells with a complete (diploid) set of chromosomes.

In meiosis, following the first round of cell division, homologous recombination occurs, in which genes are exchanged between a maternal chromatid and a paternal chromatid of a homologous chromosome pair. Following a second round of cell division, four haploid daughter cells are produced; thus, each new cell contains only one chromatid, or half of the complete chromosome complement.

Chromatids can be affected by various aberrations, resulting in significant genetic and cellular defects. For example, failure of chromatids to separate properly during cell division can result in an incorrect chromosome number in affected daughter cells, leading to aneuploidy (atypical chromosome number). Defects in sister chromatid cohesion are associated with the development of certain types of cancer.

Histone is a type of protein that plays a critical role in the structural organization and regulation of DNA within the nucleus of eukaryotic cells. Histones were discovered in avian red blood cell nuclei by German biochemist Albrecht Kossel about 1884.

Histones are water-soluble and contain large amounts of basic amino acids, particularly lysine and arginine. They combine ionically with DNA to form nucleoprotein complexes known as nucleosomes. Each nucleosome is made up of an octamer of two copies of four different histone proteins, designated H2A, H2B, H3, and H4. DNA winds around groups of histones, helping to organize it into a compact structure known as chromatin. A so-called linker histone, H1, binds to linker DNA, located where the strand enters and exits the nucleosome, and thereby adds stability to the chromatin structure.

The DNA-packaging function of histones allows long DNA molecules, in the form of chromatin, to fit neatly into the cell nucleus. Histones, through the formation of chromatin, also influence the accessibility of DNA to transcription machinery and therefore serve an essential role in controlling gene expression. Histones also assist with managing chromatin structure during DNA repair processes and with organizing chromatin during cell division, ensuring correct chromosome segregation.

Chemical modifications on histones, such as acetylation and methylation, can further activate or repress gene expression through a process known as epigenetic regulation, in which gene activity is altered without changing the underlying DNA sequence. Epigenetic regulation is carried out by specialized enzymes that act on histones, such as histone acetyltransferases (HATs), which add acetyl groups and promote gene activation, and histone deacetylases (HDACs), which remove acetyl groups and generally block gene activity.

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