Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Chromatin shopping experience:

1. Compare - without doubt the biggest advantage that the Chromatin offers shoppers today is the ability to compare thousands of Chromatin at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Chromatin? Wrong! If the Chromatin is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Chromatin then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Chromatin? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Chromatin and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Chromatin wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Chromatin then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Chromatin site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Chromatin, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Chromatin, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

Chromatin is the complex of DNA and protein that makes up chromosomes. It is found inside the cell nucleus of Eukaryote cell (biology), and within the nucleoid in prokaryotes. The nucleic acids are in the form of double-stranded DNA (a double helix). The major proteins involved in chromatin are histone proteins, although many other chromosomal proteins have prominent roles too. The 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 to serve as a mechanism to control gene expression. Changes in chromatin structure are affected mainly by methylation (DNA and proteins) and acetylation (proteins). Chromatin structure is also relevant to DNA replication and DNA repair.

Chromatin is easily visualised by staining, hence its name, which literally means coloured material.

, the nucleosome, the 10nm "beads-on-a-string" fibre, the 30nm fibre and the metaphase chromosome.

Basic Structure Simplistically, there are three levels of chromatin organization (Fig. 1):
  • DNA wrapping around nucleosomes - The "beads on a string" structure.
  • A 30 nanometer condensed chromatin fiber consisting of nucleosome arrays in their most compact form.
  • Higher level DNA packaging into the metaphase chromosome.


    • These structures do not occur in all eukaryotic cells; there are examples of more extreme packaging, for example spermatozoa and avian red blood cells.



    The different levels of chromatin compaction are clearly visible in cells. In non-dividing cells there are two types of chromatin: euchromatin and heterochromatin. These correspond to uncompacted actively transcribed DNA and compacted untranscribed DNA.

    The structure of chromatin varies considerably as the cell (biology) progresses through the cell cycle. The changes in structure are required to allow the DNA to be used and managed, whilst minimising the risk of damage.

    History In 1882 Walther Flemming used the term chromatin for the first time. Flemming assumed that within the nucleus there was some kind of nuclear-scaffold. Furthermore, there were nucleoli, the nuclear plasm and the nuclear membranes. He wrote (transl. from German): “The scaffold owes its capability of refraction, the way how it behaves, and in particular its colorability to a substance which, with regard to its latter attribute, I have termed Chromatin. It is possible that this substance is really identical with the Nuclein-bodies. .... I’ll retain the name Chromatin as long as Chemistry has decided about it, and I empirically refer to it as that substance in the cell's nucleus which takes up the dye upon staining the nucleus ("Kerntinktionen"). Interphase chromatin The structure of chromatin during interphase is optimised to allow easy access of transcription (genetics) and DNA repair factors to the DNA while compacting the DNA into the nucleus (cell). The structure varies depending on the access required to the DNA. Genes that require regular access by RNA polymerase require the looser structure provided by euchromatin.

    Change in chromatin structure Chromatin undergoes various forms of change in its structure. Histone proteins, the foundation blocks of chromatin, are modified by various post-translational modification to alter DNA packing. Acetylation results in the loosening of chromatin and lends itself to replication and transcription. When methylated they hold DNA together strongly and restrict access to various enzymes. A recent study showed that there is a bivalent structure present in the chromatin: methylated lysine residues at location 4 and 27 on histone 3. It is thought that this may be involved in development; there is more methylation of lysine 27 in embryonic cells than in differentiated cells, whereas lysine 4 methylation positively regulates transcription by recruiting nucleosome remodeling enzymes and histone acetylases.

    DNA structure The vast majority of DNA within the cell is the normal DNA structure. However in nature DNA can form three structures, A-, B- and Z-DNA. A and B-DNA are very similar, forming right handed helices, while Z-DNA is a more unusual left handed helix with a zig-zag phosphate backbone. Z-DNA is thought to play a specific role in chromatin structure and transcription (genetics) because of the properties of the junction between B- and Z-DNA.

    At the junction of B- and Z-DNA one pair of bases is flipped out from normal bonding. These play a dual role of a site of recognition by many proteins and as a sink for torsional stress from RNA polymerase or nucleosome binding.



    The nucleosome and "beads-on-a-string" Main articles: Nucleosome, Chromatosome and Histone The basic repeat element of chromatin is the nucleosome, interconnected by sections of linker DNA, a far shorter arrangement than pure DNA in solution.

    In addition to the core histones there is the linker histone, H1, which contacts the exit/entry of the DNA strand on the nucleosome. The nucleosome, together with histone H1, is known as a chromatosome. Chromatosomes, connected by about 20 to 60 base pairs of linker DNA, form an approximately 10 nm "beads-on-a-string" fibre. (Fig. 1-2).

    The nucleosomes bind DNA non-specifically, as required by their function in general DNA packaging. There is, however, some preference in the sequences the nucleosomes will bind. This is largely through the properties of DNA; adenosine and thymine are more favorably compressed into the inner minor grooves. This means nucleosomes bind preferentially at one position every 10 base pairs - where the DNA is rotated to maximise the number of A and T bases which will lie in the inner minor groove. (See mechanical properties of DNA.)

    30 nm chromatin fibre The "beads-on-a-string" structure in turn coils into a 30 nm diameter helical structure known as the 30nm fibre or filament. The precise structure of the chromatin fibre in the cell is not known in detail, and there is still some debate over this.

    This level of chromatin structure is thought to be the form of euchromatin, which contains actively transcribed genes. EM studies have demonstrated the 30 nm fibre is highly dynamic such that it unfold into a 10 nm fiber ("beads-on-a-string") structure when transversed by an RNA polymerase engaged in transcription.

    ]The existing models commonly accept that the nucleosomes lie perpendicular to the axis of the fibre, with linker histones arranged internally.A stable 30 nm fibre relies on the regular positioning of nucleosomes along DNA. Linker DNA is relatively resistant to bending and rotation. This makes the length of linker DNA critical to the stability of the fibre, requiring nucleosomes to be separated by lengths that permit rotation and folding into the required orientation without excessive stress to the DNA.In this view, different length of the linker DNA should produce different folding topologies of the chromatin fiber. Recent theoretical work, based on electron-microscopy imagesof reconstituted fibers support this view.

    Spatial organization of chromatin in the cell nucleus

    The layout of the genome within the nucleus is not random - specific regions of the genome are always found in certain areas. Specific regions of the chromatin are thought to be bound to the nuclear membrane, while other regions are bound together by protein complexes. The layout of this is not, however, well characterised apart from the compaction of one of the two X chromosomes in mammalian females into the Barr body. This serves the role of permanently deactivating these genes, which prevents females getting a 'dosage compensation of relative to males.

    Metaphase chromatin staining, showing the classic metaphase chromatin structure.The metaphase structure of chromatin differs vastly to that of interphase. It is optimised for physical strength and manageability, forming the classic chromosome structure seen in karyotypes. The structure of the condensed chromosome is thought to be loops of 30nm fibre to a central scaffold of proteins. It is, however, not well characterised.

    The physical strength of chromatin is vital for this stage of division to prevent shear damage to the DNA as the daughter chromosomes are separated. To maximise strength the composition of the chromatin changes as it approaches the centromere, primarily through alternative histone H1 anologues.

    Non-histone chromosomal proteins The proteins that are found associated with isolated chromatin fall into several functional categories:

    Enzymes associated with chromatin are those involved in DNA transcription, replication and repair, and in post-translational modification of histones. They include various types of nucleases and proteases. Scaffold proteins encompass chromatin proteins such as chromatin insulation, domain boundary factors and cellular memory modules (CMMs).

    Chromatin: alternative definitions
  • Simple and concise definition: Chromatin is DNA plus the proteins (and RNA) that package DNA within the cell nucleus.
  • A biochemists’ operational definition: Chromatin is the DNA/protein/RNA complex extracted from eukaryotic lysed interphase nuclei. Just which of the multitudinous substances present in a nucleus will constitute a part of the extracted material will depend in part on the technique each researcher uses. Furthermore, the composition and properties of chromatin vary from one cell type to the another, during development of a specific cell type, and at different stages in the cell cycle.
  • The DNA – plus – histone – equals – chromatin definition: The DNA double helix in the cell nucleus is packaged by special proteins termed histones. The formed protein/DNA complex is called chromatin. The structural entity of chromatin is the nucleosome.


    • Nobel Prizes The following scientists were recognized for their contributions to what we know about chromatin:

    Albrecht Kossel (University of Heidelberg) was awarded the Nobel Prize in Physiology or Medicine 1910 "in recognition of the contributions to our knowledge of cell chemistry made through his work on proteins, including the nucleic substances"

    Thomas Hunt Morgan (California Institute of Technology) was awarded the Nobel Prize in Physiology or Medicine 1933 "for his discoveries concerning the role played by the chromosome in heredity".

    Francis Crick, James Watson and Maurice Wilkins (MRC Laboratory of Molecular Biology, Harvard University, London University) were awarded the Nobel Prize in Physiology or Medicine 1962 "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".

    Aaron Klug (MRC Laboratory of Molecular Biology) was awarded the Nobel Prize in Chemistry 1982 "for his development of crystallographic electron microscopy and his structural elucidation of biologically important nucleic acid-protein complexes".

    Roger Kornberg (Stanford University) was awarded the Nobel Prize in Chemistry in 2006 "for his studies of the molecular basis of eukaryotic transcription". In 1974 published the discovery that the nucleosome consists of a tetramer of histones H3 and H4 and two dimers of histones H2A and H2B.

    See also

    References

    External links

    Chromatin is the complex of DNA and protein that makes up chromosomes. It is found inside the cell nucleus of Eukaryote cell (biology), and within the nucleoid in prokaryotes. The nucleic acids are in the form of double-stranded DNA (a double helix). The major proteins involved in chromatin are histone proteins, although many other chromosomal proteins have prominent roles too. The 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 to serve as a mechanism to control gene expression. Changes in chromatin structure are affected mainly by methylation (DNA and proteins) and acetylation (proteins). Chromatin structure is also relevant to DNA replication and DNA repair.

    Chromatin is easily visualised by staining, hence its name, which literally means coloured material.

    , the nucleosome, the 10nm "beads-on-a-string" fibre, the 30nm fibre and the metaphase chromosome.

    Basic Structure Simplistically, there are three levels of chromatin organization (Fig. 1):
  • DNA wrapping around nucleosomes - The "beads on a string" structure.
  • A 30 nanometer condensed chromatin fiber consisting of nucleosome arrays in their most compact form.
  • Higher level DNA packaging into the metaphase chromosome.


    • These structures do not occur in all eukaryotic cells; there are examples of more extreme packaging, for example spermatozoa and avian red blood cells.



    The different levels of chromatin compaction are clearly visible in cells. In non-dividing cells there are two types of chromatin: euchromatin and heterochromatin. These correspond to uncompacted actively transcribed DNA and compacted untranscribed DNA.

    The structure of chromatin varies considerably as the cell (biology) progresses through the cell cycle. The changes in structure are required to allow the DNA to be used and managed, whilst minimising the risk of damage.

    History In 1882 Walther Flemming used the term chromatin for the first time. Flemming assumed that within the nucleus there was some kind of nuclear-scaffold. Furthermore, there were nucleoli, the nuclear plasm and the nuclear membranes. He wrote (transl. from German): “The scaffold owes its capability of refraction, the way how it behaves, and in particular its colorability to a substance which, with regard to its latter attribute, I have termed Chromatin. It is possible that this substance is really identical with the Nuclein-bodies. .... I’ll retain the name Chromatin as long as Chemistry has decided about it, and I empirically refer to it as that substance in the cell's nucleus which takes up the dye upon staining the nucleus ("Kerntinktionen"). Interphase chromatin The structure of chromatin during interphase is optimised to allow easy access of transcription (genetics) and DNA repair factors to the DNA while compacting the DNA into the nucleus (cell). The structure varies depending on the access required to the DNA. Genes that require regular access by RNA polymerase require the looser structure provided by euchromatin.

    Change in chromatin structure Chromatin undergoes various forms of change in its structure. Histone proteins, the foundation blocks of chromatin, are modified by various post-translational modification to alter DNA packing. Acetylation results in the loosening of chromatin and lends itself to replication and transcription. When methylated they hold DNA together strongly and restrict access to various enzymes. A recent study showed that there is a bivalent structure present in the chromatin: methylated lysine residues at location 4 and 27 on histone 3. It is thought that this may be involved in development; there is more methylation of lysine 27 in embryonic cells than in differentiated cells, whereas lysine 4 methylation positively regulates transcription by recruiting nucleosome remodeling enzymes and histone acetylases.

    DNA structure The vast majority of DNA within the cell is the normal DNA structure. However in nature DNA can form three structures, A-, B- and Z-DNA. A and B-DNA are very similar, forming right handed helices, while Z-DNA is a more unusual left handed helix with a zig-zag phosphate backbone. Z-DNA is thought to play a specific role in chromatin structure and transcription (genetics) because of the properties of the junction between B- and Z-DNA.

    At the junction of B- and Z-DNA one pair of bases is flipped out from normal bonding. These play a dual role of a site of recognition by many proteins and as a sink for torsional stress from RNA polymerase or nucleosome binding.



    The nucleosome and "beads-on-a-string" Main articles: Nucleosome, Chromatosome and Histone The basic repeat element of chromatin is the nucleosome, interconnected by sections of linker DNA, a far shorter arrangement than pure DNA in solution.

    In addition to the core histones there is the linker histone, H1, which contacts the exit/entry of the DNA strand on the nucleosome. The nucleosome, together with histone H1, is known as a chromatosome. Chromatosomes, connected by about 20 to 60 base pairs of linker DNA, form an approximately 10 nm "beads-on-a-string" fibre. (Fig. 1-2).

    The nucleosomes bind DNA non-specifically, as required by their function in general DNA packaging. There is, however, some preference in the sequences the nucleosomes will bind. This is largely through the properties of DNA; adenosine and thymine are more favorably compressed into the inner minor grooves. This means nucleosomes bind preferentially at one position every 10 base pairs - where the DNA is rotated to maximise the number of A and T bases which will lie in the inner minor groove. (See mechanical properties of DNA.)

    30 nm chromatin fibre The "beads-on-a-string" structure in turn coils into a 30 nm diameter helical structure known as the 30nm fibre or filament. The precise structure of the chromatin fibre in the cell is not known in detail, and there is still some debate over this.

    This level of chromatin structure is thought to be the form of euchromatin, which contains actively transcribed genes. EM studies have demonstrated the 30 nm fibre is highly dynamic such that it unfold into a 10 nm fiber ("beads-on-a-string") structure when transversed by an RNA polymerase engaged in transcription.

    ]The existing models commonly accept that the nucleosomes lie perpendicular to the axis of the fibre, with linker histones arranged internally.A stable 30 nm fibre relies on the regular positioning of nucleosomes along DNA. Linker DNA is relatively resistant to bending and rotation. This makes the length of linker DNA critical to the stability of the fibre, requiring nucleosomes to be separated by lengths that permit rotation and folding into the required orientation without excessive stress to the DNA.In this view, different length of the linker DNA should produce different folding topologies of the chromatin fiber. Recent theoretical work, based on electron-microscopy imagesof reconstituted fibers support this view.

    Spatial organization of chromatin in the cell nucleus

    The layout of the genome within the nucleus is not random - specific regions of the genome are always found in certain areas. Specific regions of the chromatin are thought to be bound to the nuclear membrane, while other regions are bound together by protein complexes. The layout of this is not, however, well characterised apart from the compaction of one of the two X chromosomes in mammalian females into the Barr body. This serves the role of permanently deactivating these genes, which prevents females getting a 'dosage compensation of relative to males.

    Metaphase chromatin staining, showing the classic metaphase chromatin structure.The metaphase structure of chromatin differs vastly to that of interphase. It is optimised for physical strength and manageability, forming the classic chromosome structure seen in karyotypes. The structure of the condensed chromosome is thought to be loops of 30nm fibre to a central scaffold of proteins. It is, however, not well characterised.

    The physical strength of chromatin is vital for this stage of division to prevent shear damage to the DNA as the daughter chromosomes are separated. To maximise strength the composition of the chromatin changes as it approaches the centromere, primarily through alternative histone H1 anologues.

    Non-histone chromosomal proteins The proteins that are found associated with isolated chromatin fall into several functional categories:

    Enzymes associated with chromatin are those involved in DNA transcription, replication and repair, and in post-translational modification of histones. They include various types of nucleases and proteases. Scaffold proteins encompass chromatin proteins such as chromatin insulation, domain boundary factors and cellular memory modules (CMMs).

    Chromatin: alternative definitions
  • Simple and concise definition: Chromatin is DNA plus the proteins (and RNA) that package DNA within the cell nucleus.
  • A biochemists’ operational definition: Chromatin is the DNA/protein/RNA complex extracted from eukaryotic lysed interphase nuclei. Just which of the multitudinous substances present in a nucleus will constitute a part of the extracted material will depend in part on the technique each researcher uses. Furthermore, the composition and properties of chromatin vary from one cell type to the another, during development of a specific cell type, and at different stages in the cell cycle.
  • The DNA – plus – histone – equals – chromatin definition: The DNA double helix in the cell nucleus is packaged by special proteins termed histones. The formed protein/DNA complex is called chromatin. The structural entity of chromatin is the nucleosome.


    • Nobel Prizes The following scientists were recognized for their contributions to what we know about chromatin:

    Albrecht Kossel (University of Heidelberg) was awarded the Nobel Prize in Physiology or Medicine 1910 "in recognition of the contributions to our knowledge of cell chemistry made through his work on proteins, including the nucleic substances"

    Thomas Hunt Morgan (California Institute of Technology) was awarded the Nobel Prize in Physiology or Medicine 1933 "for his discoveries concerning the role played by the chromosome in heredity".

    Francis Crick, James Watson and Maurice Wilkins (MRC Laboratory of Molecular Biology, Harvard University, London University) were awarded the Nobel Prize in Physiology or Medicine 1962 "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".

    Aaron Klug (MRC Laboratory of Molecular Biology) was awarded the Nobel Prize in Chemistry 1982 "for his development of crystallographic electron microscopy and his structural elucidation of biologically important nucleic acid-protein complexes".

    Roger Kornberg (Stanford University) was awarded the Nobel Prize in Chemistry in 2006 "for his studies of the molecular basis of eukaryotic transcription". In 1974 published the discovery that the nucleosome consists of a tetramer of histones H3 and H4 and two dimers of histones H2A and H2B.

    See also

    References

    External links



    Chromatin - Wikipedia, the free encyclopedia
    Chromatin is the complex of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells, and within the nucleoid in prokaryotic cells. [1] The ...

    Babraham Institute Chromatin Laboratory
    The Laboratory of Chromatin and Gene Expression Genes are encoded in long DNA strands that make up chromosomes in the cell nucleus. Each chromosome contains hundreds of genes ...

    Chromatin
    In the movie above, N-Myc levels correlate with active chromatin and larger nuclear size, while repressive chromatin increases in knockout nuclei with progressively less N-Myc.

    Babraham Institute Peter Fraser Chromatin Laboratory
    Peter Fraser - Head of Laboratory Tel. (01223) 496644 • Contact via email • Group web pages • Recent, selected Publications • Group Members • Epigenome Network of ...

    Chromatin Overview
    Chromatin: a powerful new avenue of oncology research Chromatin, a complex of proteins and DNA, is the format in which genetic information is stored within all living cells.

    MRC Clinical Sciences Centre Eukaryotic Chromatin Dynamics Group ...
    The Eukaryotic Chromatin Dyanmics Group, led by Veronica Yu, was established in 2005 and is part of the Experimental and Physiological Medicine Section at the Medical Research ...

    Niall Dillon - Gene Regulation and Chromatin Group
    Niall Dillon, Head of the Gene Regulation and Chromatin Group at the MRC Clinical Sciences Centre

    Chromatin Structure and Function
    Information regarding histones, histone modifications and their biological roles, and related links.

    GO:0000785 chromatin
    The European Bioinformatics Institute ... You can reach the search box using alt+9 (Win/IE), alt+shift+9 (Win/Firefox) or ctrl+9 (Mac)

    Definition: sex chromatin from Online Medical Dictionary
    The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

     

    Chromatin



     
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