X Chromosome Inactivation

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&nbsp;<u>X-chromosome inactivation</u> is the process in which one of the two X chromosomes in a female mammal becomes inactivated. This process takes place due to <u>dosage compensation<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref></u>&nbsp;- the mechanism which maintains '''similar amounts of X-linked gene products''' between males and females. This is required because with females having 2 X chromosomes (XX), they have the potential for producing '''twice the amount''' of X-linked gene products.<br>
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[[X_chromosome|X-chromosome]] inactivation is the process in which one of the two X-chromosomes in a [[female|female]] [[mammal|mammal]] becomes inactivated. This process takes place due to dosage compensation<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref> - the mechanism which maintains similar amounts of X-linked gene products between [[males|males]] and females. This is required because with females having 2 X [[chromosomes|chromosomes]] (XX), they have the potential for producing twice the amount of X-linked gene products.  
  
X-chromosome inactivation happens through forming <u>Barr bodies<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref></u>&nbsp;- these are '''inactive X chromosomes''', seen as condensed &amp; darkly staining bodies in the nuclei of cells in females.<br>
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X-chromosome inactivation happens through forming [[Barr bodies|Barr bodies]]<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref>&nbsp;- these are inactive X chromosomes, seen as condensed and darkly staining bodies in the [[Nuclei|nuclei]] of cells in females.  
  
As females have 2 X chromosomes (one from the father and one from the mother), how do we know which one is inactivated? The <u>Lyon Hypothesis<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref></u>&nbsp;is a theory for this (widely accepted now for placental mammals), which states that inactivation takes place at '''random''' at an early stage during development. It also hypothesises that once that X-chromosome has been inactivated, the '''same''' X-chromosome will be inactivated in all of the offspring cells.  
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As females have 2 X chromosomes (one from the father and one from the mother), how do we know which one is inactivated? The [[Lyon Hypothesis|Lyon Hypothesis]]<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref> is a theory for this (widely accepted now for placental mammals), which states that inactivation takes place at random at an early stage during development. It also hypothesises that once that X-chromosome has been inactivated, the same X-chromosome will be inactivated in all of the offspring cells.  
  
 
X-chromosome inactivation is carried out in 2 main steps:  
 
X-chromosome inactivation is carried out in 2 main steps:  
  
(1) The cell checks the number of X-chromosomes present in the cell  
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#The cell checks the number of X-chromosomes present in the cell  
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#The cell selects 1 X-chromosome to be active, while the rest are inactivated
  
(2) The cell selects 1 X-chromosome to be active, whilst the rest are inactivated<br>
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The mechanism begins at a site called the XIC - [[X inactivation centre|X inactivation centre]]. This produces two main [[ncRNA|non-coding RNA]] transcripts - [[Xist gene|Xist gene]] and [[Tsix gene|Tsix gene]]<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref>. It is known that the Xist gene is active on the X-chromosome which is to be inactivated. This leads to the gene producing an [[RNA|RNA]] [[molecule|molecule]] which coats that X-chromosome (spreading outwards from the XIC). Probably by recruiting other [[protein|protein]] complexes, this X-chromosome is packaged into a dense and compacted form of [[chromatin|chromatin]], which is called [[heterochromatin|heterochromatin]].
  
The mechanism begins at a site called the '''XIC - X inactivation centre'''. This produces 2 main non-coding RNA transcripts - '''Xist gene''' and '''Tsix gene<ref>Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013</ref>'''.&nbsp;It is known that the Xist gene is active on the X-chromosome which is to be inactivated. This leads to the gene producing a RNA molecules which coats that X-chromosome (spreading outwards from the XIC). Probably by recruting other protein complexes, this X-chromosome is packaged into a dense &amp; compacted form of chromatin, which is called '''heterochromatin'''.
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=== References  ===
  
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== References:  ==
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Latest revision as of 21:50, 4 December 2018

X-chromosome inactivation is the process in which one of the two X-chromosomes in a female mammal becomes inactivated. This process takes place due to dosage compensation[1] - the mechanism which maintains similar amounts of X-linked gene products between males and females. This is required because with females having 2 X chromosomes (XX), they have the potential for producing twice the amount of X-linked gene products.

X-chromosome inactivation happens through forming Barr bodies[2] - these are inactive X chromosomes, seen as condensed and darkly staining bodies in the nuclei of cells in females.

As females have 2 X chromosomes (one from the father and one from the mother), how do we know which one is inactivated? The Lyon Hypothesis[3] is a theory for this (widely accepted now for placental mammals), which states that inactivation takes place at random at an early stage during development. It also hypothesises that once that X-chromosome has been inactivated, the same X-chromosome will be inactivated in all of the offspring cells.

X-chromosome inactivation is carried out in 2 main steps:

  1. The cell checks the number of X-chromosomes present in the cell
  2. The cell selects 1 X-chromosome to be active, while the rest are inactivated

The mechanism begins at a site called the XIC - X inactivation centre. This produces two main non-coding RNA transcripts - Xist gene and Tsix gene[4]. It is known that the Xist gene is active on the X-chromosome which is to be inactivated. This leads to the gene producing an RNA molecule which coats that X-chromosome (spreading outwards from the XIC). Probably by recruiting other protein complexes, this X-chromosome is packaged into a dense and compacted form of chromatin, which is called heterochromatin.

References

  1. Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013
  2. Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013
  3. Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013
  4. Pierce BA. Genetics: A Conceptual Approach. 5th Ed, New York: W.H. Freeman and Company. 2013
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