X Chromosome Inactivation

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 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 & 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, whilst the rest are inactivated

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[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 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 & 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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