X chromosome Inactivation

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To establish equal levels of expressed genes between males and females, a mechanism known as dosage compensation is necessary. In mammals this process is called X chromosome inactivation.

X chromosome inactivation occurs in mammals with two or more X chromosomes. In a normal female, there are two X chromosomes and in a normal male, there is an X chromosome and a Y chromosome. The X chromosome contains about 1000 genes and is much larger than the Y chromosome which contains less than 100 genes[1].

Since females have two X chromosomes, they contain significantly more genes than males. To establish equal levels of expressed genes between males and females, a mechanism known as dosage compensation is necessary. In mammals, dosage compensation is achieved by X chromosome inactivation.

During the process of X chromosome activation, one of the two X chromosomes in each cell become highly condensed into a structure known as Barr Body[2]. This process takes place in all embryos and the choice of whether to inactivate the paternally or maternally inherited X chromosome is generally random (with the exception of marsupial mammals like the kangaroo, the koala and the wombat, where X chromosome that is inactivated is always the paternally inherited one[3].). Once one of the X chromosomes is inactivated, the same one is inactivated in all preceding cell divisions of that cell and its progeny. Although X chromosome inactivation is permanent, there is an exception where the process is reversed during the formation of germ line cell. Therefore all haploid cell in ovary that may undergo meiosis will contain an active X-chromosome.

X chromosome inactivation allows two X chromosomes to be present in the nucleus. It starts from a site in the middle of the chromosome called the X-inactivation centre (XIC). An RNA molecule, XIST RNA, is expressed only from the inactive X chromosome and will remain in the nucleus, coating the entire inactive X chromosome with heterochromatin, which results in gene silencing. About 10% of the genes on the X-chromosome escape this silencing and remain active. X chromosomes lacking XIC will not be inactivated and will cause the organism to die[4].

Visualisation of such an inactivation is shown under further analysis. This is visualised in the nucleus of the cell whereupon a condensed structure of Barr Body is located. This assembly is a particular type of heterochromatin.

X chromosome inactivation has been widely researched, including with mouse X chromosomes.

References

  1. Alberts, B. et al., 2008. Molecular Biology of The Cell. 5th ed. New York: Garland Science. p 473
  2. Alberts, B. et al., 2008. Molecular Biology of The Cell. 5th ed. New York: Garland Science. p473
  3. Hartyl, D.L. and Ruvolo, M., 2011. Genetics Analysis of Genes and Genomes. 8th ed. London: Jones and Bartlett Learning International. p264
  4. Alberts, B. et al., 2008. Molecular Biology of The Cell. 5th ed. New York: Garland Science. p 474

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