Barr Body

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A Barr Body is an inactivated, condensed X chromosome found in female cells.

Since females possess two X chromosomes and males have one X chromosome and a Y chromosomes, Barr bodies are essential to regulate the amount of X-linked gene product being transcribed. To ensure that X-linked gene product doses are kept similar between males and females, one of the X chromosomes in a female becomes very condensed - the Barr body. This results in the genetic information on the chromosome being inaccessible to proteins that cause gene transcription. This is called dosage compensation.

X chromosome Inactivation is random and occurs at an early point of development, however, about 10% of the genes on the inactivated X chromosome avoid being silenced[1].

The number of Barr bodies in a cell is one less than the number of X chromosomes. For example:

However in a male with Klinefelter's syndrome (where the genotype is 47XXY), the number of Barr bodies would also be 1[2][3].

To work out the number of Barr bodies an individual has the formula: Xn-1 can be used.

Lyonization

Lyonization was discovered by British geneticist Mary Lyon when she found that chromosome sets with more than one X chromosomes undergo X-inactivation. She consequently came up with the Lyon Hypothesis of which her discovery is based.

This is a conservative method in which an X chromosome is shut down, in order to form a Barr body. Lyonization is the process where the chromosome is compacted into a small, dense Barr body. Here most of the genes are inactivated so that they are not transcribed.

Lyonization allows human females to have the usual 'dosage' of genes as males; males already have fewer genes due to the presence of the Y chromosome which is smaller than the X chromosome; females have two XX chromosomes[4].

The Lyon Hypothesis:

Non-coding RNA and X inactivation

The inactivation process is controlled by 2 genes: Xist and Tsix ( which if you noticed are the opposites of each other)

Xist is only expressed in cells containing 2 X chromosomes( females) and it has the ability to recruit various silencing proteins to mark the future non-coding X chromosome[5].

Reference

  1. Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walter P (2008) Molecular Biology of The Cell, 5th edition, New York: Garland Science
  2. Page 262, Hartl D.L and Ruvolo M (2012) Genetics, Analysis of Genes and Genomes, 8th edition, USA: Jones and Bartlett
  3. Alberts, B. Johnnson, A. Lewis, J. Raff, M. Roberts, K and Walter, P. (2008) Molecular Biology of The Cell 5th Edition, New York:Garland Science. (page 473)
  4. X-inactivation. Khan Academy. https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-sex-linkage/a/x-inactivation
  5. Penny, G. D., et al. Requirement for Xist in X chromosome inactivation. Nature 379, 131–137 (1996) doi:10.1038/379131a0
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