Barr Body
A Barr Bodyis an inactivated, condensed X chromosome found in female cells.
Since females posses 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:
- In a normal female with the genotype 46XX , the number of Barr bodies would be 1.
- In a normal male with the genotype 46XY, the number of Barr bodies would be 0.
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
Discovery of barr bodies leads to thousands more question for scientist. The changes that can lead to a permanent impact, is it formed randomly? And are they affecting all cells, or only a selective amount? Thus, Mary Lyon comes with her own ideology to answer these question, and her hypothesis is still accepted till today.
The Lyon Hypothesis:
- Inactivation is random at an early point in development
- Once inactivated, all progeny cells have the same X-chromosome inactivated
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 an ability to recruit various silencing proteins to mark the future non-coding X chromosome.
- ↑ 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
- ↑ Page 262, Hartl D.L and Ruvolo M (2012) Genetics, Analysis of Genes and Genomes, 8th edition, USA: Jones and Bartlett
- ↑ 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)
Penny, G. D., et al. Requirement for Xist in X chromosome inactivation. Nature 379, 131–137 (1996) doi:10.1038/379131a0