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.
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.
To work out the number of Barr bodies an individual has the formula: Xn-1 can be used.
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.
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 the 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)
- ↑ X-inactivation. Khan Academy. https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-sex-linkage/a/x-inactivation
- ↑ Penny, G. D., et al. Requirement for Xist in X chromosome inactivation. Nature 379, 131–137 (1996) doi:10.1038/379131a0