Barr Body: Difference between revisions
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A | A Barr Body is an inactivated, condensed X [[Chromosome|chromosome]] found in female cells. | ||
Since X chromosomes | Since females possess two X chromosomes and males have one X chromosome and a [[Y chromosome|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 chromosome|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|gene]] transcription. This is called [[Dosage Compensation Mechanism|dosage compensation]]. | ||
Inactivation | [[X chromosome Inactivation|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<ref>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</ref>. | ||
The number of Barr bodies in a cell is one less than the number of X chromosomes. | 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|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 syndrome|Klinefelter's syndrome]] (where the genotype is 47XXY), the number of Barr bodies would also be 1<ref>Page 262, Hartl D.L and Ruvolo M (2012) Genetics, Analysis of Genes and Genomes, 8th edition, USA: Jones and Bartlett</ref><ref>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)</ref>. | |||
To work out the number of Barr bodies an individual has the formula: Xn-1 can be used. | |||
Reference | === 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<ref>X-inactivation. Khan Academy. https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-sex-linkage/a/x-inactivation</ref>. | |||
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<ref>Penny, G. D., et al. Requirement for Xist in X chromosome inactivation. Nature 379, 131–137 (1996) doi:10.1038/379131a0</ref>. | |||
=== Reference === | |||
<references /> | <references /> | ||
Latest revision as of 06:51, 5 December 2018
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:
- 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
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:
- 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[5].
Reference
- ↑ 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