- Barr body
A Barr body (named after discoverer Murray Barr) is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species (including humans) in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis. This happens early in embryonic development at random in mammals, except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the father's X chromosome is always deactivated.
In men and women with more than one X chromosome, the number of Barr bodies visible at interphase is always one less than the total number of X chromosomes. For example, men with a 47,XXY karyotype have a single Barr body, whereas women with a 47,XXX karyotype have two Barr bodies. Barr bodies can be seen on the nucleus of neutrophils.
Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere. The center contains twelve genes, seven of which code for proteins, five for untranslated RNAs, of which only two are known to play an active role in the X inactivation process, Xist and Tsix. The centre also appears to be important in chromosome counting: ensuring that random inactivation only takes place when two or more X-chromosomes are present. The provision of an extra artificial Xic in early embryogenesis can induce inactivation of the single X found in male cells.
The roles of Xist and Tsix appear to be antagonistic. The loss of Tsix expression on the future inactive X chromosome results in an increase in levels of Xist around the Xic. Meanwhile, on the future active X Tsix levels are maintained; thus the levels of Xist remain low. This shift allows Xist to begin coating the future inactive chromosome, spreading out from the Xic. In non-random inactivation this choice appears to be fixed and current evidence suggests that the maternally inherited gene may be imprinted.
It is thought that this constitutes the mechanism of choice, and allows downstream processes to establish the compact state of the Barr body. These changes include histone modifications, such as histone H3 methylation and histone H2A ubiquitination, as well as direct modification of the DNA itself, via the methylation of CpG sites. These changes help inactivate gene expression on the inactive X-chromosome and to bring about its compaction to form the Barr body.
Links to full text articles are provided where access is free, in other cases only the abstract has been linked.
- ^ Barr, M. L., Bertram, E. G., (1949), A Morphological Distinction between Neurones of the Male and Female, and the Behaviour of the Nucleolar Satellite during Accelerated Nucleoprotein Synthesis. Nature. 163 (4148): 676-7.
- ^ Lyon, M. F., (2003), The Lyon and the LINE hypothesis. j.semcdb 14, 313-318. (Abstract)
- ^ Lyon, M. F. (1961), Gene Action in the X-chromosome of the Mouse (Mus musculus L.) Nature. 190 (4773): 372-3. doi:10.1038/190372a0 PMID 13764598 (Abstract)
- ^ a b Brown,C.J., Robinson,W.P., (1997), XIST Expression and X-Chromosome Inactivation in Human Preimplantation Embryos. Am. J. Hum. Genet. 61, 5-8 (Full Text PDF)
- ^ Lee, J. T., (2003), X-chromosome inactivation: a multi-disciplinary approach. j.semcdb 14, 311-312. (doi:10.1016/j.semcdb.2003.09.025 Abstract)
- ^ a b c Rougeulle, C., Avner, P. (2003), Controlling X-inactivation in mammals: what does the centre hold?. j.semcdb 14, 331-340. (Abstract)
- ^ Lee, J. T., Davidow, L. S., Warshawsky, D., (1999), Tisx, a gene antisense to Xist at the X-inactivation centre. Nat. Genet. 21, 400-404. Full Text
- ^ Lyon, M. F., (2003), The Lyon and the LINE hypothesis. j.semcdb 14, 313-318. (Abstract
- ^ Heard, E., Rougeulle, C., Arnaud, D., Avner, P., Allis, C. D. (2001), Methylation of Histone H3 at Lys-9 Is an Early Mark on the X Chromosome during X Inactivation. Cell 107, 727-738. (Full Text)
- ^ de Napoles,M., Mermoud,J.E., Wakao,R., Tang,Y.A., Endoh,M., Appanah,R., Nesterova,T.B., Silva,J., Otte,A.P., Vidal,M., Koseki,H., Brockdorff,N., (2004), Polycomb Group Proteins Ring1A/B Link Ubiquitylation of Histone H2A to Heritable Gene Silencing and X Inactivation. Dev. Cell 7, 663-676 (Abstract)
- ^ Chadwick,B.P., Willard,H.F., (2003), Barring gene expression after XIST: maintaining faculative heterochromatin on the inactive X. j.semcdb 14, 359-367 (Abstract)
- Alberts,B., Johnson,A., Lewis,J., Raff,M., Roberts,K., Walter,P., (2002), Molecular Biology of the Cell, Fourth Edition, (428-429) Garland Science, 0-8153-4072-9 (Web Edition, Free access)
- Turnpenny & Ellard: Emery's Elements of Medical Genetics 13E (http://www.studentconsult.com/content/default.cfm?ISBN=9780702029172&ID=HC006029)
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Look at other dictionaries:
Barr body — [bär] n. [after M. L. Barr (1908 95), Cdn anatomist] a structure found on the inside of the nuclear membrane of female cells that takes a dark stain and indicates by its presence the sex of the individual … English World dictionary
Barr body — Barr body. = sex chromatin (см.). (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
Barr body — Barr body. См. половой хроматин. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
Barr body — A microscopic feature of female cells due to the presence of two X chromosomes in the female. One of these X chromosomes is inactive and is crumpled up to form the Barr body. * * * Barr body bär n a densely staining inactivated condensed X… … Medical dictionary
Barr body — Barr′ bod y [[t]bɑr[/t]] n. gen an inactiveXchromosome present in the nuclear membrane of female somatic cells, used for verifying the sex of an individual • Etymology: 1960–65; after Murray L. Barr (born 1908), Canadian physician … From formal English to slang
Barr body — noun Etymology: Murray Llewellyn Barr died 1995 Canadian anatomist Date: 1961 a densely staining inactivated condensed X chromosome that is present in each somatic cell of most female mammals and is used as a test of genetic femaleness (as in a… … New Collegiate Dictionary
Barr body — /bahr/ a condensed, inactivated X chromosome present in most female mammals in the nuclei of all cells except the germ cells: used, as in sports competitions, to verify that an individual is genetically female. Also called sex chromatin. [1960… … Universalium
Barr body — inactivated X chromosome (for Murray L Barr) … Eponyms, nicknames, and geographical games
Barr body — noun Physiology a densely staining structure in the cell nuclei of females, consisting of a condensed, inactive X chromosome and regarded as diagnostic of genetic femaleness. Origin 1960s: named after the Canadian anatomist M. L. Barr … English new terms dictionary
Barr body — see sex chromatin [M. L. Barr (1908–95), Canadian anatomist] … The new mediacal dictionary