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A Barr body (named after discoverer Murray Barr)〔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.〕 is the inactive X chromosome in a female somatic cell,〔Lyon, M. F., (2003), The Lyon and the LINE hypothesis. j.semcdb 14, 313-318. ((Abstract ))〕 rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y (including humans) or W chromosome rather than the diploidy of the X. The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis.〔Lyon, M. F. (1961), Gene Action in the ''X''-chromosome of the Mouse (''Mus musculus'' L.) ''Nature''. 190 (4773): 372-3. PMID 13764598 ((Abstract ))〕 This happens early in embryonic development at random in mammals,〔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 ))〕 except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the father's X chromosome is always deactivated.〔Lee, J. T., (2003), X-chromosome inactivation: a multi-disciplinary approach. j.semcdb 14, 311-312. ( Abstract)〕 In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes. For example, men with Klinefelter syndrome (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. ==Mechanism== A typical human female has only one Barr body per somatic cell, while a typical human male has none. Mammalian X-chromosome inactivation is initiated from the X inactivation centre or ''Xic'', usually found near the centromere.〔Rougeulle, C., Avner, P. (2003), Controlling X-inactivation in mammals: what does the centre hold?. j.semcdb 14, 331-340. ((Abstract ))〕 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.〔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 )〕 This shift allows ''Xist'' to begin coating the future inactive chromosome, spreading out from the ''Xic''.〔Lyon, M. F., (2003), The Lyon and the LINE hypothesis. j.semcdb 14, 313-318. ((Abstract )〕 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〔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 ))〕 and histone H2A ubiquitination,〔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 ))〕 as well as direct modification of the DNA itself, via the methylation of CpG sites.〔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 ))〕 These changes help inactivate gene expression on the inactive X-chromosome and to bring about its compaction to form the Barr body. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Barr body」の詳細全文を読む スポンサード リンク
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