Navigation Links
Barr body


In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male. 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.

The inactivation state of chromosomes is passed on to daughter cells during mitosis. Since random chromosomes are selected for inactivation early in embryonic development, this results in different regions of the adult body having different chromosomes inactivated. This can be significant if different alleles of a gene are present on the different chromosomes; in some regions of the body one allele will be active, and in other regions the other will. This is what results in the coloration pattern of female calico cats; pigmentation genes on the X chromosome are activated in different patches of skin based on which chromosome is condensed in those regions.

The Barr body chromosome is generally considered to be inert, but in fact a small number of genes remain active and expressed in some species. These genes are generally those which are present on the other sex chromosome (Y or W).

Mechanism

Mammalian X-chromosome inactivation is initiated from the X inactivation centre or Xic, usually found near the centromere. The centre 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 X-chromosomes are present. The provision of an extra artificial Xic in early embryogenisis 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 ubiquitylation , 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.

See also


'"/>


(Date:7/31/2014)... State University researchers have completed a 20-year study ... Hills prairie at different times of the year. ... late spring time frame has no measurable negative ... have multiple benefits. , The study was conducted ... Prairie Biological Station fire chief, and Joseph Craine, ...
(Date:7/31/2014)... German . , ... a sponge that soaks up liquids. Hence, these highly ... gases. However, loading of many MOFs is inhibited by ... report in Nature Communications that the barriers ... can be prevented by water-free synthesis and storing strategies. ...
(Date:7/31/2014)... July 31, 2014 -- Do people make a rational ... raise them that way? Is it a matter of ... and a colleague from Rice University say that neither ... people lean left while others lean right. , A ... deep-seated psychology are at the core of political differences, ...
Breaking Biology News(10 mins):Study finds benefits to burning Flint Hills prairie in fall and winter 2Study finds benefits to burning Flint Hills prairie in fall and winter 3Free pores for molecule transport 2Scientists find growing consensus: Political attitudes derive from body and mind 2
... COLLEGE STATION A team of scientists studying the ... has had its research published in the February issue ... Microbiology , said the study,s lead investigator. The ... illness through the use of electron-beam irradiation. "The ...
... have developed a methodology for governments and corporations to ... paper in Environmental Science & Technology . ... the importance of scarce metals using a methodology that ... supply restriction. "In the past few years, ...
... in German . To explore the most ... functions Stefan Hell,s team of researchers at the Max Planck ... closer to this goal. Using the STED microscopy developed by Hell, ... live images inside the brain of a living mouse. Captured in ...
Cached Biology News:Study shows electron-beam irradiation reduces virus-related health risk in lettuce, spinach 2Study shows electron-beam irradiation reduces virus-related health risk in lettuce, spinach 3New methodology assesses risk of scarce metals 2Sharp images from the living mouse brain 2Sharp images from the living mouse brain 3
Other biology definitionOther Tags