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The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation).
Xist (X-inactive specific transcript) is a non-coding RNA transcribed from the X chromosome of the placental mammals that acts as a major effector of the X-inactivation process. [5] It is a component of the Xic – X-chromosome inactivation centre [ 6 ] – along with two other RNA genes ( Jpx and Ftx ) and two protein genes ( Tsx and Cnbp2 ).
The partial inactivation of the X-chromosome is due to repressive ... The following is a partial list of genes on human chromosome X. For complete list, see the link ...
X chromosome reactivation (XCR) is the process by which the inactive X chromosome (the Xi) is re-activated in the cells of eutherian female mammals. Therian female mammalian cells have two X chromosomes, while males have only one, requiring X-chromosome inactivation (XCI) for sex-chromosome dosage compensation. In eutherians, XCI is the random ...
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. [7] This shift allows Xist to begin coating the future inactive chromosome, spreading out from the Xic. [2]
The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, [17] but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).
Additionally, 10-25% of human X chromosome genes, [12] and 3-7% of mouse X chromosome genes [13] outside of the PARs show weak expression from the inactive X chromosome. Random X-inactivation demands that the cell can determine if it contains more than one active X-chromosome before acting to silence any extraneous X-chromosome(s).
Heterochromatin mainly consists of genetically inactive satellite sequences, [11] and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all. [12] Both centromeres and telomeres are heterochromatic, as is the Barr body of the second, inactivated X-chromosome in a female.