|MadSci Network: Genetics|
X chromosome inactivation in female mammals results from condensation or packaging of one of the two copies of the X chromosome into heterochromatin; this highly-packaged X chromosome is observable as a darkly stained Barr body in interphase cells. The reason that one of the two X chromosomes in females is inactivated is to provide "dosage compensation" of gene products encoded on the X; since females have two Xs, if both remained transcriptionally active, females would have twice the level of X-encoded gene products as males, thus one copy of the X chromosme is inactivated by packaging it into heterochromatin. The "decision" of which of the two X chromosomes is inactivated is made in the early stages of female development and occurs at radom in each of the cells of the developing embryo. However, subsequent cells that arise by mitosis inherit the inactive X from these progenitor cells. Each embryonic cell decides independently which X will be inactivated; in some cells it will be the maternal X in others the paternal X. Regardless, every cell that arises after the decision will inherit that "decision". A small number of genes on the inactivated X chromosome remain transcriptionally active even after packaging into heterochromatin; most of these genes are active on the active copy of X too. One gene that is not transcribed on the unpackaged X, but is active on the packaged X chromosome is called Xist (X inactivation specific transcript). Deletion of this gene abolishes the ability of that X chromosome from being packaged into heterochromatin. The Xist transcript does not appear toleave the nucleus and it has been suggested that the Xist transcript may associate with the X chromosome that produces it and in association with other proteins lead to the inactivation of that X chromosome.
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