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X-inactivation
Inactivation of copies of X chromosome / From Wikipedia, the free encyclopedia
X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. 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).
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1.Early stage embryonic cell of a female human
2.Maternal X chromosome
3.Paternal X chromosome
4.Mitosis and random X-chromosome inactivation event
5.Paternal chromosome is randomly inactivated in one daughter cell, maternal chromosome is inactivated in the other
6.Paternal chromosome is randomly inactivated in both daughter cells
7.Maternal chromosome is randomly inactivated in both daughter cells
8.Three possible random combination outcomes
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Left: DNA (DAPI)-stained nucleus. Arrow indicates the location of Barr body(Xi). Right: DNA associated histones protein detected
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The choice of which X chromosome will be inactivated in a particular embryonic cell is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism (its cell line). The result is that the choice of inactivated X chromosome in all the cells of the organism is a random distribution, often with about half the cells having the paternal X chromosome inactivated and half with an inactivated maternal X chromosome; but commonly, X-inactivation is unevenly distributed across the cell lines within one organism (skewed X-inactivation).
Unlike the random X-inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally-derived X chromosome.