X Inactivation Cycle
Xist expression and X inactivation change throughout embryonic development. In early embryogenesis, the oocyte and sperm do not express Xist and the X chromosome remains active. After fertilization, when the cells are in the 2 to 4 cell stage, Xist transcripts are expressed from the paternal X chromosome, which in return cause the paternal X chromosome to become imprinted and inactivated. The cells develop into pluripotent cells (the inner cell mass) where the imprint is removed, which leads to the downregulation of Xist and thus reactivation of the inactive X chromosome. Recent data suggests that Xist activity is regulated by an anti-sense transcript. The epiblast cells are then formed and they begin to differentiate, and the Xist is upregulated from either of the two X chromosomes and at random, an X is inactivated and the Xist allele is turned off in the active X chromosome. In maturing XX primordial germ cells, Xist is downregulated and X reactivation occurs once again.
Read more about this topic: XIST (gene)
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