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=== Cycle of X-chromosome activation in rodents ===
The paragraphs below have to do only with rodents and do not reflect XI in the majority of mammals.
X-inactivation is part of the activation cycle of the X chromosome throughout the female life. The egg and the fertilized zygote initially use maternal transcripts, and the whole embryonic genome is silenced until zygotic genome activation. Thereafter, all mouse cells undergo an early, [[Imprinting (genetics)|imprinted]] inactivation of the paternally-derived X chromosome in [[mammalian embryogenesis|
In the early [[blastocyst]], this initial, imprinted X-inactivation is reversed in the cells of the [[inner cell mass]] (which give rise to the embryo), and in these cells both X chromosomes become active again. Each of these cells then independently and randomly inactivates one copy of the X chromosome.<ref name=okamoto/> This inactivation event is irreversible during the lifetime of the individual, with the exception of the germline. In the female [[germline]] before meiotic entry, X-inactivation is reversed, so that after meiosis all haploid [[oocyte]]s contain a single active X chromosome.
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# '''Xa<sup>M</sup>''' AND '''Xa<sup>P</sup>''' haploid germ cells (eggs).
The X activation cycle has been best studied in mice, but there are multiple studies in humans. As most of the evidence is coming from mice, the above scheme represents the events in mice. The completion of the meiosis is simplified here for clarity. Steps
===== Timing =====
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|Imprinted (paternal) X-inactivation
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|Unclear if it takes place in humans<ref name=":5">{{cite journal | vauthors = Deng X, Berletch JB, Nguyen DK, Disteche CM | title = X chromosome regulation: diverse patterns in development, tissues and disease | language = En | journal = Nature Reviews. Genetics | volume = 15 | issue = 6 | pages = 367–78 | date = June 2014 | pmid = 24733023 | pmc = 4117651 | doi = 10.1038/nrg3687 }}</ref>
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Like Xist, prior to inactivation, both X chromosomes weakly express Tsix RNA from the Tsix gene. Upon the onset of X-inactivation, the future Xi ceases to express Tsix RNA (and increases Xist expression), whereas Xa continues to express Tsix for several days.
Rep A is a long non coding RNA that works with another long non coding RNA, Xist, for X inactivation. Rep A inhibits the function of Tsix, the antisense of Xist, in conjunction with eliminating expression of Xite. It promotes methylation of the Tsix region by attracting PRC2 and thus inactivating one of the X chromosomes.<ref name=":8">Mercer, T.R., Dinger, M.E., Mattick, J.S., (2009). Long non-coding RNAs: insight into functions. Nature Reviews Genetics. (10)
===Silencing===
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