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X Chromosomes

The Disorders Associated With X Chromosomes

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X Chromosomes

What are X chromosomes? List the disorders associated with it.

In Biology, We come to know that there are a lot of chromosomes that are there in the body as thee have been some important aspects that are present in the body. Hence the identity of the chromosomes and performing the functions of the body is important and responsible for carrying out regular processes and performances in the body. Sometimes the identity of the chromosomes that are attracted to the same genome or the DNA, the behave and perform differently. Thus the matter of identifying and conducting the role that s assigned to the body needs to be changed. The body demands several identity and initial notion of the body organization and building. There are certain ideologies that will be making the identity of a person will be initiated regarding the position of the chromosomes. The type and the position of the chromosomes is a very important factor needs to be done so that the identity of the person and the perseverance of the genetic understanding can be done. There are various characteristics that the main importance of the various terms and the style of mixing of the various kinds of chromosomes needs to be analyzed as the functioning of the chromosomes. Unlike the gene-poor Y chromosome, the X chromosome contains over 1,000 genes that are essential for proper development and cell viability. However, females carry two copies of the X chromosome, resulting in a potentially toxic double dose of X-linked genes. To correct this imbalance, mammalian females have evolved a unique mechanism of dosage compensation distinct from that used by organisms such as flies and worms. In particular, by way of the process called X-chromosome inactivation (XCI), female mammals transcriptionally silence one of their two Xs in a complex and highly coordinated manner (Lyon, 1961). The inactivated X chromosome then condenses into a compact structure called a Barr body, and it is stably maintained in a silent state. XIST, or X-inactive specific transcript, was discovered due to its specific expression from inactive female X chromosomes. This RNA has four unique properties (Borsani et al., 1991; Brockdorff et al., 1991, 1992; Brown et al., 1991, 1992; Clemson et al., 1996):The XIST gene does not encode a protein but rather produces a 17 kilobase (kb) functional RNA molecule. Hence, it is a noncoding RNA (Costa, 2008). XIST RNA is only expressed in cells containing at least two Xs and is not normally expressed in male cells (Figure 2). Higher XIST expression can be seen in cells with more X chromosomes, as a counting mechanism dictates that only one X per cell can remain active. In such cells, XIST is expressed from all supernumerary Xs. XIST RNA remains exclusively in the nucleus and is able to "coat" the chromosome from which it was produced. Paradoxically, XIST RNA is expressed from an otherwise inactive X chromosome.
Research has shown that XIST RNA is both necessary and sufficient for inactivation (Penny et al., 1996; Wutz & Jaenisch, 2000), and it recruits various silencing protein complexes to label the future inactive X chromosome. Increased XIST expression represents a key initiation event in X inactivation, indicating the central role of this noncoding RNA. Random X inactivation occurs in the early female embryo, where both the maternal and the paternal X chromosome have an equal chance of becoming inactivated (Figure 4). Each female cell has the difficult task of trying to distinguish between two X chromosomes within the same nucleus, then designating one as an active X chromosome and the other as an inactive X. This complex process of silencing is accomplished independently in each cell, largely by XIST and TSIX. Embryonic stem (ES) cells can undergo random X inactivation when differentiated in vitro (Martin et al., 1978; Rastan & Robertson, 1985), and they thus serve as a good model system with which to study this phenomenon. In fact, the use of ES cells along with early mouse embryos has enabled geneticists to dissect the different phases of the random XCI pathway. It seems that each cell first counts its number of X chromosomes, then randomly chooses one X to remain active, and, finally, silences the future inactive X (Bourmil & Lee, 2001). Whole-chromosome silencing involves the recruitment of many specialized factors, such as histone variants and chromatin modifiers (Lucchesi et al., 2005). In addition to silencing one of the two Xs, the cell must also make sure that the other X remains active. Thus, there must be a way for the two Xs to communicate with each other to designate mutually exclusive fates. Interestingly, recent evidence suggests that this communication is mediated by protein- and transcription-dependent pairing between the Xs during early development (Bacher et al., 2006; Xu et al., 2006, 2007). The random XCI story becomes even more complex with the discovery of various enhancers and modifiers that can alter or skew inactivation of one X chromosome over the other.

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