X inactivation is the mechanism by which mammals adjust the genetic imbalance that arises from the different numbers of gene-rich X chromosomes between the sexes. The dosage difference between XX females and XY males is functionally equalized by silencing one of the two X chromosomes in females. This dosage-compensation mechanism seems to have arisen concurrently with early mammalian evolution and is based on the long functional Xist RNA, which is unique to placental mammals. It is likely that previously existing mechanisms for other cellular functions have been recruited and adapted for the evolution of X inactivation. Here, we critically review our understanding of dosage compensation in placental mammals and place these findings in the context of other cellular processes that intersect with mammalian dosage compensation.
In mammals, silencing of one of the two X chromosomes in female cells provides dosage compensation between the sexes. The noncoding Xist RNA localizes over the inactive X chromosome and initiates gene silencing. Visualization of Xist in living cells is used to study the mechanism of localization and measure the dynamics of chromosome-bound Xist.
INTRODUCTIONFluorescence in situ hybridization (FISH) has become a widely used method in genome and molecular genetic studies. The technique is highly versatile and has been adapted to carry out genome-wide screenings, microarray quantifications, cancer cytogenetics analysis, and RNA expression and localization studies. The study of intracellular RNA localization using RNA FISH provides insights into the in situ physical characteristics of transcription and intracellular RNA transport in individual cells. In our lab, we use RNA FISH to detect the localization of Xist RNA, a nuclear noncoding transcript that coats the entire chromosome from which it is transcribed. The advantage of using RNA FISH in our case is to extract precise molecular information directly in the context of cellular structure. The RNA FISH technique requires the generation of a labeled probe, hybridization of the probe to a fixed sample, and detection of the labeled probe using microscopy.
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