Mosaicism

How does a cell manage to silence one X chromosome in a cell but not the other even though the two chromosomes are almost identical? A clue to this puzzle came from the discovery of a gene named XIST (X inactive specific transcripts). This is a gene that is expressed only on the inactive X chromosome. It is transcribed into an RNA that does not code for protein, unlike most genes. Instead, the RNA associates with the X chromosome from which it is made, resulting in silencing of the chromosome.

We know many of the components of this silencing process, and they are proteins that have been implicated in the silencing of other genes or regions of chromosomes as well. They are predominantly factors that influence the structure of the chromatin, which is the complex of DNA and proteins that is found in chromosomes. For example, the chromatin structure can be changed by adding methyl groups to the DNA, or by adding acetyl or methyl groups to the histone proteins with which the DNA interacts.