Question

What is the inactivation of the X chromosome? What is a Barr body?

Answer 1

INACTIVATION OF THE X CHROMOSOME

Inactivation of the X chromosome is a process required to equalize the dosage of X-encoded genes between female and male cells. Iactivation of X chromosome is started very early during female embryonic development or upon differentiation of female embryonic stem (ES) cells and results in inactivation of one X chromosome in every female somatic cell. The regulation of inactivation of X chromosome (XCI) involves factors that also play a crucial role in embroynic stem cell maintenance and differentiation and the inactivation of X chromosome process therefore provides a beautiful concept to study embryonic stem cell biology.

From the time inactivation of X chromosome was initially described, it was clear that several mechanisms must be precisely integrated to achieve correct regulation of this complex process. X-inactivation appears to be triggered upon differentiation, suggesting its regulation by developmental hints. Whereas any number of X chromosomes greater than one is silenced, only one X chromosome remains active. Silencing on the inactive X chromosome coincides with the accession of a multitude of chromatin changes, resulting in the formation of extraordinarily stable facultative heterochromatin that is faithfully propagated through subsequent cell divisions. The integration of all these processes requires a region of the X chromosome known as the X-inactivation center, which contains the Xist gene and its cis-regulatory elements. Xist encodes an RNA molecule that plays critical roles in the choice of which X chromosome remains active, and in the initial spread and establishment of silencing on the inactive X chromosome.

BARR BODY

A Barr Body or sex chromatin is an inactivated, condensed X chromosome found in female cells.Since females posses two X chromosomes and males have one X chromosome and a Y chromosomes, Barr bodies are essential to regulate the amount of X-linked gene product being converted. To ensure that X-linked gene product doses are kept similar between males and females, one of the X chromosomes in a female becomes very compressed. This results in the genetic information on the chromosome being inconvenient to proteins that cause gene transcription. This is called dosage compensation.