Barr body:
Barr and Bertram observed a darkly stained body in G-0 stage of nerve cells of female cats that were absent in similar cells of males. This body was described as a sex chromatin body or simply Barr body. The Barr body is one of the two inactive X-chromosomes in mammalian female cells. It remains condensed during G-0 stage and is almost completely silent in transcription, where as the other X-chromosome in the cell is de-condensed and transcriptionally very active. The inactivation of X-chromosome is termed as lyonization, which is an epigenetic process: that is, a process that affects the expression of genes without any change in DNA sequence. The number of the Barr body depends on the number of X-chromosomes in a cell. It follows an N-1 rule where N is the total number of X-chromosomes. For example, Turner(45, X) female, contains no Barr body where as Klinefelter(47, XXY) male contains one Barr body.
Mammalian X-chromosome inactivation is initiated from the X-inactivation center(XIC), which is located on the long arm of the X-chromosome. The X-inactivation center contains twelve genes, seven of which code for proteins, five for non-coding RNAs, of which only two are known to play an active role in the X-inactivation process, XIST and TSIX. The XIC also appears to be important in chromosome counting; ensuring that random inactivation only takes place when more than one X-chromosome are present.
The roles of XIST and TSIX appear to be antagonistic. The XIST gene does not encode a protein but rather produces a long non coding RNA(17kb). XIST RNA is only expressed in cells containing at least two X-chromosomes and is not normally expressed in normal male cells where only one X-chromosome present per cell. Higher XIST expression can be seen in cells with more X-chromosomes. XIST RNA is expressed from an future inactive X-chromosome. It remains exclusively in the nucleus and is able to 'coat' the chromosome from which it was produced i,e cis-acting.
How is XIST expressed from one inactive X-chromosome, while it remains silent on the other X? The answer to this question came after the discovery of XIST's antisense with respect to each other. The term 'antisense' refers to the fact that TSIX is complementary in sequences to XIST. TSIX is also a long non coding RNA(40). Like XIST, TSIX only acts on the chromosome that produces it i,e cis-acting. Moreover, there is an inverse relationship between TSIX and XIST expression. When TSIX transcription is reduced on one X, XIST expression increases and leads to inactivation of that same X-chromosome. The mechanism for triggering expression of the XIST gene from only one chromosome is incompletely understood. It appears that XICs on the two X-chromosomes sense each other and pair. The transient pairing results in transcription of XIST in one chromosome and TSIX on the other.