Barr Body Definition
The Barr body, also sometimes called the sex chromatin, is the inactive X chromosome in female somatic cells. Human females have two X chromosomes, while males have one X and one Y. In all of the female somatic cells, which don’t take part in sexual reproduction, one of the X chromosomes is active, and the other is inactivated in a process called lyonization, becoming the Barr body. The reason for shutting off one X chromosome is so that only the necessary amount of genetic information is expressed, rather than double or even more. This is why X-inactivation doesn’t only occur in humans, but in all organisms whose gender is determined by the presence or absence of a Y or W chromosome in the cell. In short, the amount of X chromosome genes expressed has to be equal in both males and females.
The Barr body is packaged in heterochromatin, while the active X chromosome is packaged in euchromatin. This means that although both X chromosomes have the same gene content, the inactive X chromosome is condensed and not easily accessible to molecules involved in transcription, while the active X chromosome has a larger volume, and is more dispersed, or open, allowing for transcription to take place.
Which X Chromosome Becomes the Barr Body?
It is totally random which chromosome becomes inactivated, but within any particular cell, the inactivated X chromosome remains inactive for the cell’s whole life. Furthermore, the same chromosome will be inactive in all the cells that descend from the original one, and, needless to say, the same applies to the active X chromosome. Therefore, there’s only ever one active X chromosome in any given cell, but it varies which X chromosome it is. For that reason, the number of Barr bodies is always one less than the total number of X chromosomes. This is true even when an individual has a mutation that has led to the presence of extra X chromosomes, as in the case of Klinefelter syndrome in males, where an extra X chromosome is found in all cells.
Mechanism of X-inactivation
There is an X-inactivation center (XIC) found near the centromere on the X chromosomes, containing a gene called X-inactive specific transcript (Xist), and another called Tsix (Xist reversed). Xist leads to gene inactivation on the X chromosome, and so we find that the Xi is coated with Xist RNA, while the Xa is not. Tsix, on the other hand, is a negative regulator of Xist, meaning that it opposes its function. Since Xist leads to the silencing of genes, then the presence of Tsix prevents that. Therefore, we find that chromosomes with more Tsix expression are far less likely to become inactivated than those with less Tsix expression.
To start, both X chromosomes equally express the Xist RNA, but as the inactivation process begins, the Xi to-be increases the amount of Xist RNA being produced, which then coats this chromosome, silencing its genes. On the other hand, the future Xa stops the expression of the Xist gene, which means that its genes are not silenced, and the chromosome remains active. Of course, the Xist gene on the Barr body is not silenced, and a percentage of the rest of the genes have also been found to escape inactivation. This means that a female who is born with only one chromosome will still have less genes being expressed than a female who has both X chromosome, even though only one X chromosome is fully active in normal females. Like Xist, Tsix RNA is equally expressed on both X chromosomes to start, but that changes as the inactivation process begins taking place.
Genetic Disorders and the Barr Body
Because only one X chromosome is active in every cell, females with Turner’s syndrome can in rare cases make it to birth even though their cells only contain one X chromosome to begin with. They can also live out their lives and be functional despite their cells containing no Barr body and therefore missing out on the few genes that are expressed from the inactive X chromosome. Females with this disorder don’t develop like the average female, but they still express enough X chromosome genes to get by. In the case of disorders where there are extra X chromosomes in the cells of either males or females, only one X chromosome remains active, and the rest become Barr bodies. This can also lead to developmental problems.
Finally, problems can also arise in females whose cells have both X chromosomes. Normally, the paternal X chromosome is silenced half of the time, and the maternal X chromosome is silenced the other half. However, in some cases one or the other is silenced more frequently. Depending on how skewed the ratio of inactivation of the paternal X chromosome to inactivation of the maternal X chromosome is, females can either have no symptoms, minor problems, or they can face problems that lead to death.