The male and females of a species are distinct and each will have its one genetic content. The numbers of autosomes in both organisms are same in diploid conditions and there is no mechanism to change level of expression of gene products on autosomes. The situation is not the same in case of Allosomes or X-chromosomes, the males and females will have different numbers sex chromosomes either as XY or XX, ZZ or ZW, etc. The Y chromosome contains high heterochromatin content and has only few genes, while X-chromosome contains hundreds of genes, some of which are very essential for the survival of the organisms. X-chromosome contains genes that are required for brain function and sexual reproduction. Females have two X-chromosomes and hence have double the content of genetic material with respect to X-chromosome by comparing with males which have only one X-chromosomes. Reading on the genetic content, the females should express double the gene products of the genes that are present in the X-chromosome. This might have lead to favoring of one particular sex in nature. To tackle this phenomenon, nature has developed a fabulous phenomenon called Dosage Compensation. Dosage Compensation is the mechanism of equalizing the amount of expressible genetic content in both sexes.
Mary Lyon to explain the fur coat color of mice hypothesized random inactivation of X-chromosomes in female. Further Murray Barr identified highly condensed heterochromatin in the somatic cells of female called Barr body. Barr bodies are localized in the periphery of the resting somatic cells. In heterozygous organism, except for one X-chromosome, all additional X-chromosomes becomes inactive and forms a highly condensed Heterchromatin in the somatic phase. In gametic cells or germ cells, there is no X-chromosome inactivation or Lyonization. Hence Lyonization mechanism is observed only in somatic cells. The Barr body is utilized in detecting number of X-chromosomes present in individuals during the initial level of screening for chromosomal variations. Normal females will have two X-chromosomes and has one Barr body and the number of Barr bodies increases with the increase in the number of X-chromosomes, which can be detected using buccal smear technique.
The pattern of Inactivation of X-chromosome differs from organisms to organisms. In some organism like humans, the pattern of X-inactivation is highly random, mosaic pattern is observed in the expression of genes that are coming from paternal/ father side and maternal / mother side. Coat color of mammals is classical example of X-chromosome inactivation where in few somatic cells X-chromosome from father is inactivated, while in other somatic cells, the X-chromosome from mother is inactivated. Though the procedure for selection of X-chromosome for inactivation is random, but the process once occurred is irreversible in somatic cells. The cells having inactivated paternal X-chromosome will always have the functional maternal X-chromosome and vice versa throughout the life cycle of the organism.
In mammals, the inactivation of X-chromosome is regulated by Xic loci (X-inactivation centre). Xic codes for a non protein coding RNA of 19Kb called Xist. Xist RNA binds to the same X-chromosome that expresses it in cis and triggers X-chromosome inactivation. The chromatin of the X-chromosome after Xist RNA coating/binding changes into tight heterochromatin. Histone methylation and acetylation modifications are observed after Xist binding to X-chromosome and play a critical role in X-chromosome inactivation. Further the inactivated X-chromosome replicates asynchronously in somatic cells compared to the active X-chromosome homologue during cell replication. To explain the mechanism of X-inactivation many theories have been proposed and to date it is very difficult to explain the complete mechanism of X-inactivation and selection of X-chromosome for inactivation.
X chromosomal inactivation is the wonderful mechanism developed in nature to unify the level of gene expressions in both sexes. The research on X-chromosomal inactivation has many applications on human health. Due to X-chromosome inactivation, alleles which are recessive get expressed because of suppression of dominant gene in heterozygous conditions. The reactivation suppressed dominant/wild type gene because of X-inactivation can cure the diseases like muscular dystrophy, fragile X-syndrome, etc., that are X-linked in females.
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