Barr body and Dosage compensation Principles of Genetics

X-Inactivation, Barr body and Dosage Compensation

X-inactivation:

is a process by which one copy of the X chromosome is inactivated during early embryonic development in female mammals. The inactive X chromosome is silenced and it is a transcriptionally inactive structure called heterochromatin.

All female mammals have two X chromosomes, any gene encoded by the X chromosome will be expressed at twice the level in females. This imbalance will effect cellular processes. X-inactivation prevents them from having two X chromosome gene products. This is known as Dosage Compensation.

The change that occurs during X inactivation are epigenetic modifiications. X inactivation also occurs in males with Klinefelter syndrome and super females who have more than one X chromosome. This process was described by Mary Lyon. It is also known as lyonization

Barr body A Barr body discoverered by Murray Barr is a small densely stained structure in the cell nuclei of females. It consists of a condensed, inactive X chromosome. In humans with more than one X chromosome, the number of Barr bodies is N- 1 where N is number of X chromosomes. Klinefelter syndrome (47,XXY ) have a one Barr body, Superfemales( 47, XXX karyotype) have two Barr bodies.

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Calico Cats

A good example of this phenomenon is the calico cat. In cats, one gene on the X chromosome is responsible for fur colour orange or black. In cats heterozygous for this gene, the different patches of black and orange fur appears. It indicates that calico cats are female in which one of the X chromosome is inactivated. The white cats are due to effects of an autosomal gene.

Calico Cat

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MECHANISM OF X-INACTIVATION:

Non coding RNA Xist RNA plays role in X –inactivation in mammalian females. This RNA is encoded within the locus the Xic (X-inactivation center) on the X chromosome. The Xic is present on q arm of 13 X chromosome. Xic contains two regions a. Xist (X inactivation specific transcript) and Tsix. b. Downstream of Xic is present X Control element.

In the early embryonic stage, at the 32- to 64-cell stage, the one of the X chromosome is inactivated randomly. Blocking factor produced during morula stage block only one chromosome. This factor binds the upstream of Xist and inhibit transcription of RNA. Inactivated X chromosome does not bind blocking factor have stable Xist RNA which results in X inactivation.

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