Sex chromatin bodies ((Barr body Practical Of Genetics Sex chromatin bodies ((Barr body

History 1940’s two Canadian scientists (Murray L. Barr and E. G. Bertram) noticed a dark staining mass in the nuclei of cat brain cells. Found these dark staining spots in female but not males. This held for cats and humans. They thought the spot was a tightly condensed X chromosome . كتلة صبغية مظلمة

Mary Lyon The Lyon Hypothesis (Lyonization) definition: condensation of X chromosome mechanism for inactivating the genes on the chromosome Named inactive X chromosome a Barr body

Barr Bodies Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells.

Mechanism of X-chromosome Inactivation A region of X chromosome near the centromere called the X-inactivation center (XIC) is the control unit. Contains 12 genes 7 genes code for proteins 5 genes code for untranslated RNA XIC

Xi   Xist expression Xa   Xist expression This region contains two genes actively involved in inactivating an X chromosome Xist (X-inactive specific transcript ) Tsix Antagonistic roles Xi   Xist expression  Tsix expression Xa   Xist expression  Tsix expression Xa = active X chromosome Xi = inactive X chromosome

Xist and Tsix The Xist gene is the only gene which is expressed from the Xi but not from the Xa. X chromosome lacking Xist gene cannot be inactivated

Xist gene (pronounced “exist”) Encodes a large RNA molecule Coats Xi from the XIC near the centromere outward along the X chromosome

Mechanism for compacting Xi (Barr body) The Xi has high levels DNA methylation (CH3) locks the chromosome in the inactive state. (associated with gene silencing )

A woman with the chromosome constitution 47, XXX should have 2 Barr bodies in each cell. XXY individuals are male, but have a Barr body. XO individuals are female but have no Barr bodies.

46, XX 1 Barr body 45, X 0 Barr body 47, XXY 1 Barr body 47, XXX 2 Barr bodies 48, XXXX 3 Barr bodies # Barr bodies = N-1 (N = # X cs. present)

Gene Representation Males and females have equal dose of gene representation Levels of enzymes or proteins encoded by genes on the X chromosome are the same in both males and females Even though males have 1 X chromosome and females have 2.

G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosome This gene codes for an enzyme that breaks down sugar which helps red blood cells work properly. Females produce the same amount of G6PD enzyme as males XXY and XXX individuals produce the same about of G6PD as anyone else

The process is independent from cell to cell. In cells with more than two X chromosomes, only one X remains genetically active and all the others become inactivated. The process is independent from cell to cell. A maternal or paternal X is randomly chosen to be inactivated.