Barr body Practical Of Genetics History 1940s 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 ID: 595696
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Slide1
Sex chromatin bodies((Barr body
Practical Of GeneticsSlide2
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 .Slide3
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 bodySlide4
Barr bodies represent the inactive X chromosome and are normally found only in female somatic cells.
Barr BodiesSlide5
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 genes7 genes code for proteins5 genes code for untranslated RNA
XICSlide6
This region contains two genes actively involved in inactivating an X chromosomeXist
(X-inactive specific transcript )Tsix
Antagonistic rolesXi
Xist
expression
Tsix
expression
Xa
Xist
expression
Tsix
expressionSlide7
Xist and TsixThe Xist
gene is the only gene which is expressed from the Xi but not from the Xa. X chromosome lacking Xist
gene cannot be inactivatedSlide8
Xist gene (pronounced “exist”)Encodes a large RNA molecule
Coats Xi from the XIC near the centromere outward along the X chromosomeSlide9
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 )Slide10
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
.Slide11
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)Slide12
Males and females have equal dose of gene representationLevels 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.Gene RepresentationSlide13
G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosomeThis 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 malesXXY and XXX individuals produce the same about of G6PD as anyone elseSlide14
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.Slide15Slide16Slide17Slide18Slide19