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Sex chromatin bodies Sex chromatin bodies

Sex chromatin bodies - PowerPoint Presentation

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Uploaded On 2017-10-13

Sex chromatin bodies - PPT Presentation

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

barr chromosome bodies gene chromosome barr gene bodies xist males genes body inactive cells expression females tsix xic female individuals xxy mechanism

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Presentation Transcript

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.Slide15
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