Chromosome

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Slide 108

One end of each loop is thinner (thin end) than the other end (thick end).

There is extensive RNA synthesis at the thin end of the loops, while there is little or no RNA synthesis at the thick end.

Slide 109

Phase-contrast and fluorescent micrographs of

lampbrush chromosomes

Slide 110

Dosage Compensation

Sex Chromosomes: females XX, males XY

Females have two copies of every X-linked gene; males have only one.

How is this difference in gene dosage compensated for? OR

How to create equal amount of X chromosome gene products in males and females?

Slide 111

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.

Slide 112

G6PD, glucose 6 phosphate dehydrogenase, gene is carried on the X chromosome

This gene codes for an enzyme that breaks down sugar

Females produce the same amount of G6PD enzyme as males

XXY and XXX individuals produce the same about of G6PD as anyone else

Slide 113

In cells with more than two X chromosomes, only one X remains genetically active and all the others become inactivated.

In some cells the paternal allele is expressed

In other cells the maternal allele is expressed

In XXX and XXXX females and XXY males only 1 X is activated in any given cell the rest are inactivated

Slide 114

Slide 115

Barr Bodies

1940’s two Canadian scientists 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

Slide 116

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

Barr Bodies

Slide 117

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.

Slide 118