Lab # 11 :

Sex chromatin bodiesSex chromatin bodies

Barr bodyBarr body

OrganismOrganism No. No. chromosomeschromosomes

HumanHuman 4646 ChimpanzeeChimpanzee 4848 DogDog 7878 HorseHorse 6464 ChickenChicken 7878 GoldfishGoldfish 9494 Fruit flyFruit fly 88 MosquitoMosquito 66 NematodeNematode 11(m), 12(f)11(m), 12(f) HorsetailHorsetail 216216 SequoiaSequoia 2222 Round worm Round worm 22

What Exactly is a What Exactly is a chromosomechromosome??

Chromosomes are the Chromosomes are the rod-shapedrod-shaped, , filamentous bodies filamentous bodies present in the present in the nucleusnucleus, which become visible , which become visible during during cell divisioncell division . .

They are the They are the carriers of the gene carriers of the gene or or unit of heredityunit of heredity..

Chromosome are Chromosome are not visible not visible in active in active nucleus due to their nucleus due to their high water high water contentcontent, but are clearly seen during , but are clearly seen during cell divisioncell division . .

Chromosomes were first described Chromosomes were first described

by by Strausberger Strausberger in in 18751875. .

The term The term ““ChromosomeChromosome””, however , however

was first used by was first used by Waldeyer Waldeyer in in 18881888. .

They were given the name They were given the name

chromosome (chromosome (ChromoChromo = colour; = colour;

Soma Soma = body) due to their marked = body) due to their marked

affinity for basic dyesaffinity for basic dyes. .

Their number can be counted easily Their number can be counted easily

only during only during mitotic metaphase.mitotic metaphase.

This X-Y system of This X-Y system of mammals is not the mammals is not the only chromosomal only chromosomal mechanism of mechanism of determining sex.determining sex.

Other options Other options include the X-0 include the X-0 system, the Z-W system, the Z-W system, and the system, and the haplo-diploid haplo-diploid system.system.

Barr BodiesBarr Bodies 19401940’’s two Canadian scientists s two Canadian scientists

noticed a noticed a dark staining massdark staining mass in in the nuclei of cat brain cells.the nuclei of cat brain cells.

Found these Found these dark staining spots dark staining spots in female but not males.in female but not males.

This held for cats and humans.This held for cats and humans. They thought the spot was a They thought the spot was a

tightly condensed tightly condensed X chromosome X chromosome ..

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

Barr Bodies

X Inactivation of FemalesX Inactivation of Females

During embryonic development During embryonic development one X chromosome becomes one X chromosome becomes inactive (Barr Body).inactive (Barr Body). All mitotic divisions create cells All mitotic divisions create cells

with the same inactive X.with the same inactive X.Males and females have equal Males and females have equal

dose of gene representationdose of gene representation

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

To compensate for females having 2 X chromosomes vs. males having only 1 X chromosome could do one of two things :

•Double the amount of transcription of X-chromosome genes in males.

• Inactivate one of the X-chromosomes in females.

OR

Dosage Compensation

Levels of enzymes or proteins Levels of enzymes or proteins encoded by genes on the X encoded by genes on the X chromosome are the same in chromosome are the same in both males and femalesboth males and females

Even though males have 1 X Even though males have 1 X chromosome and females have 2.chromosome and females have 2.

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 body45, X 0 Barr body47, XXY 1 Barr body47, XXX 2 Barr bodies48, XXXX 3 Barr bodies

# Barr bodies = N-1 (N = # X cs. present)

G6PD, glucose 6 phosphate dehydrogenase, G6PD, glucose 6 phosphate dehydrogenase,

gene is carried on the X chromosomegene is carried on the X chromosome

This gene codes for an enzyme that breaks down This gene codes for an enzyme that breaks down

sugar which helps red blood cells work properly.sugar which helps red blood cells work properly.

Females produce the same amount of G6PD Females produce the same amount of G6PD

enzyme as malesenzyme as males

XXY and XXX individuals produce the same XXY and XXX individuals produce the same

about of G6PD as anyone elseabout of G6PD as anyone else

Dosage compensationDosage compensation

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

In some cells the In some cells the paternal paternal allele is allele is expressedexpressed

In other cells the In other cells the maternalmaternal allele is allele is expressedexpressed

In XXX and XXXX females and XXY males In XXX and XXXX females and XXY males only only 1 X is activated1 X is activated in any given cell the in any given cell the rest are inactivatedrest are inactivated

X-inactivationX-inactivation

Which chromosome is inactive is a matter of Which chromosome is inactive is a matter of

chance, but once an X has become chance, but once an X has become

inactivated , all cells arising from that cell inactivated , all cells arising from that cell

will will keep the same inactive X keep the same inactive X

chromosome. chromosome.

In human embryos, sex chromatin bodies In human embryos, sex chromatin bodies have been observed by the 16have been observed by the 16thth day of day of gestation. gestation.

Mechanism of X-chromosome Mechanism of X-chromosome InactivationInactivation A region of X chromosome near the

centromere called the X-inactivation center (XIC) is the control unit.

This region contains the gene for X-inactive specific transcript (XIST RNA). The Xist gene is the only gene which is expressed from the XiXi but not from the XaXa.

The silencing of genes along the Xi occurs soon after coating by Xist RNA. (repressive ) (repressive )

The Xi has high levels DNA methylation locks the chromosome in the inactive state. (associated with gene silencing ) )

This occurs about 16 days after This occurs about 16 days after fertilization in a female embryo.fertilization in a female embryo.

The process is independent from cell The process is independent from cell to cell. to cell.

A maternal or paternal X is randomly A maternal or paternal X is randomly chosen to be inactivated.chosen to be inactivated.

Rollin Hotchkiss first discovered Rollin Hotchkiss first discovered methylated DNA in 1948.methylated DNA in 1948.

He found that DNA from certain sources He found that DNA from certain sources contained, in addition to the standard four contained, in addition to the standard four bases, a fifth: bases, a fifth: 5-methyl cytosine5-methyl cytosine. .

It took almost three decades to find a role It took almost three decades to find a role for it.for it.

In the mid-1970s, Harold Weintraub and his In the mid-1970s, Harold Weintraub and his colleagues noticed that colleagues noticed that active genes are active genes are low in methyl groups or under methylated. low in methyl groups or under methylated.

Therefore, a relationship between under Therefore, a relationship between under methylation and gene activity seemed methylation and gene activity seemed likely, as likely, as if methylation helped repress if methylation helped repress genes. genes.

This would be a valuable means of This would be a valuable means of keeping genes inactive if methylation keeping genes inactive if methylation passed on from parent to daughter cells passed on from parent to daughter cells during cell division. during cell division.

Each parental strand retains its methyl Each parental strand retains its methyl groups, which serve as groups, which serve as signalssignals to the to the methylating apparatus to place methyl methylating apparatus to place methyl groups on the newly made progeny groups on the newly made progeny strand. strand.

Thus methylation has two of the Thus methylation has two of the requirements for mechanism of requirements for mechanism of determination:determination:

1. It represses gene activity1. It represses gene activity 2. It is permanent.2. It is permanent.

To check this hypothesis To check this hypothesis Peter Jones Peter Jones and Lawrence Shapiro grew cells in the and Lawrence Shapiro grew cells in the presence of drug presence of drug 5-azacytosine5-azacytosine, which , which prevents DNA methylation. prevents DNA methylation.

This reactivated the lyonized (Mary This reactivated the lyonized (Mary lyon) the X chromosome.lyon) the X chromosome.

Furthermore, Shapiro showed these Furthermore, Shapiro showed these reactivated chromosomes could be reactivated chromosomes could be transferred to other cells and still transferred to other cells and still remain active. remain active.

X-inactivation facts about calico cats

What determines maleness and femaleness in humans?

2 possibilities:

1. Two X chromosomes are needed to make a female. If you have one X chromosome then you default to male. No, not the mechanism.

2. The Y chromosome is needed to make a male. Lack of the Y chromosome and you default to female. Yes, this is the mechanism.

X-chromosome Y-chromosome

22.322.2

22.1

21.321.221.1

11.411.311.2311.2211.2111.111.212

13

21.121.2

21.3

22.122.222.323

24

25

26

27

28

p

q

11.3

11.2

11.111.2111.22111.22211.22311.23

12

p

q

Duchenne Muscular Dystrophy locus

pseudoautosomal regionregion that escapes X-inactivation

X-inactivation center, XIST

critical region for ovarian development

Fragile X (FMR-1) locuspseudoautosomal region

pseudoautosomal regionSRY (testes determining factor)

AZF (azoospermia factor) (sperm development)heterochromatin

Presence of Barr bodies in a buccal smear was

introduced by the International Olympic Committee

Medical Commission as a sex determination (gender

verification) test in 1968.

This test, known as the Barr test or buccal smear sex

test, was responsible for excluding about one female

competitor in 400 from international competition.

Six female competitors failed the test in the 1984

Los Angeles Olympic games.

At the 1992 Barcelona Olympics, the Barr test

was replaced by the polymerase chain reaction

test.

Someone with a mutation in the SRY

gene can develop into a female even

though there are SRY and the Y

chromosome are present.

In rare cases, the SRY gene can be

transferred to the X chromosome by

chromosomal crossover crossover during the

production of sperm, and the resulting

XX individual would be phenotypically

male.

During the 1992 Olympics five women

failed the PCR test; in 1996, eight.

Testicular feminization syndrome (TFS), a genetic condition in which an XY (male) zygote develops as a phenotypically female adult, due to failure of androgen receptors (Androgen

insensitivity syndrome AIS).

Dora Ratjen, November 20, 1918 Bremen, Germany - April 22, 2008) was a German athlete who competed for Germany in the Women's High Jump at the 1936 Summer Olympics at Berlin, finishing fourth, but was later discovered to be male.

Stella Walsh won a silver medal at the 1936 Olympics - but was later found to be a man

Stella Walsh was killed in a random actof violence in 1968.9 As part of a routine autopsy, coroners discovered that Stella was amasculine pseudohermaphrodite (mosaicism)

800m gold medal in Berli 2009

Semenya ( south Africa) told

to take gender test

Reports in two newspapers in September said the

results of the tests showed Semenya has both male

and female characteristics. The IAAF has declined

to confirm those reports.