03 - X-inactivation

Question 1

(Xi-inactivation)

1. what is the process that equalizes differences in genetic matieral?
2. What this create in female?

Answer 1

1. X-inactivation
2. barr body

Question 2

(X-inactivation)

(involves several steps)

1. X chromosomes need to be counted so that what?
2. What choice must be made?
3. developmental cue present at only specific stages of early embyrogenesis has to trigger inactivation which starts where?
4. Inactivation spreads from X-inactivation center to cause what?
5. inactivation stabliized so inactivae X is clonally transmitted… will same X be inactivated in subsequent mitotic divisions?

Answer 2

1. all but one is inactivated
2. which one to deactivate
3. the so-called X-inactivation center (Xic)
4. inactivation of the vast majority of the several thousand genes on X
5. yes

Question 3

(X-inactivation)

1. Does inactivation include all somatic cells of the female embryo?

Answer 3

1. yes

(mechanisms behind X chromosome counting, choice of which to inactivae and the mechanisms of inactivation and its stabbilization are still works in various stages of progress.

Question 4

In females one X chromosome is maternally derived (Xm), and one is paternal (Xp), as indeed are all the autosomes. They are said to be parentally imprinted (in normal XY males all X chromosomes are maternally derived and all Y’s are paternal). The current model for X-inactivation is as follows:

1. Xp is inactivated in which cells during the early cleavage divisions, prior to differentiation of early primitive tissue layers?
2. At what stage is Xp reactivated and in what?
3. The cells of the inner cells mass then undergo what?

Answer 4

1. all
2. At early blastocyst stage in the cells of the inner cell mass

[so both Xp and Xm are briefly active in cells that will form the embryo proper, while Xp remains inactive in cells that will form the placenta (the now differentiated trophoblast
and primitive endoderm)].

3. random X inactivation (either Xm of Xp)

Question 5

1. Parts of the affected chromosome escape inactivation, this appears especially true of what region?
2. It is currently thought up to how many genes on the inactivated X actually escape inactivation?

Answer 5

1. the region carrying homologous genes (the psedoautosomal region) that is involved in chromosome pairing
2. up to 1 in 5

Question 6

The primordial germ cells of females also undergo X inactivation… probably during migration to the urogenital ridge

1. However both X chormosomes are required to be active for normal meiotic chromosome pairing during oogenesis, so what happens as cell enters meiosis?
2. What happens to these ooctye X chromosomes?

Answer 6

1. it is reactivated (~12.5 dpc in mice)
2. they rmain active until they in turn are involved in formation of a new embryo in the next generation, wen lineages of their daughter cells are inactivated