X-inactivation center and Xist gene

A short region on the X chromosome called the X-inactivation center (Xic) s known to play a critical role in inactivation, (169).
Counting of human X chromsomes is accomplished by counting the number of Xics (169)
An Xic must be found for inactivation to occur (169),
The gene within Xic that contols x-INACTIVATION is called the Xist gene. (169)
- This is active on the inactivated X chromosome.
- Produces a molecule that coats the X chromosome and inactivates it.
- After coating, other proteins assicuate with the Xist RNA and promote chromosomal compaction into a Barr body.
Another gene found within Xic is Tsix. This also plays a role in X inactivation (169)
- Overlaps with Xist and is transcribed in an opposite direction.
- Role not completely understood, but it appears to decrease expression of Xist.
- On inactive X it is not expressed.
- X chromosomes carrying a mutated form of tgis are preferentially inactivated.
The X chromosomal controlling element (Xce) also affects the choice of X chromosome to be inactivated (169)
- Genetic variation in this gene- X chromosome carrying stronlg Xce is more likely to remain acive, thereby leading to skewed X inactivation.
- Very close to end of Xic
- Mechanism is not well understood, but it might serve as abinding site proteins that regulate expression of genes in Xic.
- Genetic variation in Xce that enhances Xist expression would promote Barr body formation, whereas Xce variation that enhances Tsix expression preevents X inactivation.
3 phases of X inactivation (169):
1. Initiation: One of the X chromosomes is targeted to remain active.
  - Occurs during embryonic development.
2. Spreading: Chosen X chromosome is inacivated, which requires expression of Xist gene. Xist RNA coats inactivated X and recruits proteins that promote compaction.
  - Compaciton occurs via modification of histone proteins.\
  - Also occurs during embryonic development.
3. Maintenance: Inactivated X chromosome is maintained during future cell divisions
Some genes on inactivated X are expressed in somatic cells of adult female mammals (169),

	Created by Olivia McRitchie about 6 years ago

NCLEX Biology (Genetics) Note on Ch 7: Non-Mendelian Inheritance, created by Olivia McRitchie on 02/05/2018.