one of the 2
XX's becomes transcriptionally inactive,
which X is inactive seems to be randomized,
in interphase cells the inactive X-chromosome
can be visualized & is called a BARR BODY
|Inactivation of the X chromosome
occurs due to
chromatin condensation via 3 mechanisms:
1. methylation (adding CH3's) of 5'Cysteine of X-chromosome DNA
2. presence of chromatin proteins that promote heterochromatinization
3. action of a single gene on one X, when active makes an siRNA microRNA
Xist gene - only lightly methylated --> & makes siRNA (interference RNA; not a mRNA)
Xist-siRNA binds to X-chromosome from which it is transcribed and favors inactivation
of the X-chromosome. fig of inactivation of X by RNAi*
once inactivated all progeny cells are also inactivated.
| How do active
X-chromosomes prevent the action of siRNA?
anti-Xist gene called Tsix is involved...
Tsix makes an RNAi that binds complementarily to the siRNA-Xist molecule,
making a double stranded RNA and thereby inactivating it.
Inactivation of normal alleles -
GENE IMPRINTING* in
organisms exhibiting live birth