Barr Body-Gene/Chromosome miRNA inactivation example


Euchromatin is the
active form of chromosomal DNA,
Heterochromatin is the inactive form of chromosomal DNA.

           Barr body:  the inactive X-chromosome of mammals...
                 XX- 2 doses of gene activity, while Xy = 1 dose of X genes
                        yet, gene expression in males & females is about the same...  HOW? 

one of the 2 XX's becomes
transcriptionally inactive,

which X is inactive seems to be randomized.
During interphase in cells an inactive   X-chromosome can be visualized & is called a








Inactivation of X chromosome is due to chromatin condensation via 3 mechanisms:

  1.   methylation (adding CH3's)  on 5'Cysteine of X-chromosome DNA

  2.  presence of chromatin proteins that promote heterochromatinization

  3.  one gene on an X chromosome, when active makes an microRNA = miRISC
       Xist gene - makes miRISC (interference RNA) that binds to X-chromosome
favoring inactivation of the X-chromosome.

             Once inactivated all progeny cells are also inactivated.
          fig of inactivation of X by RNAi*          
  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 miRNA-Xist molecule,
          making a double stranded RNA and thereby inactivating it.












         Inactivation of normal alleles - GENE IMPRINTING* in organisms exhibiting live birth