Barr Body - Gene/Chromosome In activation - Interference RNA example
  
        Euchromatin is the active form of chromosomal DNA,
        Heterochromatin is the inactive form of chromosomal DNA.
  
           an example:  the inactive X-chromosome of mammals
                              XX- 2 doses of gene activity, while Xy = 1 dose of X genes
                                      yet, gene expression of males & females is about the same...  HOW? 

         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
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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
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