Re: How do cells which contain Barr bodies divide mitotically?

Lerro,
     Good question.  I assume you have been studying about Barr bodies in 
school, but just for making sure I have covered all my bases I'll give a 
brief description.  Barr Bodies are inactivated "X" chromosomes.  I don't 
know what you have learned about them, but let's take a look at why they 
are inactive.  Well, you probably know that they are inactive only in 
females of all mammal species.  Females have 2 "X" chromosomes, and it is 
thought that a double dose of otherwise normal gene products from an "X" 
chromosome might be lethal, but lets not get into that cause there is still 
some speculation on the true nature of this.  
     Anyway, which "X" chromosome becomes inactive is the intersting thing.
During the division stage of the developing embyro, each cell decides to 
inactivate one of its "X" chromosomes, creating what we refer to as a Barr 
Body.  Now every cell that comes after keeps the same chromosome 
inactivated.  It isn't really apparant in humans on the surface, but 
perhaps the best model to use is the calico cat, you know, the ones that 
are patches of orange, black, and white.  First question for you, what 
gender are all calico cats?  You are probably saying to yourself, "female 
of course."  Well, during the development of the cat, while it is still 
just a ball of cells(I am not sure how many), each cell inactivates one "X" 
chromosome.  It turns out that on the "X" chromosome is the gene that codes 
for orange or black fur.  The white is probably from other genes located on 
an autosome(specifically not on the "X" or "Y") that is dominant over 
orange or black, but I don't really know the answer for that.  So lets just 
talk about orange and black.
     As you know, a calico is a mosaic of black and orange.  The reason why 
is that each spot of black fur originated from tissue that is derived from 
cells that inactivated the chromosome that coded for the orange gene, 
thereby preventing any orange color expressed in these cells.  The opposite 
goes for the patches of orange.  Human females are mosaics of cells with 
inactivated "X" chromosomes as well, however it isn't as obvious as the 
calico cat.
    So, the mechanism of "X" inactivation is that key places on the 
chromosome become methylated.  This is a process by which the cell marks 
places on a strand of DNA to be shut off in a sense.  This happens on other 
chromosomes as well, but in those cases it is specific genes instead of a 
whole chromosome(as in one of the "X's".)  Mitosis, to answer your question 
briefly is when a cell divides as makes two new daughter cells that have 
the exact same genetic material.  This also includes the location of all 
the genes that have been methylated(inactivated) as well as inactivating 
the same "X" chromosome.  It does this by enzymes that read the DNA, 
looking for where those methylation flags are, and making sure the new DNA 
that the daughter cells get have the same spots flagged.  The parent cells 
does not need to reactivate the inactive "X" chromosome for this process at 
all, only copy it.  
     However, during the process of meiosis, the inactive "X" chromosome is 
awakened once again.  Meiosis is the process by which eggs( and sperm) are 
made for sexual reproduction.  It is similar to mitosis, however, each egg 
contains only have the genetic material as the parent because it will fuse 
with a sperm that also contains half the DNA of the father.  It seems that 
during the process of meiosis, both "X" chromosomes are needed to produce a 
proper egg, and also you want to make sure that the "X" chromosome that is 
being used for the egg is active so that it can do its job when a new 
zygote(fusion of egg and sperm) is made.  After the zygote divides into 
enough cells, and of course is also a female, the whole process of "X" 
inactivation will start again in this new individual.        
     You can certainly read more about this in a book called "Molecular 
Biology of the Cell" by Bruce Alberts.  It can be quite a challenge, even 
for the college level student.  Another book is simply called "Biology" by 
Neli A. Campbell.  It is also a college level book, but it does provide a 
lot of great information.  I hope this has helped answer you questions 
about Barr bodies, and mitosis.  If you have any further questions, feel 
free to write to my e-mail address.  Good luck.

Mark Sullivan