MadSci Network: Cell Biology
Query:

Re: How often are all the cells in our bodies replaced?

Date: Tue Oct 17 12:15:32 2000
Posted By: Matthew Eveland, Undergraduate, molecular genetics,biology, pensacola junior college
Area of science: Cell Biology
ID: 958518337.Cb
Message:

It could take months for some cells to divide. It really has a lot to do 
with what kind of tissue environment the cells are in. For instance, skin cells 
will divide very rapidly because they are constantly exposed to a bunch of 
hermful things that can kill a cell like sun light and polution, so damaged 
cells are replaced from the lower layers of skin cells. On the 
other hand some cells like brain cells hardely ever divide. They do not 
divide because the connections "wiring" in the brain needs to be maintained in a 
very specific way. Now this does not mean that you 
can just put some brain cells in a hostile environment and they will start 
to divide more often.Expression of the cells genes tell it how often to divide. 
A cells genes also tell the cell how many times it can divide before it can not 
divide any more ( I include this because it seems that you are interested 
in aging). One gene that plays a role in aging is telomerase.  Telomerase is a 
protein which adds telomere DNA to the end of chromosomes, sort of like caps at 
the end of shoelaces. Telomeres are on the ends of every chromosome. each 
telomere has a repeating pattern. In vertebrates the pattern is TTAGGG. Every 
time the cell divides one TTAGGG is lost.  When the telomeres get too short, the 
cells stop dividing. No one is really sure exactly why some cells divide more 
than others, but telomerase and telomeres are part of the answer. Skin cells 
will be able to divide more than brain cells. The human body lives much longer 
than its individual cells. There is a balance between how often they can divide, 
and how many times they can divide. OF course this is just one of several 
theories of aging.



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