Austin So (Hae Jin)
haejin at netinfo.ubc.caX
Thu May 27 13:04:28 EST 1999
LJ Smith wrote:
> Assuming that cells are "programmed" to undergo a certain number of divisions
> and then they die, how is it that we have a continuous amount of cells to
> replace the dead ones, such as with skin cells? Where do these "fresh" cells
> come from?
Cells are believed to be able to undergo 40-50 rounds of mitosis...so that's 2
exp(n)...that's *a lot* of cells that can be generated from one cell. This is
determined by the size of chromatid ends called telomeres. But only a subset
continue through mitosis and some can be considered as in a state of
"quiescence"....otherwise you'd just be a mass of tissue. That's why you have a
"renewable" cell population. Also, stem cells are believed to have an enzyme
called telomerase that is active only in the stem cell population and not the
differentiated cell population that maintains the size of these
telomeres...preventing what is referred to as the end-replication problem. There
is a recent article on the cloned sheep "Dolly" that is related to this...you
might want to have a good read.
> If you start with a single cell that undergoes mitosis and you now have 2
> cells, each with its own genome, did the repicated chromatids assort in such a
> manner that some of the original strands and some of the replicated strands
> end up in both cells?
DNA has two strands. Each one of those strands acts as a template for replication,
resulting in two "daughter" DNA molecules, which are segregated into the 2 (i.e.
daughter) cells. So each daughter cell has a strand from its parents and the
complementary strand is replicated. However, and theoretically, in the "nth"
generation, only two of the cells in that nth generation will have the original
"founding" DNA content...
Austin P. So (Hae Jin)
University of British Columbia
E-mail: haejin at netinfo.ubc.ca
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