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Cell Aging with Mitosis
Name: David
Status: Educator
Grade: 9-12
Location: PA
Country: United States
Date: May 2005
Question:
Thank you for taking the time to do this kind of
thing! I had question about cell biology. I've done some Googling, so
this is more of a 'do I have it right?' question.
My original question was: If cells only divide (1 parent -> 2 daughter),
and the daughters are identical to each other, then how can any cell be
any older than any other cell? Wouldn't they technically all be as old as
the original fertilized egg? I know that some kinds of cells divide more
often than others, but even if we limit the discussion to, say, skin
cells: Wouldn't they all be the same age, because they have all simply
divided from the same original specialized stem cell a long time back? And
therefore shouldn't every skin cell die of old age at about the same time?
From Googling I have learned:
1. Cell age is dependent on the number of divisions a cell has undergone,
not its chronological age.
2. Cells typically go into senescence after about 50 divisions (Hayflick's
limit), and are eventually removed by apoptosis.
3. But most cells do not reach Hayflick's limit.
4. Therefore, even though all the cells of a given kind (like skin cells)
might all be about the same age (having divided approx. the same number of
times), most cells do not die of "old age", but more likely from apoptosis
(from other causes) or from necrosis.
Are my points 1,2,3,4 right?
Replies:
I don't think you can assume that all skin cells have undergone the same
number of cell divisions. The dermal cells which give rise to the epidermis
may not all have undergone the same number of divisions. A possible reason
why not all cells reach the Hayflick number is that something else kills the
organism first like cancer, atherosclerosis, cardiac insufficiency or
infection.
Regards, Ron Baker, Ph.D.
Remember that matter is never created nor destroyed, it only changes form.
So you and your cells are made of matter and that matter will eventually be
recycled into the environment. Remember also that you started as one cell
from the union of two cells that had half the genetic material from each
parent, but most of the cytoplasm from the mother and almost none from your
father. So at one point in time, you were one cell. So let's start there.
That first cell can divide 50 times before it reaches its Hayflick limit and
will be destroyed. This is determined by little "caps" on the ends of the
chromosomes called telomeres. Telomeric DNA protects the genetic material
that codes for traits from being dropped off the ends of your chromosomes
each time they divide. It is noncoding DNA that is repetitive.
The first
approximately 32 cells that are produced are called embryonic stem cells
because they haven't started to specialize yet. ESC's have an enzyme called
telomerase that allows them to repair their telomeres so they don't shorten.
(Your germline cells, ie. gametes, also don't lose their telomeres as you
age). Once the cells begin to specialize they turn off certain genes
permanently and become a cell of a certain kind. Each cell type has stem
cells that replace lost cells. At this point, these stem cells can only
replace cells of the same type-that is why embryonic stem cells are so
highly prized by researchers) Each cell type divides at different rates.
Your example, skin cells, divide very rapidly to replace cells that are
lost. The cells in your muscles and certain nervous tissue cells don't ever
divide again. So the first cell divides and creates 2-they each divide and
create 4 and then 8 and then 32 and so on. The first cell can divide 50
times, but the new ones can each also divide 50 times. Eventually, it is
hypothesized, there will reach a point where all the cells are getting older
and the original cells aren't replacing the dead ones as fast or there
aren't as many around to replace the dead ones.
This is what happens as we
age. Eventually, you, a walking mass of cells, will die and all of your
cells will die with you. The matter that makes up your cells will then be
recycled and made available to other life that comes after you.
vanhoeck
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Update: June 2012
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