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Name: David
Status: Educator
Grade:  9-12
Location: PA
Country: United States
Date: May 2005

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?

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.


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