2006-07-21 11:33:32 · 8 answers · asked by icky_sticky_008 1 in Science & Mathematics ➔ Biology
when you die
2006-07-21 11:36:47 · answer #1 · answered by calli 2 · 1⤊ 0⤋
Normal cells have a limited number of divisions. I'm not too sure about the exact number, but I think it is 60-70. So, a cell divides so many times and is then destroyed by programmed cell death.
The regulation of the number of cell divisions is done by a portion of the chromosomes known as th telomere. A telomere is a sequence of repeating nucleotides present at the tip of a chromosome. With each division of a cell, the length of the telomere gets decreased by about 200 basepairs. So, when the cell loses all of its telomeric DNA, it stops dividing. This is because, the enzyme telomerase is no longer produced, when the Telomeric DNA is all gone.
Cancer cells, in some cases, find a mechanism to have constitutive telomerase producion, thus bypassing the restriction imposed on the number of divisions of the cell. They, thus, become immortal.
2006-07-21 22:13:10 · answer #2 · answered by Anonymous · 0⤊ 0⤋
I'm guessing that you're not looking for the obvious answer of "death" but are asking about when a specific cell stops dividing, right?
Attached to the chromosome is a structure called a telomere, which allows the DNA to replicate all the way to the end of the chain. But in every generation of a cell line, the telomere gets shorter. When it gets too short, the cell (rather the DNA in the cell) can no longer divide without losing genetic information and it stops. I've provided the Wikipedia citation--it's pretty interesting stuff and I encourage you to read more about it.
2006-07-21 11:55:21 · answer #3 · answered by Pepper 4 · 0⤊ 0⤋
The process of aging at a cellular level in cells is "senescence" and is essentially a genetically encoded lifespan. If you remove some skin cells from an average adult and grow them in a culture dish, they can be passaged (split) about 40 times. If you remove them from an elderly person, they will divide significantly fewer times. Culturing cells from mice ( lifespan 2yrs) can be passaged fewer times and cells from a galapagos tortoise (lifespan of 200yrs) can be passaged many many many times. So, the cells have a internal clock for self-renewal and it is genetically encoded.
What the guy said about telomeres is correct. The chromosomes in our cells have "caps" of repetitive G/C-rich DNA sequences that intiate duplication of the DNA ends during cells division (known as priming). At every cell division this sequence gets shorter and shorter, until eventually they get so short that the end cannot be copied. The chromosome end is thought to become destabilized and the cell falls into genetic catastrophe.
Cancer cells and stem cells are two notable exceptions to this rule. Both of these cells can theoretically be passaged an infinite amount of times, and are therefore referred to as "immortal" (ie cancer cells become "immortalized"). One reason for this, is that there is an enzyme known as telomerase that is absent from most somatic cells and present in cancer/stem cells that enlongates these telomeres. One mutation that occurs early in the evolution of a cancer case, is that telomerase comes "switched back on" and the cells become immortalized. Therefore, turning telomerase on exogenously (on all the times, decoupled from its normal gene regulation) in animals will not only elongate lifespan, but result in increase in tumorogenesis in those individuals.
2006-07-21 12:41:29 · answer #4 · answered by Entropy 2 · 0⤊ 0⤋
cells are constantly being formed and are continuously dying. therefore the cycle never ends until you die
2006-07-21 11:37:53 · answer #5 · answered by cuckoo meister 3 · 0⤊ 0⤋
immediately after you die.
2006-07-21 11:37:31 · answer #6 · answered by Sandra G 2 · 0⤊ 0⤋
When you're are dead.
2006-07-21 11:37:03 · answer #7 · answered by Anonymous · 0⤊ 0⤋
When you die.
2006-07-21 11:36:47 · answer #8 · answered by Meg...Out of Hybernation 6 · 0⤊ 0⤋