Cancer cells and embryonic stem cells r immortal. How they escape death ?

Monday, June 15, 2009

In 1961 ,Dr, Leonard Hayflick discovered that when cells are taken from the body and grown in tissue culture on a laboratory dish, there is a limit to how long the cell line lives . After about 50 (the num varies with the kind of tissue) divisions, the aging cell line dies. Its known as Hayflick limit. This Hayflick limit has puzzled developmental biologists for decades. What causes the cells, after so many successful divisions, to abruptly break down ? How does a cell know its time is up?

But scientist now know all the answers !!!

The secret to immortality proved to be a short tag of DNA attached on the tips of the cell's chromosomes,known as telomere. This segment of DNA plays a key role in cell division. The telomere provides a place for the cell's DNA-copying machinery to latch onto the chromosome when the time comes for the chromosomal DNA to be copied into daughter chromosomes. However, every time the machinery attaches, the short bit of the telomere where the machinery sits down on the DNA is not itself copied, so the telomere gets a little shorter each time the cell divides. When the telomere reaches a minimal length after some 50 divisions, the cell can no longer replicate its DNA and lapses into senescence.

So, why Cancer and ESC cells are immortal ?

All cells possess a gene, known as the telomerase gene, which can add DNA back to the tip of telomeres. In almost all cells this gene is turned off early in development. If somehow, this gene is turned on then the cell become a cancer cell.

Embryonic Stem Cells also have the telomerase activity. But when they differentiate into other specialized cells they lose their telomerase activity ,as well as immortality. A few, however, are set aside, protected from the influences that trigger differentiation and shutting down of the telomerase gene. Called germ line cells, these embryonic stem cells have a fully functional telomerase gene and continue to divide, producing eggs and sperm that in their turn produce more stem cells in the next generation.

reference :

http://txtwriter.com/onscience/index.html