ABSTRACT
Embryonic and cancer stem cells have been observed to replicate without limit when cultured in a medium with suitable nourishment and growth factors. In contrast, other cells with the ability to divide stop growing after 40-60 cycles of division, regardless of the contents of their medium. To control cell division, cells communicate with each other via growth factors, which as the name implies, signal other cells to divide and grow. Cell proliferation produces two cells from one. It involves an increase in cell size followed by cell division. The observation of tumor cells with unlimited capacity to divide suggests the existence of a mechanism to stabilize telomere length. There is an enzyme, telomerase reverse transcriptase that repairs chromosomes by adding back TTAGGG repeats. Apoptosis forms a barrier to tumor development and is a key contributor to the mechanism through which chemotherapy and radiation treatments work.
