Senescence is the gradual and progressive slowing of cellular activity, including cell division, that occurs with aging. Cells lose the ability to divide over time, a phenomenon researchers call Hayflick’s limit. The limit relates to the number of times the cell divides. During cell division, fibers of DNA called telomeres attach to the chromatids, facilitating their separation from each other to enter the new daughter cells. The process destroys the segment of the TELOMERE attached to the CHROMATID, causing the telomere to shorten with each cell division. When the cell runs out of telomeres it can no longer divide and it dies.
The exceptions are cancer cells, which seem to be nonsenescent. Cancer cells produce increased levels of an enzyme called telomerase, which acts to restore the length of the telomeres and gives cancer cells the ability to endlessly divide. Normal cells also produce telomerase but not in quantities sufficient to regenerate telomeres. Researchers do not know what causes cancer cells to increase the amount of telomerase they produce. As well, other factors are at play in the processes of senescence, which researchers continue to study.