AS.020.305 Midterm: cell bio final study sheet (dragged) 8
Document Summary
Human*cells*have*a*built*in*limitation*on*the*number*of*times*they*can*divide: fibroblasts taken from normal human tissue, for example, go through only about 25-50 population doublings when cultured in a standard mitogenic medium. Toward the end of this time, proliferation slows down and finally halts, and the cells enter a non-dividing state from which they never recover. Telomerase also promotes the formation of protein cap structures that protect the chromosome ends. Because human fibroblasts, and many other human somatic cells, are deficient in telomerase, their telomeres become shorter with every cell division, and their protective protein caps progressively deteriorate. Eventually, the exposed chromosome ends are sensed as dna damage, which activates a p53-dependent cell-cycle arrest that resembles the arrest caused by other types of. This strategy provides a single window of opportunity for pre-replicative complexes to form (g1phase,when cdk activity is low) and a second window for them to be activated and subsequently disassembled (sphase, when cdk activity is high).
