Practice question

Hayflick limit defines finite number of times normal somatic cells can divide before entering senescence, discovered by Leonard Hayflick and Paul Moorhead in 1961 using human fetal lung fibroblasts WI-38. They observed progressive decline in doubling capability leading to arrest after approximately 40-60 population doublings, refuting earlier claims by Alexis Carrel that cultured cells were immortal. Molecular basis is incomplete replication of linear chromosome ends due to end-replication problem: DNA polymerase cannot fully replicate 3 prime overhang, causing loss of 50-200 base pairs of telomeric TTAGGG repeats per division. When telomeres become critically short, shelterin proteins like TRF2 no longer protect ends, they are recognized as double-strand breaks activating ATM/ATR kinases, stabilizing p53, inducing cell cycle inhibitors p21 and p16, leading to permanent G1 exit. Cancer cells circumvent limit via reactivation of telomerase hTERT or alternative lengthening telomeres mechanism maintaining telomere length indefinitely. Measurement of telomere length by terminal restriction fragment analysis or qPCR confirms progressive loss across passages. This knowledge strengthens laboratory safety, protocol reproducibility, and regulatory compliance critical for translational research and clinical applications, ensuring reliable data and workforce protection.