senescence

Meaning: Cellular senescence is a stable, irreversible arrest of the cell cycle in which a cell stays alive but no longer divides. In most somatic cells, DNA polymerase cannot fully copy the 3′ end of a linear chromosome, so telomeres shorten with each division. Critically short telomeres trigger a DNA-damage response that halts the cycle. DNA damage and oncogene activation can also induce senescence, which acts as a barrier to tumor growth. At the organism level, senescence means aging; in plants, programmed deterioration such as leaf yellowing.

On the MCAT: A passage shows cultured fibroblasts that stop dividing after a limited number of doublings (the Hayflick limit). Normal fibroblasts have little or no telomerase, so adding it lets them divide more. Germ cells, stem cells and most cancer cells keep telomerase and divide far longer; blocking it in such cells brings senescence sooner. A growth curve that plateaus while cells stay alive points to senescence.

Common confusion: Senescent cells are not dead, so senescence is not apoptosis or necrosis. It also differs from G0 quiescence, where cells can re-enter the cycle with growth signals. Telomeres are noncoding repeats, so shortening does not delete genes.