On the other hand, Leonard Hayflick first described the Hayflick limit, a phenomenon where normal human cells stop dividing after 40 to 60 passages . The "replication end problem" causes telomeres to shorten during cell division. As a result, the majority of eukaryotes rely on telomerase to maintain chromosomes in dividing cells, as excessively short telomeres impede proliferation and induce senescence . Telomerase preferentially lengthens the shortest telomeres, but it does not act on every telomere in every cell cycle . The number of repeats added by telomerase is a controlled phenomenon that maintains a specific equilibrium. Any disruption in this equilibrium results in a variety of telomere-related diseases .