THE AGING CLOCK. “Telomeric shortening, which occurs in several classes of dividing normal somatic cells, may be the replicometer that determines the number of times that a normal cell is able to divide. Once a critical or threshold number of telomeric TTAGGG repeats is reached, cells will then be unable to divide.”
http://protein.bio.msu.ru/biokhimiya/contents/v62/full/62111380.html