Telomeres, the repeated sequence found at the ends of chromosomes, shorten in many normal human cells with increased cell divisions. Statistically, older people have shorter telomeres in their skin and blood cells than do younger people.53,54 In the animal kingdom, though, long-lived species often have shorter telomeres than do short-lived species, indicating that telomere length probably does not determine life span.55,56,57 Solid scientific evidence has shown that telomere length plays a role in determining cellular life span in normal human fibroblasts and some other normal cell types.58 Increasing the number of times a cell can divide, however, may predispose cells to tumor formation.59,60 Thus, although telomere shortening may play a role in limiting cellular life span, there is no evidence that telomere shortening plays a role in the determination of human longevity.

53Harley CB, Futcher AB, Greider CW. Telomeres shorten during ageing of human fibroblasts. Nature 1990;345:458-460.

54Vaziri H, Dragowska W, Allsopp RC, Thomas TE, Harley CB, Lansdorp PM. Evidence for a mitotic clock in human hematopoietic stem cells: loss of telomeric DNA with age. Proc Natl Acad Sci USA 1994;91:9857-9860.

55Hemann MT, Greider CW. Wild-derived inbred mouse strains have short telomeres. Nucleic Acids Res 2000;28:4474-4478.

56Kakuo S, Asaoka K, Ide T. Human is a unique species among primates in terms of telomere length. Biochem Biophys Res Commun. 1999;263(2):308-14.

57Holliday R. Endless quest. Bioessays. 1996;18(1):3-5.

58Bodnar AG, Ouellette M, Frolkis M, et al. Extension of life span by introduction of telomerase into normal human cells. Science. 1998;279:349-352.

59Wang J, Hannon GJ, Beach DH. Risky immortalization by telomerase. Nature 2000;405:755-756.

60de Lange T, Jacks T. For better or worse? Telomerase inhibition and cancer. Cell 1999;98:273-275.

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