Editing Telomere (section)

== Lengthening ==
[[File:Hayflick Limit (1).svg|thumb|The average cell will divide between 50 and 70 times before cell death. As the cell divides the telomeres on the end of the chromosome get smaller. The [[Hayflick limit]] is the theoretical limit to the number of times a cell may divide until the telomere becomes so short that division is inhibited and the cell enters senescence.]]
The phenomenon of limited cellular division was first observed by [[Leonard Hayflick]], and is now referred to as the [[Hayflick limit]].<ref name="pmid13905659">{{cite journal | vauthors = Hayflick L, Moorhead PS | title = The serial cultivation of human diploid cell strains | journal = Experimental Cell Research | volume = 25 | issue = 3 | pages = 585–621 | date = December 1961 | pmid = 13905658 | doi = 10.1016/0014-4827(61)90192-6 }}</ref><ref>{{cite journal | vauthors = Hayflick L | title = The limited in vitro lifetime of human diploid cell strains | journal = Experimental Cell Research | volume = 37 | issue = 3 | pages = 614–36 | date = March 1965 | pmid = 14315085 | doi = 10.1016/0014-4827(65)90211-9 }}</ref> Significant discoveries were subsequently made by a group of scientists organized at [[Geron Corporation]] by Geron's founder [[Michael D. West]], that tied telomere shortening with the Hayflick limit.<ref name="pmid7544491">{{cite journal | vauthors = Feng J, Funk WD, Wang SS, Weinrich SL, Avilion AA, Chiu CP, Adams RR, Chang E, Allsopp RC, Yu J | display-authors = 6 | title = The RNA component of human telomerase | journal = Science | volume = 269 | issue = 5228 | pages = 1236–41 | date = September 1995 | pmid = 7544491 | doi = 10.1126/science.7544491 | bibcode = 1995Sci...269.1236F | s2cid = 9440710 }}</ref> The cloning of the catalytic component of telomerase enabled experiments to test whether the expression of telomerase at levels sufficient to prevent telomere shortening was capable of immortalizing human cells. Telomerase was demonstrated in a 1998 publication in ''[[Science (journal)|Science]]'' to be capable of extending cell lifespan, and now is well-recognized as capable of immortalizing human somatic cells.<ref>{{cite journal | vauthors = Bodnar AG, Ouellette M, Frolkis M, Holt SE, Chiu CP, Morin GB, Harley CB, Shay JW, Lichtsteiner S, Wright WE | s2cid = 35667874 | display-authors = 6 | title = Extension of life-span by introduction of telomerase into normal human cells | journal = Science | volume = 279 | issue = 5349 | pages = 349–52 | date = January 1998 | pmid = 9454332 | doi = 10.1126/science.279.5349.349 | bibcode = 1998Sci...279..349B }}</ref>

Two studies on long-lived [[seabird]]s demonstrate that the role of telomeres is far from being understood. In 2003, scientists observed that the telomeres of [[Leach's storm-petrel]] (''Oceanodroma leucorhoa'') seem to lengthen with chronological age, the first observed instance of such behaviour of telomeres.<ref>{{cite journal | vauthors = Nakagawa S, Gemmell NJ, Burke T | title = Measuring vertebrate telomeres: applications and limitations | journal = Molecular Ecology | volume = 13 | issue = 9 | pages = 2523–33 | date = September 2004 | pmid = 15315667 | doi = 10.1111/j.1365-294X.2004.02291.x | bibcode = 2004MolEc..13.2523N | s2cid = 13841086 | url = http://eprints.whiterose.ac.uk/353/1/burket16.pdf }}</ref>

A study reported that telomere length of different mammalian species correlates inversely rather than directly with lifespan, and  concluded that the contribution of telomere length to lifespan remains controversial.<ref>{{cite journal | vauthors = Gomes NM, Ryder OA, Houck ML, Charter SJ, Walker W, Forsyth NR, Austad SN, Venditti C, Pagel M, Shay JW, Wright WE | display-authors = 6 | title = Comparative biology of mammalian telomeres: hypotheses on ancestral states and the roles of telomeres in longevity determination | journal = Aging Cell | volume = 10 | issue = 5 | pages = 761–8 | date = October 2011 | pmid = 21518243 | pmc = 3387546 | doi = 10.1111/j.1474-9726.2011.00718.x }}</ref> There is little evidence that, in humans, telomere length is a significant biomarker of normal aging with respect to important cognitive and physical abilities.<ref>{{cite journal | vauthors = Harris SE, Martin-Ruiz C, von Zglinicki T, Starr JM, Deary IJ | s2cid = 10309423 | title = Telomere length and aging biomarkers in 70-year-olds: the Lothian Birth Cohort 1936 | journal = Neurobiology of Aging | volume = 33 | issue = 7 | pages = 1486.e3–8 | date = July 2012 | pmid = 21194798 | doi = 10.1016/j.neurobiolaging.2010.11.013 }}</ref>