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A telomere is a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos (''τέλος'') 'end' and merοs (''μέρος'', root: ''μερ-'') 'part.' For vertebrates, the sequence of nucleotides in telomeres is TTAGGG. This sequence of TTAGGG is repeated approximately 2,500 times in humans.〔Sadava, D., Hillis, D., Heller, C., & Berenbaum, M. (2011). ''Life: The science of biology''. (9th ed.) Sunderland, MA: Sinauer Associates Inc.〕 During chromosome replication, the enzymes that duplicate DNA cannot continue their duplication all the way to the end of a chromosome, so in each duplication the end of the chromosome is shortened〔(AtGoogleTalks, August 20, 2008 Molecular biologist Elizabeth Blackburn )〕 (this is because the synthesis of Okazaki fragments requires RNA primers attaching ahead on the lagging strand). The telomeres are disposable buffers at the ends of chromosomes which are truncated during cell division; their presence protects the genes before them on the chromosome from being truncated instead. Over time, due to each cell division, the telomere ends become shorter.〔Passarge, Eberhard. Color atlas of genetics, 2007.〕 They are replenished by an enzyme, telomerase reverse transcriptase. ==Discovery== In the early 1970s, Russian theorist Alexei Olovnikov first recognized that chromosomes could not completely replicate their ends. Building on this, and to accommodate Leonard Hayflick's idea of limited somatic cell division, Olovnikov suggested that DNA sequences are lost every time a cell/DNA replicates until the loss reaches a critical level, at which point cell division ends. However, Olovnikov's prediction was not widely known except by a handful of researchers studying cellular aging and immortalization. In 1975–1977, Elizabeth Blackburn, working as a postdoctoral fellow at Yale University with Joseph Gall, discovered the unusual nature of telomeres, with their simple repeated DNA sequences composing chromosome ends. Blackburn, Carol Greider, and Jack Szostak were awarded the 2009 Nobel Prize in Physiology or Medicine for the discovery of how chromosomes are protected by telomeres and the enzyme telomerase. Nevertheless, in the 1970s there was no recognition that the telomere-shortening mechanism normally limits cells to a fixed number of divisions, and no animal study suggesting that this could be responsible for aging on the cellular level and sets a limit on lifespans.〔Harrison's Principles of Internal Medicine, Ch. 69, Cancer cell biology and angiogenesis, Robert G. Fenton and Dan L. Longo, p. 454.〕 It remained for a privately funded collaboration from biotechnology company Geron to isolate the genes for the RNA and protein component of human telomerase in order to establish the role of telomere shortening in cellular aging and telomerase reactivation in cell immortalization. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「telomere」の詳細全文を読む スポンサード リンク
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