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Mega-telomere
A mega-telomere (also known as an ultra-long telomere or a class III telomere), is an extremely long telomere sequence that sits on the end of chromosomes and prevents the loss of genetic information during cell replication. Like regular telomeres, mega-telomeres are made of a repetitive sequence of DNA and associated proteins, and are located on the ends of chromosomes. However, mega-telomeres are substantially longer than regular telomeres, ranging in size from 50 kilobases to several megabases (for comparison, the normal length of vertebrate telomeres is usually between 10 and 20 kilobases).
Telomeres act like protective caps for the chromosome. During cell division, a cell will make copies of its DNA. The enzymes in the cell that are responsible for copying the DNA cannot copy the very ends of the chromosomes. This is sometimes called the "end replication problem". If a cell did not contain telomeres, genetic information from the DNA on the ends of chromosomes would be lost with each division. However, because chromosomes have telomeres or mega telomeres on their ends, repetitive non-essential sequences of DNA are lost instead (See: Telomere shortening). While the chromosomes in most eukaryotic organisms are capped with telomeres, mega-telomeres are only found in a few species, such as mice and some birds. The specific function of mega-telomeres in vertebrate cells is still unclear.
==Discovery==
Telomeric regions of DNA were first identified in the late 1970s (See: Discovery of Telomeric DNA). However, extremely long regions of telomere sequence were not recognized in vertebrates until over a decade later. These sequences, which ranged from 30 to 150 kilobases in size, were first identified in laboratory mice by David Kipling and Howard Cooke in 1990.〔
In 1994, extremely long telomeric regions were identified in chickens. Telomeric sequences ranging from 20 kilobases to several megabases have also been identified in several species of birds.〔 These large regions were termed "ultra-long" telomeres in the literature when they were identified using southern blotting〔 and "mega-telomeres" when identified by cytogenetic methods. The currently accepted terminology for these sequences is "mega-telomeres"〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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