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Telomere Function and Chromosomes
The DNA inside the chromosomes of eukaryotic organisms is linear, that is it has two ends. This is in contrast to most, but not all prokaryotic DNA which is circular. Telomeres play a vital role in the life of the cell, protecting chromosomes from molecular attack, and preventing DNA strands from sticking to each other. Molecular biologist Elizabeth Blackburn has compared telomeres to the tips of shoelaces that prevent them from fraying.
Telomeres are 'written' in the language of DNA and their length varies between species. Though they consist of nucleotides, they do not code for any proteins. Telomeres consist of repetitive sequences of six nucleotides - TTAGGG.
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Telomeres and Cell Division
Telomeres are finite. With every new cell division telomere length shortens. During cell division the chromosomes are replicated, faithfully producing copies of the DNA strands, albeit with mutations sometimes. However, the telomeres at the tips of the chromosomes cannot be reproduced faithfully, and this results in daughter chromosomes that are slightly shorter than the original. This puts a natural limit on the lifespan of the cell as eventually after repeated divisions the telomeres are too short to provide protection.
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Telomeres and Ageing
Cell senescence is linked to telomere length. When telomeres are too short the cell dies. But could the process be reversed? Prevent telomere shortening or increase telomere length and you have an eternal cell, and hey presto! we get to live forever. Some scientists are giving serious thought to this, and a small handful believe it may one day be possible; either by gene therapy or by activating telomerase, the enzyme that maintains telomere length, and minimises the amount that is lost during cell division.
However, this has not been observed in humans, though the relationship between lengthy telomeres and longer life spans has been demonstrated in the roundworm C. elegans. A team of researchers writing in Nature Genetics in 2004 reported that worms with longer telomeres lived on average 20 per cent longer than normal worms.
But there could be a price to pay if we tinker with telomere length - cancer. Cancerous cells can continue to divide indefinitely by maintaining telomere length. The enzyme telomerase is not active in most differentiated cells, however, it is active in stem cells, germ cells, and tumour cells, and adds bases to telomeres. Lengthening these structures could promote tumour genesis and prevent malfunctioning cells from dying. Perhaps nature prefers us to shuffle off this mortal coil when we reach our natural sell-by date.
Although it seems highly unlikely that immortality will ever be achieved, telomeres could possibly be manipulated by drugs or gene therapy to minimise the effects of ageing and to increase lifespans by several years.
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Joeng KS, Song EJ, Lee KJ, Lee J (2004). "Long lifespan in worms with long telomeric DNA". Nature Genetics 36 (6): 607–11.