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What are Telomeres?
Chromosomes are long strands of tightly coiled DNA. At the end of each strand is a telomere that protects the chromosome from molecular attack and stops it from sticking to other DNA strands. It acts like a bookend, and consists of repetitive sequences of six nucleotides - TTAGGG. If our cells did not have telomeres the main section of a chromosome would get shorter during cell division and key genes would be lost. Telomeres have been compared to the tips of shoelaces that prevent them from fraying and unravelling.
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What Happens During Cell Division?
Cell division is cell replication. Meiosis is the cell division associated with gamete formation and mitosis is the cell division that replaces old and dying cells. Somatic cells have a lifespan, a finite number of cell divisions; some estimates say between 60 and 100. Before a cell divides the DNA unzips and the genetic information is copied. However, not every nucleotide is replicated, and when the DNA strands are put back together telomere length has shortened. It shortens after every cell division.
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What Happens When Too Short?
When the telomeres are too short there is a danger that essential genes could be damaged and so the cell dies. It has reached its Hayflick limit, which is the number of times a cell can divide before it dies. It was named after Leonard Hayflick who discovered the process in 1965.
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Are They the Secret to Longevity? Can they Change the Longevity of an Organism?
The jury is well and truly out on this one, but there are some people who believe that telomeres hold the secret to longevity. Conceptually it's rather elegant. Cells die as telomeres shorten; find out a way of extending telomere length and cells live for longer, and potentially so do the bodies housing those cells. And there is actually a way of extending telomeres. Our cells make use of an enzyme known as telomerase to maintain and extend telomere length, but it only happens in certain types of cells, and they are germ cells and stem cells. Telomerase is not active in adult somatic cells.
Experiments have shown that round worms with longer telomeres live 20% longer than normal roundworms. so if we could find a way of turning on telomerase maybe we could extend the shelf-life of cells, and ourselves too.
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There's always an exception to every rule and it does not always follow that telomeres shorten with age. A study of 959 individuals from Sweden appeared in PLoS Genetics in February 2009 which showed that whilst age-related telomere decline occurred in some people, it was not found in all subjects. About one third of people in this study had stable or increased telomere length over a 10-year period. Such a phenomenon can usually be seen in cancer patients, where telomerase has been switched on in tumour cells, but there was no association here between telomere length and later cancer diagnosis.
Scientists are unsure as to what this means. Perhaps the people have some special anti-ageing mechanism? What is happening though is that telomeres and telomerase are attractive research targets in cancer studies and research programs that are trying to understand the ageing process and telomere biology.
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Nordfjäll K, Svenson U, Norrback K-F, Adolfsson R, Lenner P, Roos G. The individual blood cell telomere attrition rate is telomere length dependent, PLoS Genetics 2009, published February 13.