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The end of a chromosome, which consists of tandemly repeated short sequences of DNA that perform the function of ensuring that each cycle of DNA replication has been completed. Each time a cell divides some sequences of the telomere are lost; eventually (after 60–100 divisions in an average cell) the cell dies (the telomere theory of ageing (see senescence) is based on this phenomenon). Replication of telomeres is directed by telomerase, an enzyme consisting of RNA and protein that is inactive in normal cells of higher organisms. Its presence in tumours is linked to the uncontrolled multiplication of cancer cells. In yeasts, protists, and germ cells of higher organisms, telomerase is normally active, so that the chromosomes are kept at their appropriate lengths.

Subjects: Genetics and Genomics.

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