In all mammals, in most animals, in some fungi, and in some protozoa, chromosome ends {telomere}| have TTAGGG repeated 2000 times. In plants, TTTAGGG repeats. In ciliates, TTTTGGGG or TTGGGG repeats. Telomeres are nucleoprotein complexes at chromosome ends and can have caps.
Chromosome copying starts just inside chromosome ends, so copies are shorter than copied chromosome. Telomeres decrease in length with each replication. Over a lifetime, humans make 150 copies and shorten telomeres by half. Telomerase enzyme restores telomere length in sperm and egg cells, and in cancer cells, using RNA as template for reverse transcriptase. Telomerase prevents shortening in human immune system, hemopoietic system, germline cells, embryonic cells, stem cells, skin cells, intestinal-lining cells, hair-follicle cells, and cancer cells.
After telomeres reach threshold length, cells can have senescence. Perhaps, telomere shortening ends at age 40, related to cell-turnover reduction.
Rodent cells do not have telomere shortening, stop dividing after 10 to 15 doublings, have telomerase, and grow indefinitely in culture.
Biological Sciences>Genetics>Chromosome
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Date Modified: 2022.0224