Telomeres Can Be Maintained Without Telomerase

In 1972, James Watson noted that DNA polymerase, the enzyme responsible for DNA replication, could not copy linear DNA all the way to the tip. This would result in a steady shortening of chromosomes unless there was some mechanism for protecting the integrity and length of each chromosome's telomeres.(1)Telomeres are structures located at the ends of eukaryotic chromosomes and consist of short repeated DNA sequences. In humans, this sequence is TTAGGG, and is repeated about 2,500 times. When the telomeres repeat, they bind special proteins that stabilize the chromosome ends. Each time a cell divides, its DNA must be copied through a process called DNA replication.

After each instance of DNA replication, 50-200 base pairs of telomeric DNA can be lost. After 20-30 cell divisions, the chromosomes become unstable and are no longer able to take part in cell division resulting in cellular death.(2)In contrast to Watson's observations, stem cells in bone marrow and reproductive cells maintain their telomeric DNA indefinitely. Comparisons of telomeric DNA in several different species provided evidence that all cells have the telomere structure. Some cells can maintain the telomere better than others across species and within the same organism.

In 1984, Blackburn and Greider discovered an enzyme they dubbed "telomerase". The enzyme was able to add the repeat sequences, one at a time, to the ends of chromosome telomeres. Cells that have active telomerase enzymes appear to live indefinitely thus staving off cellular death.(1) 

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