Each eukaryotic chromosome consists of a single molecule of DNA associated with a variety of proteins.
The telomeres of humans consist of as many as 2000 repeats of the sequence 5 GGTTAG 3.
In the figure on the right, the horizontal black arrows show the direction that the replication forks are moving. Wherever the replication fork of a strand is moving towards the 3 end, the newly-synthesized DNA (red) begins as Okazaki fragments (red dashes).
This continues until close to the end of the chromosome. Then, as the replication fork nears the end of the DNA, there is no longer enough template to continue forming Okazaki fragments. So the 5 end of each newly-synthesized strand cannot be completed. Thus each of the daughter chromosomes will have a shortened telomere.
It is estimated that human telomeres lose about 100 base pairs from their telomeric DNA at each mitosis.
This represents about 16 GGTTAG repeats. At this rate, after 125 mitotic divisions, the telomeres would be completely gone.
Is this why normal somatic cells are limited in the number of mitotic divisions before they die out?
Telomeres are important so their steady shrinking with each mitosis might impose a finite life span on cells. This, in fact, is the case. Normal (non-cancerous) cells do not grow indefinitely when placed in culture.
Cells removed from a newborn infant and placed in culture will go on to divide almost 100 times. Well before the end, however, their rate of mitosis declines (to less than once every two weeks). Were my cells to be cultured (I am 81 years old), they would manage only a couple of dozen mitoses before they ceased dividing and died out.
This phenomenon is called replicative senescence [More]. Could shrinkage of telomeres be a clock that determines the longevity of a cell lineage and thus is responsible for replicative senescence?
It turns out that these cells are able to maintain the length of their telomeres. They do so with the aid of an enzyme telomerase.
Telomerase is an enzyme that adds telomere repeat sequences to the 3 end of DNA strands. By lengthening this strand, DNA polymerase is able to complete the synthesis of the "incomplete ends" of the opposite strand.
When normal somatic cells are transformed in the laboratory with DNA expressing high levels of telomerase, they continue to divide by mitosis long after replicative senescence should have set in. And they do so without any further shortening of their telomeres. This remarkable demonstration (reported by Bodnar et. al. in the 16 January 1998 issue of Science) provides the most compelling evidence yet that telomerase and maintenance of telomere length are the key to cell immortality.
What if their cells could be transformed not only with the therapeutic gene but also with an active telomerase gene? This should give them an unlimited life span.
The now-famous sheep Dolly was cloned using a nucleus taken from an adult sheep cell that had been growing in culture. The cell donor was 6 years old, and its cells had been growing in culture for several weeks.
What about Dollys telomeres? Analysis of telomere length in Dollys cells reveals that they were only 80% as long as in a normal one-year-old sheep. Not surprising, since the nucleus that created Dolly had been deprived of telomerase for many generations.
But her short telomeres do add another question to the debate about cloning mammals from adult cells.
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