Telomeres shorten in part because of the ''end replication problem'' that is exhibited during DNA replication in eukaryotes only.
Because DNA replication does not begin at either end of the DNA strand, but starts in the center, and considering that all DNA polymerases that have been discovered move in the 5' to 3' direction, one finds a leading and a lagging strand on the DNA molecule being replicated.
On the leading strand, DNA polymerase can make a complementary DNA strand without any difficulty because it goes from 5' to 3'. However, there is a problem going in the other direction on the lagging strand.
To counter this, short sequences of RNA acting as primers attach to the lagging strand a short distance ahead of where the initiation site was.
The DNA polymerase can start replication at that point and go to the end of the initiation site.
This causes the formation of Okazaki fragments. More RNA primers attach further on the DNA strand and DNA polymerase comes along and continues to make a new DNA strand.
Eventually, the last RNA primer attaches, and DNA polymerase, RNA nuclease and DNA ligase come along to convert the RNA (of the primers) to DNA and to seal the gaps in between the Okazaki fragments. But in order to change RNA to DNA, there must be another DNA strand in front of the RNA primer.
This happens at all the sites of the lagging strand, but it does not happen at the end where the last RNA primer is attached. Ultimately, that RNA is destroyed by enzymes that degrade any RNA left on the DNA. Thus, a section of the telomere is lost during each cycle of replication at the 5' end of the lagging strand.
However, in vitro studies (von Zglinicki et al. 1995, 2000) have shown that telomeres are highly susceptible to oxidative stress.
Telomere shortening due to free radicals explains the difference between the estimated loss per division because of the end-replication problem (ca. 20 bp) and actual telomere shortening rates (50-100 bp), and has a greater absolute impact on telomere length than shortening caused by the end-replication problem.
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