Cellular Aging The End Replication Problem

James D. Watson, while investigating the replication pattern of linear molecules of T7 DNA, noted that leading strand replication in the 5' to 3' direction should proceed smoothly to the end of its template. However, lagging strand synthesis is unable to copy the parental strand in its entirety (Watson 1972). Lagging strand synthesis is initiated by RNA primers, which are extended to Okazaki fragments that are ligated together after removal of the initiating RNA. This mechanism does not allow for fill-in synthesis of the gap left by the most distal RNA primer, inevitably leaving the daughter strand shorter than the parental strand. The inability to fully replicate the template, leading to terminal sequence loss during each replication cycle, is termed the end replication problem.

Following an independent approach, Alexey Olovnikov at the Russian Academy of Sciences published "A Theory of Marginotomy" (Olovnikov 1973). His hypothesis holds that the limited doubling potential of primary somatic cells can be explained by terminal sequence loss of the daughter DNA strand. To buffer the progressive erosion, Olovnikov proposed that so-called telogenes - vital genes without any coding information - are located at opposite ends of the linear chromosome. Once they are lost due to the end replication problem, the coding chromosomal DNA is no longer protected, replication ceases, and the cells enter what Hayflick called Phase 3 (Hayflick and Moorhead 1961).

Now, of course, we know that the ends of chromosomes do not consist of the hypothesized telogenes, but are specialized structures, called telomeres, consisting of G-rich DNA repeats and proteins that bind to these repeats. Olovnikov was partially correct in suggesting that they fulfill a buffer function, and Watson had precisely predicted the reason for terminal sequence loss. However, at the time it was unclear that telomeres and replication-associated telomere loss represent the counting mechanism that monitors and limits the proliferative potential of primary cells as first discovered by Hayflick. At this point in time we can define the Hayflick limit as a function of initial telomere length and rate of terminal sequence loss per cell division, rendering the telomere the genetic clock that measures cellular replicative aging (Fig. 1.1).

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