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I’m reading Molecular Biology of the Cell by Alberts et. Al and at one point the authors mention the following:

We saw earlier that synthesis of the lagging strand at a replication fork must occur discontinuously through a backstitching mechanism that produces short DNA fragments. This mechanism encounters a special problem when the replication fork reaches an end of a linear chromosome. The final RNA primer synthesized on the lagging-strand template cannot be replaced by DNA because there is no 3ʹ-OH end available for the repair polymerase. Without a mechanism to deal with this problem, DNA would be lost from the ends of all chromosomes each time a cell divides [emphasis mine].

They use this explanation as a motivation for why telomeres are needed. But I don’t quite understand it. Below is a diagram that they show earlier. It seems to me like the process shown can just as easily be done at the ends of chromosomes. In particular, if we imagine that the left-most primer has no DNA following it, it seems to me like polymerase can just do what it normally does to replicate the DNA until the end is reached. Where’s the problem? enter image description here

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    $\begingroup$ How is the 3’ end of the orange strand replicated? $\endgroup$
    – canadianer
    May 14 at 12:31
  • $\begingroup$ @canadianer I see. But what is meant by "there is no 3ʹ-OH end available for the repair polymerase"? $\endgroup$ May 14 at 19:55
  • $\begingroup$ It means that DNA polymerase only extends existing chains and doesn’t create new chains de novo. In other words, it needs a primer. $\endgroup$
    – canadianer
    May 16 at 11:41
  • $\begingroup$ @canadianer right but why is there no 3’-OH end available? $\endgroup$ 7 hours ago
  • $\begingroup$ This just means that because the final part of the 'reading' is in the 3'-OH to 5'-OH direction (R to L in the orange lagging strand), the last available component for synthesis of an RNA primer by the DNA primase is from that latter 5'-OH towards the 3'-OH end of the strand. So, the final 3'-OH component of that strand cannot be synthesised: there is no 5'-OH component to its right in the diagram. Telomeres perform other functions; nature is fantastically efficient. The whole thing is an immensely beautiful wave (interference) dynamic in 3-D, wrapped around the tetrahedral form of histones. $\endgroup$
    – jeremiah
    6 hours ago

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