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How is the primer present in the daughter, leading strand replaced? I've circled the primer in question.

enter image description here

I don't see how this primer can be replaced by DNA polymerase; there is no free 3' end for DNA pol to act on.

My book, however, says there is no telomere problem with the leading strand.

Other sources say there is a telomere problem.

So what is it?

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Replication doesn't start at the very end of a chromosome, so there is no problem with leading strand synthesis. It's probably easiest to see if the other half of the image was there:

enter image description here

Forgive the lion; plain white paper seems to be in short supply. Also my writing. Too much tea this morning. Hopefully you can see that leading strand synthesis can continue right to of the end of the chromosome as the fork progresses. The leading primers at the replication origin can be replaced once the first Okazaki fragment from the other replication fork is synthesized.

There is, however, a problem, aptly termed the end replication problem, with lagging stand synthesis:

enter image description here

The dots just mean the chromosome continues in that direction. Important to notice is the lagging strand RNA primer at the end of the chromosome. When this is removed, there is apparently no way to synthesize DNA there. Repeated replication cycles progressively shorten the chromosome and cause the Hayflick limit to cell division (the number of times a cell can divide before dying). To solve this problem, some cell types express an enzyme called telomerase. This is an RNA directed DNA polymerase that adds deoxyribonucleotide repeats to the 3' end of a DNA strand, using its own internal RNA template, to prevent information loss.

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  • $\begingroup$ Actually, the lion is positioned pretty well to be a topoisomerase. Just about to cut… $\endgroup$ – canadianer Oct 18 '15 at 20:08
  • $\begingroup$ thank you so much for this helpful answer. I think I see it now. You are saying that the primers for leading strand synthesis are always sandwiched between DNA segments, while one primer for lagging strand synthesis is unfortunately found alone and un-sandwiched? And that it is these sandwiched primers, that, upon removal, can be replaced by DNA because there is a free -OH end on one side of the sandwich? $\endgroup$ – Dissenter Oct 18 '15 at 22:52
  • $\begingroup$ @Dissenter You're welcome and yes I think you've got it. $\endgroup$ – canadianer Oct 18 '15 at 22:56
  • $\begingroup$ Cells need not die, the will enter into senescence when their telomeres get too short. Also, this may confuse the issue slightly, but the 5' end, though synthesized to the end of the chromosome, is processed to create a 3' overhang. Both ends need a 3'overhang in order to strand invade the telomere to form the T-loops at their respective ends of the chromosome. If the T-loop does not form, this can lead to chromosomal translocations or it can cause the cellular repair mechanisms to recognize the ends as DS breaks and that can trigger the apoptotic pathway. $\endgroup$ – AMR Oct 19 '15 at 0:43
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I think the confusion is coming from the way this picture is drawn. The leading strand does not begin at the end of a chromosome, there is in fact more DNA to the right of your picture that is not shown. This picture is showing the left half of a replication bubble.

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I think I understand this. Replication will occur bidirectionally from the the origin inside the replication bubble. If we focus just on 1 parent strand being replicated, on one side of the origin, the daughter stand replication will be continuous. However, on the other side of the same parent strand, the replication will require Okazaki fragments.

So for the same daughter strand, on one side of the replication origin, it will be leading, and on the other side of the origin it will be lagging!

Back to the telomere shortening (end replication problem), this will mean that each daughter strand will end up too short on it's 5' end after the RNA primer (Okazaki fragment) is removed.

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    $\begingroup$ Welcome to Bio. We encourage the use of references and sources to allow people to read the backgrounds of your answer. $\endgroup$ – AliceD Mar 17 '16 at 12:02

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