DNA Structure and Replication

ID #2233

I'm having trouble figuring out how the telomerase works. From the book readings, I understand why it's needed, but how is it working? In one of the figures, they show the end of the template strand with bases TTG hanging off at the 3' end. Then a telomerase comes in and it just happens to have the complementary AAC sequence? Are there many different telomerases and this one just happened to be complementary? Also, once it adds all those bases onto the end of the template, it looks like it's leaving the lagging end unchanged? what's the point of making the template strand longer but not doing anything to the lagging strand?


The ends of telomeres are composed of short repeated sequences of DNA. The RNA template carried by telomerase is complementary to part of the repeated sequence, so it can anneal to the telomere and use its RNA template to synthesize DNA bases, thus lengthening the template strand (shown in Figure 19-16 of your Becker textbook). I don't know if the repeated sequences are the same in all telomeres. If I had to guess, I would guess that they are and cells only make one type of telomerase, but that is only conjecture on my part. 

What I call the "lagging strand template" is lengthened by telomerase. As shown in Figure 19-16, when the primer at the very 5' end of the lagging strand is removed there is no free 3'OH group to replace the RNA bases with DNA, so there is an overhang on the top strand. By lengthening this "top (template) strand," there is more room for the standard DNA replication machinery to add a primer and DNA bases to fill the missing DNA bases. 

I know this seems confusing, but you can try drawing it out if you are still confused, and that may help you to see how telomerase solves what would be a very serious problem for eukaryotic cells. 

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