Telomeres and Telomerase

Hi in this video, I'm gonna be talking about telomeres and telomerase. So the ends of chromosomes which are called telomeres are replicated a bit differently than the rest of the chromosome now in the leading strand? This actually isn't true because if we look at this image here, let me back up. So here's the image that we saw before. Here's our leading strand. So we have our primer here that tells DNA plum rice where to start? Didn't D. N. A. Plum rice comes on and it starts replicating and it'll replicate all the way to the end, Right? And then it eventually it'll just fall off when it ends Now. Um for the lagging strand, this is where telomeres and telomerase come into extreme importance because you have to have these primers that are required to create the Okazaki fragments. But what about the last primer? So for instance? So here this is replicating this way. It will say that, let's say that once this gets unwound there's a primer here and it replicates this way. Then we have a primer here and replicates this way. But now we're at the end, right. And there's no other place for this primer divine. It would have to bind here in order to replicate this way. But now we're just off the chromosome completely. There's no more chromosome left for the primer to bind to. So what happens to this region of the D. N. A, how does it get replicated without a primer. And so if this was just left alone, this is called the telomere here. Because this is the end. If this was just left alone and replicated normally and there was no like fixed this problem each time the chromosome or the lagging strand would shorten. And you can't have that, you can't have your chromosome shortening or else you're going to have significant medical problems and actually significant medical problems do come from improper handling of this situation. So what happens is there's this special enzyme called telomerase. Telomerase replicates these ends of the telomeres on the lagging strand or it helps do. And what it does is um uh telomerase um it adds nucleotides onto the three prime end of DNA molecules. So how it does this is because telomerase is an enzyme and inside the enzyme there's a short RNA molecule. So telomerase carries this RNA molecule around and this RNA molecule looks like this a A U c c c three prime to five prime. And what this is is actually a template for replication. And so what happens is telomerase binds on to the very end here. Right. I'll show an image of this. It binds on and it starts adding nucleotides using this RNA as a template. So this process is called reverse transcription because R N. A. Is being used as a template to synthesize more D. N. A. And this is called reverse transcription when RNA is used as a template. And so when telomerase comes on, it has this template and then it uses that template to add this nuclear type. So the compliment right? A. U. C. C. C. T. T. Um A G G G five prime, three prime three prime five prime. And it adds this repeatedly. I mean like hundreds potentially even thousands of times onto the end of the chromosome. And this prevents the telomeres from shortening because now that you're adding all of these repeats onto the end. So now we've extended the D. N. A. Strand for a very long time. So when you need this primer here D. N. A. Can now be replicated. And so when you x extending that allows you to replicate the end of the chromosome where otherwise you wouldn't have been able to. So um so that means that telomeres which are the ends of the chromosome contain a freaking crap ton of these T. T. A G G G repeats. Right? And it just contains so many of them to prevent the chromosome from shortening. So what happens like you said? And here's the end of the chromosome telomerase comes in. It uses this uh and it starts adding these sequences on then you can add an RNA primer. Let me back up and add an RNA primer. And that tells the DNA polymerase to attach here. They attach here and replicate. And so now it has it can replicate the end of the chromosome using these fragments and it does lengthen the chromosome adding these repetitive repeats. And that's OK lengthening lengthening the chromosome with you know, sort of not genetic information that's not in quoting anything. So just like random nucleotides, it's okay. It's not harming anything. It's not mean anything bad um to the chromosome or the individual. And so it's actually protecting the chromosome. So these repeats don't do anything bad. So it lengthens the chromosome and DNA polymerase can make sure that all the genetic material that is important is getting replicated. So this is super important. And like I said before, there are some serious medical conditions that occur when um this is faulty now.

Lagging strand telomeres are replicated in the same way as the rest of the chromosome

• A. True
• B. False

Which cell type contains the most telomerase

• A. Kidney cells
• B. Brain cells
• C. Egg cells
• D. Skin cells

Telomeres are composed of what type of DNA molecule?

• A. Single A repeats
• B. Repeats of a 5' TTAGGG 3' sequence
• C. Repeats of a 3' AAUCCC 5' sequence
• D. Single T repeats