Hi. In this video, I'm going to be talking about telomeres and telomerase. So the ends of chromosomes, which are called telomeres, are true, because if we look at this image here, let me back up, so here's 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 polymerase where to start. Then, DNA polymerase 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, 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 to bind. It would have to bind here in order to replicate this way, but now we're just off the chromosome completely. There's not more chromosome left for the primer to bind to, so what happens to this region of the DNA? 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, fix to this problem, each time the chromosome or the 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 the situation. So what happens is, there is this special enzyme called telomerase.
Telomerase replicates these ends of the telomerase on the lagging strand, or it helps to. And what it does is, telomerase, it adds nucleotides onto the 3′ 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, AAUCCC3′5′. 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 RNA is being used as a template to synthesize And so when telomerase comes on, it has this template, And so when telomerase comes on, it has this template, and then it uses that template to add this nucleotide, so the complement. Right? ttaggg5′3′, 3′5′, and it adds this repeatedly, I mean, like, 100, potentially even thousands of times onto the end of the chromosome, and this prevents the telomere from shortening because now that you're adding all of these repeats on to the end, so now we've extended the DNA strand for a very long time, so when you need this primer here, DNA can now be replicated. And so when you extending that allows you to replicate the end of the chromosome where otherwise you wouldn't have been able to.
So, so that means that telomeres, which are the ends of the chromosome, contain a freaking crap ton of ttaggg repeats. Right? And, it just contains so many of them to prevent the chromosome from shortening. So what happens, like we said, and here's the end of the chromosome. Telomerase comes in. It uses this, 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, you know, repetitive repeats. And that's okay. Lengthening lengthening the chromosome with, you know, sort of not, genetic information that's not encoding anything, so just like random nucleotides, is okay. It's not harming anything, it's not doing anything bad, 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, this is faulty. Now different cell types contain different amounts of telomerase, which means that different cell types replicate their telomeres with different sort of behind them. So some, if you have more telomerase, you're going to really make sure that every single chromosome is being replicated perfectly. Whereas if you have no telomerase, then you're not going to be able to replicate the end of the chromosome, and those chromosomes will be shortened. So germ cells, so these are the cells that become, you know, the egg, the sperm, or the sex cells. These contain a lot of telomerase, and you can kind of predict why that would be. You had to guess why would germ cell have a lot of telomerase. Do you have any ideas? Right. So it's because germ cells, are that's carrying the information that's going to be used to make the offspring. It's going to be used to pass on to those generations, so they need to make sure that it's 100% correct. Right? You don't want to cut off some telomeres and give that to your offspring because that could cause very damaging effects. So germ cells contain a lot of telomerase to make sure that all those chromosomes are healthy and complete. Somatic cells, so these are like all the other cells, skin, kidney, etcetera, etcetera, etcetera. They have some of them have a little telomerase, and some of them have no telomerase. If you don't have telomerase, that means that your telomeres will will shorten. And so in these somatic cells, they do continue to get shorter, and this has something to do with aging, why we age, has a little bit to do also with cancer, and, so as the telomeres get shorter, eventually the cell is like, okay, you've cut off way too much of my chromosome, I'm I'm gonna die, And this is a part of cell aging, particularly cells don't live forever for the most part. There's some exceptions like brain cells, but, you know, most of your skin cells, they they're they're passed every few days or few weeks, and they do this because they're replicated so frequently and their telomeres get too short, and the cell is like, oh, I'm going to die. There is a, vocab word here that you may see. I didn't write on here, but it's called senescence. And this is a state some cells enter into when their telomeres are really short, And it's not death, right, they're still alive, but they're old cells. They're not going to replicate anymore, and they're not going to do anything else
