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Telomeres and single copy DNA vs repetitive DNA

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Efrat Bruck.

Video transcript

- [Voiceover] Here is a pair of chromosomes as they would appear during mitosis, and the ends of chromosomes are capped with an area known as telomeres, and telomeres are mainly found in eukaryotic chromosomes because usually prokaryotes just have one circular chromosome, so it doesn't have any ends. And what do telomeres do? Well, they protect chromosomes, or protect the ends of chromosomes from deterioration. Why would the ends of chromosomes deteriorate? So, the enzymes that replicate chromosomes are not able to get to the very, very end of the chromosome. So there's gonna be, they're gonna get to, let's say this area. So there's gonna be a small spot over here that's not replicated, and since the telomeres don't have any genes in them, it's not really harmful, it doesn't really matter. So, what would happen if there were no telomeres? Well, let's take a look at the other chromosome. If there were no telomeres, and let's say, the chromosome was only replicated 'til about here, there would be this area with useful genes that wouldn't be replicated, and that would be pretty problematic. So basically, telomeres act as a buffer zone, because they do not contain any important genes. Another thing that telomeres do is they prevent chromosomes from sticking to each other. If chromosomes stuck to each other, then a lot of the genes would be scrambled and genes wouldn't be where they're supposed to be and that would be pretty problematic. And here's actually a picture of human chromosomes where the telomeres are highlighted in this florescent, so you can see the telomeres over here. So you can see how at both ends of each chromatid, there are telomeres, and so what happens is that with each time the chromosomes replicate, the telomeres get a little bit shorter and shorter and shorter, so there's an enzyme known as telomerase and telomerase is able to lengthen telomeres and bring them back to their original length. So there are some cells that replicate a lot and they have a lot of telomerase. This cell can keep on replicating and replicating, but then there are other cells that do not have a lot of telomerase, and when telomeres are basically non-existent anymore, because the chromosomes replicated many, many times, let's just get rid of the telomeres, so the chromosomes will actually not be able to replicate, and so the cell will not divide again, and it will kind of die. Now that we're talking about telomeres, I want to bring up a topic that's tangentially related, and that is single copy DNA and repetitive DNA. So, single copy DNA is when you have a DNA sequence, I'm just gonna make one up, let's say, A T C C, that basically does not repeat itself, so it might be flanked by other DNA sequences, as opposed to repetitive DNA, which is when you have a DNA sequence that keeps repeating itself, so you might have it A T C C and then again, A T C C, A T C C, et cetera. So what's the difference between single copy DNA and repetitive DNA? So here we have a spectrum. On the left we have single copy DNA, in the middle we have DNA that's somewhat repetitive, and on the right we have highly repetitive DNA. So, single copy DNA holds most of the organism's, there should be an apostrophe there, important genetic information, so basically most of the important genes are going to be single copy, so since the important genes are single copy DNA, single copy DNA is transcribed and translated and it has a low mutation rate, which is a good thing because of course, we don't want there to be mutations in the important genes. Repetitive DNA, or DNA that's somewhat repetitive is found, well at least in mammals and in insects, near the centromeres. If you recall, the centromeres are the center of the chromatid, or when you have chromatids that are duplicated, the chromatids are attached by the centromere, by the, that middle part in the chromosome, and they may contain genes that are transcribed and translated, but then there might also be parts of the repetitive DNA that don't contain genes, and those parts are not transcribed and translated, and repetitive DNA has a higher mutation rate than single copy DNA. Now let's take a look at DNA that's highly repetitive. So, it contains no genes, and because it contains no genes, it is not transcribed and not translated, and highly repetitive DNA has an even higher rate of mutation than DNA that's somewhat repetitive. So there's lots of highly repetitive DNA that we're not exactly sure what its purpose is. Scientists are currently trying to figure out what the purpose of this highly repetitive DNA is, but there are some sections of highly repetitive DNA that we do know what their purpose is. For example, telomeres. Telomeres are sections of highly repetitive DNA and as I've explained before, their purpose is to basically act as a buffer zone for the important part of the chromosome, and in fact, the DNA sequence that's repeated in telomeres is this right over here, G G T T A G, and in human chromosomes, the telomeres are made up of approximately 2,000 repeats of this DNA sequence, G G T T A G.