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Telomeres and single copy DNA vs repetitive DNA

Telomeres, the protective caps on chromosome ends, prevent deterioration and chromosome sticking. They contain no genes, acting as a buffer zone during replication. Telomerase can lengthen telomeres, allowing cells to replicate more. The video also discusses single copy and repetitive DNA, with telomeres being highly 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.

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  • blobby green style avatar for user Tatiana Betancur
    Please try to make the writing more clear. Thanks!
    (56 votes)
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  • blobby green style avatar for user Kiran S
    At , she says the telomerase has a repeating sequence of GGTTAG, that is wrong. It has a repeating sequence of TTAGGG
    (5 votes)
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    • leaf green style avatar for user Jonathan
      TTAGGG is the repeating sequence in telomeres, but in the actual lengthening process telomerase (the enzyme responsible for lengthening telomeres) repeatedly adds GGTTAG. Since the element is repeated many times, the repeating sequence just depends on where you "start" in the sequence e.g. put two GGTTAG's together and you have GG then TTAGGG then TTAG.
      (33 votes)
  • leafers seedling style avatar for user EmilyAWillard
    Hi! It's impossible to read the text on this video - it is very blurry. Is it possible to re-upload this video with better resolution? I love the MCAT videos - they are AWESOME. Thank you!
    (13 votes)
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  • duskpin ultimate style avatar for user MS17155 - Shivanshu Siyanwal
    Why are repetitive DNA so prone to mutations?
    (4 votes)
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    • mr pink red style avatar for user brooke1
      Repeats are prone to something called slipped-strand mispairing - basically they cause DNA to get misaligned easier during DNA synthesis. If not fixed, these errors cause deletions/frameshift mutations in the DNA. Interestingly, some bacteria make use of this mechanism so that they achieve constant genetic/phenotypic variability in their surface structures as to evade the immune system :)
      (17 votes)
  • blobby green style avatar for user Jose
    Great video. General question though in an experiment how would you go about manipulating a telomere binding sequence to represent another one. For example, TTAGGG to ATACGG. This was a topic that came up in my molecular biology exam.
    (6 votes)
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  • leafers sapling style avatar for user Estrella
    Is the amount of telomerase present in cells the only (or main) factor that prevents certain cells in the body (like neurons) from replicating frequently? If so, is it possible to allow more replication of these cells by increasing the amount of telomerase (and thus lengthening the telomeres) int these cells?
    (Just as a side note, how would the amount of telomerase present affect the ability of cells to retain their telomeres? -- since telomerase is an enzyme, and thus inexhaustible, the amount present would only affect the rate of telomere regeneration and not the presence of telomeres, right? )
    (4 votes)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      Enzymes aren't "exhaustible". If you study any type of molecular bio in the future, you'll see that proteins have half lives (which can range from a few seconds to a few years). So, a protein will be made and used until degraded, and more can be made when needed by signaling to the nucleus, which leads to translation of more of it. So what you really mean to ask is if certain cells stop expressing (transcribing and translating genes encoding telomerase) telomerase after some time. And the answer is yes: basically all of your cells except eggs and sperms.

      When your young, you have active telomeres, but once you grow up, telomere activity drastically decreases everywhere...except in your gametes. And yes, this leads to signs of aging, and potentially cancer, much later in life. Generally, your body detects if telomeres are getting shorter than the acceptable threshold, so they will do apoptosis. The ones that get away become cancerous.

      The reason why certain specialized cells don't divide is usually linked to their complex structure, not so much to telomere length. The reason why brain cells don't divide too often is complex. Firstly, your brain can't risk having too many brain cells since they wouldn't fit. Secondly, they don't just grow and split like skin cells. Brain cells develope in embryos in a really complex path where they travel to their destination and start extending axons and what not. So it's a little hard to make new ones at an older age. But you do have a point, because some evolutionary scientists believe that our brains evolved to not continually undergo cell division in order to decrease the chance of cancer occurrence in our most vital organ. But it's still a hypothesis.
      (4 votes)
  • leafers ultimate style avatar for user Philip Zandelin
    Is the loss of telomeres the cause of aging?
    (2 votes)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      Aging is mostly attributed to oxidative stress, but mutations do pile up throughout our lifetime, leading to genetic-based complications such as cancer when we're old. That's why cancer usually shows up later in life (more than about 6 mutations have to accumulate in a cell line for that cell to become cancerous). So telomere degradation, which leads to deletion mutations eventually, leads to such complications at older ages. I believe telomerase activity also decreases with age leading to more telomere degradation...though I may be wrong. There's definitely a link between telomere length and life expectancy, but it's not yet quite known if telomere degredation is a cause or a consequence of aging.

      If you ever look up Dolly the sheep, she was the first cloned animal. They took the body cell of another sheep, extracted its DNA, and inserted it into an empty egg. This egg became Dolly. Interestingly, once Dolly was born, she only lived to half the life span of an average sheep. One reason they think this might be is due to the fact that the DNA used to clone Dolly came from a middle-aged sheep whose telomeres had already been degraded a bit. If this does in fact end up being the reason for her short age, then telomeres would be linked to length of life! Isn't that so cool?! Genetics is honestly so cool.
      (4 votes)
  • purple pi purple style avatar for user DominiqueRC
    When you refer to single copy and highly repetitive DNA, is this the same thing as 'euchromatin' and 'heterochromatin' ?
    (1 vote)
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  • piceratops ultimate style avatar for user Samson Lopez
    Can someone explain the mechanism by which the telomeres get shorter and shorter with replication?
    (1 vote)
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  • blobby green style avatar for user girl10154
    is single copy DNA similar between people whereas the repeated DNA varies between people? and that is why forensics use repeated DNA?? thanks
    (1 vote)
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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.