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Current time:0:00Total duration:5:34

Telomeres and single copy DNA vs repetitive DNA

Video transcript

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 going to be they're going to get to let's say in this area there's going to be a small spot over here that's not replicated and since the telomeres don't have any genes in them that'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 to 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 you and chromosomes where the telomeres are highlighted in this fluorescent so you can see the telomeres over here so you can see 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 so the cell can keep on replicating and replicating but then there are other cells that do not have a lot of telomerase and when e telomeres are basically non-existent anymore because the chromosome is replicated many many times let's just get rid of the telomeres so the chromosomes will actually not be able to replicate and so this 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's when you have a DNA sequence I'm just going to make one up let's say a T CC 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 at a TCC and then again a TCC a TCC etc 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 organisms there's human ' they're 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 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 a 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 we explained before their purpose is to basically act as a buffer zone for the important part of a chromosome and in fact the DNA sequence that's repeated in telomeres is this right over here g g t t AG and in human chromosomes the telomeres are made up of approximately 2,000 repeats of this DNA sequence g g t t AG