In this video, we're going to introduce telomeres. Telomeres are non-coding DNA, which means that they do not code for any proteins. They consist of repeating sequences that are found at the tips or the ends of eukaryotic chromosomes. Once again, the telomeres are non-coding DNA found at the tips or ends of only eukaryotic chromosomes, not prokaryotic chromosomes. In many eukaryotic cells, the telomeres will actually shorten with each round of DNA replication, and so they'll get shorter and shorter with every round of DNA replication. This shortening of the telomeres has been linked to the aging process. Significant telomere loss due to shortening of the telomeres with each round of DNA replication will actually signal the cell division process to stop in normal cells. In normal cells, when the telomeres get too short, the cells will eventually stop dividing and ultimately die.
Now, if we take a look at our image down below of these telomeres, what you'll notice over here on the left-hand side, we're showing you a eukaryotic chromosome. Notice that at the ends or the tips of the eukaryotic chromosomes is where the telomeres will be found, and the telomeres are going to consist of non-coding DNA, so it does not code for any proteins. The coding DNA will actually be found in the internal region of the chromosome.
It's important to note that with each generation, which requires DNA replication, the chromosomes will shorten as you see here. They start off really long, and with each generation, they get shorter, progressively shorter and shorter. When the chromosomes' telomeres get significantly short, when there is significant telomere loss, that will signal the cell to stop dividing in a normal cell. You can see here that this chromosome over here, which has really long telomeres, is going to be much younger than the chromosome over here which has much shorter telomeres. This one is much older as you can see here with the cane and the glasses.
It's important to note is that telomere shortening is part of the normal process of aging. However, some cells have an enzyme called telomerase, which is an enzyme itself that is going to be found in some cells that catalyzes the lengthening of telomeres. Cells that express high levels of telomerase will not shorten their telomeres because the telomerase enzyme will lengthen the telomeres and keep the telomeres at the same length. Telomerase is usually expressed in germ cells, which are precursor cells for gametes or sex cells, and it'll also be expressed in cancer cells. Cancer cells are going to be able to maintain their telomere length, and that allows them to continuously divide. Remember that significant telomere loss will usually signal division to stop. But if the cells are able to lengthen their telomeres with telomerase, then cell division will not be triggered to stop, and the cells will be able to continuously divide.
This here concludes our brief introduction to telomeres, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
