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TPO12 Lecture 1
Narrator:Listen to part of a lecture in a Biology Class. Professor:
As we learn more about the DNA in human cells and how it controls the growthand development of cells, then maybe we can explain a very importantobservation, that when we try to grow most human cells in libratory, they seemprogrammed to divide only a certain number of times before they die. Now thisdiffers with the type of cell. Some cells, like nerve cells, only divide seven tonine times in their total life. Others, like skin cells, will divide many, many moretimes. But finally the cells stop renewing themselves and they die. And in thecells of the human body itself, in the cells of every organ, of almost every typeof tissues in the body, the same thing will happen eventually.
OK, you know that all of persons’ genetic information is contained on very longpieces of DNA called Chromosomes. 46 of them are in the human cells that’s23 pairs of these Chromosomes are of very lengths and sizes. Now if you lookat this rough drawing of one of them, one Chromosome is about to divide intotwo. You see that it sort of looks like, well actually it’s much more complex thanthis but it reminds us a couple of springs linked together to coil up pieces ofDNA. And if you stretch them out you will find they contain certain genes,certain sequences of DNA that help to determine how the cells of the body willdevelop. When researchers look really carefully at the DNA in Chromosomes
though, they were amazed, we all were, to find that only a fraction of it, maybe20-30%, converts into meaningful genetic information. It’s incredible; at least itwas to me. But if you took away all the DNA that codes for genes, you still havemaybe 70% of the DNA left over. That’s the so-called JUNK DNA. Though theword junk is used sort of townies cheek.
The assumption is that even these DNA doesn’t make up any of the genes itmust serve some other purpose. Anyway, if we examine these ends of thesecoils of DNA, we will find a sequence of DNA at each end of every humanChromosome, called a telomere. Now a telomere is a highly repetitious andgenetically meaningless sequence of DNA, what we were calling JUNK DNA.But it does have any important purpose; it is sort of like the plastic tip on eachend of shoelace. It means not help you tie your shoe but that little plastic tipkeeps the rest of the shoelace, the shoe string from unraveling into weak anduseless threads. Well, the telomere at the end of Chromosomes seems to doabout the same thing--- protect the genes the genetically functional parts of theChromosome from being damaged. Every time the Chromosome divides,every time one cell divides into two. Pieces of the ends of the Chromosome,the telomere, get broken off. So after each division, the telomere gets shorterand one of the things that may happen after a while is that pieces of the genes themselves get broken off the Chromosomes. So the Chromosome is nowlosing important genetically information and is no longer functional. But as longas the telomeres are at certain length they keep this from happening. So itseems that, when the, by looking at the length of the telomeres on specificChromosomes we can actually predict pretty much how long certain cells cansuccessfully go on dividing. Other some cells just seem to keep on dividingregardless which mean not be always a good thing if it gets out of control.
But when we analyze the cells chemically we find something very interesting, achemical in them, and an enzyme called telomerase. As bits of the telomerebreak off from the end of Chromosome, this chemical, this telomerase canrebuild it, can help resemble the protected DNA, the telomere that theChromosome is lost. Someday we may be able to take any cell and keep italive functioning and reproducing itself essentially forever through the use oftelomerase. And in the future we may have virtually immortal nerve cells andimmortal skin cells of whatever because of these chemical, telomerase cankeep the telomere on the ends of Chromosomes from getting any shorter.
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