Listen to part of a lecture in a Biology Class.
As we learn more about the DNA in human cells and how it controls the growth and development of cells, then maybe we can explain a very important observation, that when we try to grow most human cells in laboratory, they seem programmed to divide only a certain number of times before they die.Now this differs with the type of cell. Some cells, like nerve cells, only divide seven to nine times in their total life.Others, like skin cells, will divide many, many more times.But finally the cells stop renewing themselves and they die.And in the cells of the human body itself, in the cells of every organ, of almost every type of tissues in the body, the same thing will happen eventually.
OK, you know that all of persons' genetic information is contained on very long pieces of DNA called Chromosomes.46 of them are in the human cells that's 23 pairs of these Chromosomes are of various lengths and sizes. Now if you look at this rough drawing of one of them, one Chromosome is about to divide into two.You see that it sort of looks like, well actually it's much more complex than this but it reminds us a couple of springs linked together to coil up pieces of DNA.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 will develop.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, maybe 20-30%, converts into meaningful genetic information.It's incredible - at least it was to me.But if you took away all the DNA that codes for genes, you still have maybe 70% of the DNA left over.That's the so-called JUNK DNA. Though the word junk is used sort of tongue in cheek.
The assumption is that even these DNA doesn't make up any of the genes, it must serve some other purpose.Anyway, if we examine these ends of these coils of DNA, we will find a sequence of DNA at each end of every human Chromosome, called a telomere.Now a telomere is a highly repetitious and genetically 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 each end of shoelace.It means not help you tie your shoe but that little plastic tip keeps the rest of the shoelace, the shoe string from unraveling into weak and useless threads.Well, the telomere at the end of Chromosomes seems to do about the same thing - protect the genes the genetically functional parts ofthe Chromosome 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 shorter and 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 now losing important genetically information and is no longer functional.But as long as the telomeres are at certain length they keep this from happening.So it seems that, when the, by looking at the length of the telomeres on specific Chromosomes we can actually predict pretty much how long certain cells can successfully go on dividing.Now there are some cells just seem to keep on dividing regardless, 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 - a chemical in them, and an enzyme called telomeres. As bits of the telomere break off from the end of Chromosome, this chemical, this telomeres can rebuild it, can help reassemble the protected DNA, the telomere that the Chromosome is lost. Someday we may be able to take any cell and keep it alive functioning and reproducing itself essentially forever through the use of telomeres.And in the future we may have virtually immortal nerve cells and immortal skin cells of whatever because of these chemical - telomeres can keep the telomere on the ends of Chromosomes from getting any shorter.
Professor: As we learn more about the DNA in human cells and how it controls the growth and development of cells, then maybe we can explain a very important observation, that when we try to grow most human cells in laboratory…
But finally the cells stop renewing themselves and they die. And in the cells of the human body itself, in the cells of every organ, of almost every type of tissues in the body, the same thing will happen eventually.
此题为主旨题。这一这句话“we can explain a very important observation,”引出话题，即细胞分裂的次数是有限的，因此不能一直分裂。这种题型终归得结合全文来筛选答案，至少开头出现了一个why 暗示我们在B D里面选
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