托福听力讲座部分,想要更扎实基础,有效的提高分数,我们还是需要进行原文的练习积累。利用这些材料,更好的巩固听力,提升整体的考试水平。那么对应的这部分的练习材料都有哪些呢?下面小编为大家整理了详细的内容,供大家参考!

  Narrator: Listen to part of a lecture in a biology class.

  FEMALE 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 a 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 tissue in the body, the same thing will happen eventually.

  OK. You know that all of a person's genetic information is contained on very long pieces of DNA called chromosomes—12 of them in a human cell, that's 23 pairs of these chromosomes—of various lengths and sizes. Now, um, if you'll look at this rough drawing of one of them—one chromosome about to divide into two—you'll see that it sort of looks like…well actually it's much more complex than this…but it reminds us of a couple of springs linked together…two coiled up pieces of DNA.

  And if you stretch them out, you'll find they contain certain genes, certain sequences of DNA that help determine how the cells of the body will develop. When researchers looked really carefully at the DNA in chromosomes, though, they were amazed—we all were—to find that only a fraction of it, maybe 20 to 30 percent, converts into meaningful genetic information.

  It's incredible, at least it was to me, but if you…if you took away all the DNA that codes for genes, you'd still have maybe 70 percent 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 if this DNA doesn't make up any of the genes, it must serve some other purpose. Anyway,…if we examine the ends of these coils of DNA…, we'll 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 an important purpose. It's sort of like the plastic tip on each end of a shoelace. It may not help you tie your shoe, but that little plastic tip keeps the rest of the shoelace…the shoestring…from unraveling into weak and useless threads.

  Well, the telomeres at the ends of chromosomes seem to do about the same thing—protect the genes, the genetically functional parts of the chromosome, from being damaged. Every time the chromosome divides—every time one cell divides into two—pieces of the ends of the chromosome, the telomeres, get broken off. So after each division, the telomeres get shorter; and one of the things that may happen after a while is that pieces of the genes themselves get broken off of the chromosomes…

  so the chromosome is now losing important genetic information and is no longer functional. But as long as the telomeres are a 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 that just seem to keep on dividing, regardless…which may not always be a good thing if it gets out of control…

  but when we analyze these cells chemically, we find something very interesting—a chemical in them, an enzyme called “telomerase.”As bits of the telomere break off from the end of the chromosome, this chemical—this “telomerase”—can rebuild it…can help reassemble the protective DNA, the telomere, that the chromosome has lost.

  Someday, we may be able to take any cell and keep it alive, functioning and reproducing itself essentially forever, through the use of telomerase. And in the future we may have virtually immortal nerve cells and immortal skin cells or whatever, because this chemical, telomerase, can keep the telomeres on the ends of the chromosomes from getting any shorter.

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