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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 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 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’ll 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, two coiled 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 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 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 an important purpose. It is sort of like the plastic tip on each end of shoelace.It may 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 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 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 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 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 the cells chemically, we find something very interesting—a chemical in them, and an enzyme called telomerase.As bits of the telomere break off from the end of Chromosome, this chemical—this “telomerase”—can rebuild it…can help reassemble the protective 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 telomerase.And in the future we may have virtually immortal nerve cells and immortal skin cells of whatever, because this chemical, telomerase, can keep the telomeres on the ends of Chromosomes from getting any shorter.
旁白：听一段关于生物的讲座。
教授：我们学了人类细胞内的DNA 如何控制细胞的生长和发展，那么也许我们可以解释一个非常重要的观察研究了，当我们试图在实验室培育大多数的人类细胞时，它们像被设计好的一样在活性失去之前只分裂那么几次。在这里，不同的细胞是不同的：有些细胞，比如神经细胞，只在有活性的过程中分裂七到九次。其他的，比如皮肤细胞，分裂的次数大大高于它。但最终，细胞们都会停止更新，然后失去活性。而且在人体的细胞中，在所有器官、几乎每种身体组织中，同样的事情最后也会发生。
好，你们知道，所有人的遗传信息都包含在很长的DNA片段中，叫做染色体。人体中有46条，也就是23 对染色体，每个有特定的长度和大小。如果你看一下其中的一个草图，一个染色体正在分裂成两个。你会发现它看起来是这样的，当然它比看上去要复杂的多，它看上去像一些弹簧连在一起，盘卷成了DNA。当你把它们展开，你会发现它们含有特定的基因，特定的DNA序列，帮助决定身体细胞是如何发育的。当研究人员仔细观察染色体中的DNA时，他们惊讶地发现，大概20%-30%的片段，可以转化成有意义的基因信息。这是不可思议的，至少对于我是这样的，但如果你取走所有基因编码的DNA，还会剩下大概70%的DNA残留。这就是所谓的“垃圾DNA”（JunkDNA）尽管垃圾这个词用起来有点半开玩笑的意思。
我们假设即使这些DNA 不能构成任何基因，它肯定有点其他什么用途。不论如何，当我们检查这些盘卷的DNA 的尾部，我们发现每个人类染色体的尾部都有一个特定的DNA 序列，它叫做端区（telomere）。端区是一个高度重复，没有遗传意义的DNA 序列，也就是我们说的垃圾DNA。但是它的确有一个重要的目的。它有点像鞋带两头的塑料头。它可能不会帮助你系上鞋带，但这两个塑料头帮助鞋带的其他部分，线绳的部分，不至于脱线，变成脆弱没用的线头。染色体两头的端区（也叫端粒）看上去做的是同样的事，来保护基因，那些有基因功能的染色体部分不至于被毁坏。每次染色体分裂，每次一个细胞分裂成两个，染色体的尾部，也就是端区，就会发生断裂。所以每经过一次分裂，端区就会变得更短。一段时间后，有可能发生的是，基因会从染色体中分离出来。这样染色体就失去了重要的基因信息，导致它失去了它的功能。但只要端区维持着一定的长度，它们保证这种事不会发生。所以这看上去⋯⋯通过观察特定的染色体的端区，我们可以预测某些细胞成功分裂的时长。有一些细胞似乎在不停地分裂，这并不是一件好事，因为它有可能（由于过度分裂）而失去控制。
但当我们用化学方法分析这些细胞，我们发现了一个很有趣的事。一种化学物质，一种叫做端粒酶（telomerase）的酶。当端粒从染色体末端断裂开来，这种化学物质，这个端粒酶可以重建它，可以重新组装给予保护的DNA，也就是染色体失去的端粒。总有一天，通过使用端粒酶，我们能够使任何细胞存活，使其永久性地拥有活性和复制自己。在将来，我们可能有真正意义上的长生不老的神经细胞、皮肤细胞等等，因为这种化学物质，端粒酶，使得染色体低端的端粒不会变得更短。
题型分类：主旨题
原文定位：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.
选项分析：
文章通过“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，选项D正确。选项B中存在干扰，但是此项教授并没有解释出来，因此此项不可选。
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