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TPO12 Lecture 1 Biology
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 libratory, 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 very 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 townies 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 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 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. Other 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 telomerase. As bits of the telomere break off from the end of Chromosome, this chemical, this telomerase can rebuild it, can help resemble 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 telomerase. And in the future we may have virtually immortal nerve cells and immortal skin cells of whatever because of these chemical, telomerase can keep the telomere on the ends of Chromosomes from getting any shorter.
《生物》
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，那么就可以解释一个非常重要的观察研究了，这个观察表明当我们想要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。一些细胞，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。而且在人类体内的细胞中，在所有器官、几乎每种身体组织中，这样的事也终究会发生。
好，你们知道，所有人的遗传基因都存在于一个非常长的，叫做染色体的 DNA 中。46条，也就是 23 对染色体，在人体细胞中，每个有特定的长度和大小。如果你粗略看一下一张它的图片，一个染色体正在分裂成两个。它的样子有点让我们联想到，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，你会发现它们涵盖了一些基因，一些决定身体细胞如何生长的 DNA 序列。当学者仔细看 DNA 中的染色体，他们都惊叹了，我们都很惊叹，因为仅仅一个大概20%-30%的片段，就可以转化成涵义丰富的基因信息。这非常神奇，至少对于我是。但如果你取走所有为基因编码的 DNA，还会剩下大概 70%的 DNA，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 不能产生基因，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，一个分裂成两个，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，一些基因从染色体中被破除掉了，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一些特定的染色体的端区，我们可以预测多长的染色体可以成功分裂。有一些染色体不论如何都会进行分裂，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。一个化学物质，一种叫做端粒酶(telomerase)的酶。当端粒从染色体中断裂开，这种化学物质，这个端粒酶可以重建它，可以重新组装这个具有保护功能的 DNA，也就是染色体失去的端粒。有一天，借助端粒酶，我们或许能够取出任何一种细胞，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，因为端粒酶可以构建端粒，使得染色体不会变得更短。
以上就是托福听力TPO12原文中Lecture 1的文本内容，希望大家能够用心体会，更多TPO文本内容小编稍后为您呈现。最后，小编祝大家在托福考试中取得好成绩！
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