Lecture 4 人体密码
Wors Tips
nerve n.神经 renewv.更新；更换
tissue n.（人、动植物细胞的）组织 organ n.（人体或动植物的）器官
genetic adj.基因的；遗传学的 chromosome n.染色体
sequence n.顺序；次序；一系列；一连串 fraction n.碎片；片断；小部分
tongueincheek 挖苦的；开玩笑的 telomeren.染色体端粒
shoelace n.鞋带 unravel v.解开；解决；散开
repetitious adj.重复的；一再的；反反复复的
Listen to the following recording and answer questions 1-3.
1）A.She doubted its accuracy.
B.She was surprised by its conclusion.
C.She was concerned about its implications.
D.She thought it was unnecessary.
2）A.It causes a cell to begin dividing.
B.It separates one gene from another.
C.It is genetically meaningless.
D.It has no function.
3）A.to point out that chromosomes are arranged in pairs
B.to describe the coiled shape of a chromosome
C.to illustrate how chromosomes are protected from damage
D.to explain how chromosomes are joined before dividing
听力原文
As we learn more about the DNA and 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 the laboratory，they seem program to divide only a certain number of times before they die.Now this differs with the type of cell.Er，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，46 of them in the human cells that’s 23 pairs of these Chromosomes of various lengths and sizes.Now，en，if you look at this rough drawing of one of them，one Chromosome about to divide into two.You’ll see that it is 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 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…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 if these 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’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 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.
Questions：
1.The professor discusses research about the percentage of a chromosome’s DNA that contains genetic information.How does she feel about this research？
2.What does the professor say about theDNAin a telomere？
3.Why does the professor mention shoelaces？
参考译文
我们学了人类细胞内的DNA如何控制细胞的生长和发展，我们可以解释为什么在实验室里培植的人类细胞在死亡前分裂的次数很有限，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DNA片段，也就是染色体中。人体细胞中有46条染色体，也就是23对。每个有特定的长度和大小。如果你看一下这张粗略的图片，会发现一个染色体正在分裂成两个。它的样子有点让我们联想到一些连在一起的弹簧盘卷而成了DNA，事实上要复杂得多。当你把它们展开，你会发现它们涵盖了一些基因，也就是一些决定身体细胞如何生长的DNA序列。当学者仔细看染色体中的DNA，他们都很惊叹，因为仅仅其中的一部分，也就是大概20%-30%的DNA转化成涵义丰富的有效基因。这非常神奇，至少对于我是。但如果你拿掉所有为基因编码的DNA，还会剩下大概70%的DNA，这就是所谓的“垃圾DNA”（Junk DNA），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不是基因的组成部分，那么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末端都有一个特定的DNA序列，它叫作端粒（telomere）。端粒是一个高度重复、没有遗传意义的DNA序列，也就是我们说的垃圾DNA。但是它存在的目的就像鞋带末端的塑料头，不是帮助系鞋带，而是防止鞋带因无封口而松开和损坏，让鞋带变成脆弱没用的线头。端粒有着相似的功能，可以保护染色体那些有基因功能的部分免于被损坏。
参考答案
1.B 2.C 3.C