Narrator:Listen to part of a lecture in a Biology Class.
Professor:As we learn more about the DNA in human cells and how it controls the growth and development of cells,
教授:我们已经了解人类细胞内的 DNA 如何控制细胞的生长和发展，
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.
好了，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 of them are in the human cells that's23 pairs of these Chromosomes are of very lengths and sizes.
在人体细胞中的46条，也就是 23 对染色体，每条都有特定的长度和大小。
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.
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。
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.
当学者仔细看 DNA 中的染色体，很令人惊讶，我们也很惊讶，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。
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.
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”，
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.
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。
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.
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叫做端区的特定DNA 序列。
Now a telomere is a highly repetitious and genetically meaningless sequence of DNA, what we were calling JUNK DNA.
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。
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.