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.