新托福考试必备：新托福TPO（1-24）听力原文文本TPO12

　　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.