【托福听力备考】TPO12听力文本——Lecture 1
众所周知，托福TPO材料是备考托福听力最好的材料。相信众多备考托福的同学也一直在练习这套材料，那么在以下内容中我们就为大家带来托福TPO听力练习的文本，希望能为大家的备考带来帮助。
TPO12 Lecture 1 Biology
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, 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 a person’s 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’ll look at this rough drawing of one of them, one Chromosome
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, 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 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 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 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 an important purpose; it
is 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. Every
time the Chromosome divides, every time one cell divides into two. Pieces of the
ends of the Chromosome, the telomeres, get broken off. So after each division,
the telomeres get 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 genetic 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 that 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 these cells chemically, we find something very
interesting, a chemical in them, an enzyme called telomerase. As bits of the
sort out 和divide into
telomere break off from the end of the Chromosome, this chemical, this
telomerase can rebuild it, can help reassemble the protective DNA, the telomere
that the Chromosome has 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 or whatever, because this chemical, telomerase, can keep the
telomeres on the ends of Chromosomes from getting any shorter.

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