og 10th 第5篇

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Passage 5
            

Nearly a century ago, biologists found that if they

separated an invertebrate animal embryo into two parts

at an early stage of its life, it would survive and develop

as two normal embryos. This led them to believe that the

(5) cells in the early embryo are undetermined in the sense

that each cell has the potential to develop in a variety of

different ways. Later biologists found that the situation

was not so simple. It matters in which plane the embryo

is cut. If it is cut in a plane different from the one used

(10) by the early investigators, it will not form two whole

embryos.

A debate arose over what exactly was happening.

Which embryo cells are determined, just when do they-

become irreversibly committed to their fates, and what

(15) are the “morphogenetic determinants” that tell a cell

what to become? But the debate could not be resolved

because no one was able to ask the crucial questions

in a form in which they could be pursued productively.

Recent discoveries in molecular biology, however, have

(20) opened up prospects for a resolution of the debate.

Now investigators think they know at least some of the

molecules that act as morphogenetic determinants in

early development. They have been able o show that,

in a sense, cell determination begins even before an egg

(25) is fertilized.

Studying sea urchins, biologist Paul Gross found

that an unfertilized egg contains substances that func-

tion as morphogenetic determinants. They are located

in the cytoplasm of the egg cell; i.e., in that part of the

(30) cell’s protoplasm that lies outside of the nucleus. In the

unfertilized egg, the substances are inactive and are not

distributed homogeneously. When the egg is fertilized,

the substances become active and, presumably, govern

the behavior of the genes they interact with. Since the

(35) substances are unevenly distributed in the egg, when the

fertilized egg divides, the resulting cells are different

from the start and so can be qualitatively different in

their own gene activity.

The substances that Gross studied are maternal

(40) messenger RNA’s --products of certain of the maternal

genes. He and other biologists studying a wide variety

of organisms have found that these particular RNA’s

direct, in large part, the synthesis of histones, a class

of proteins that bind to DNA. Once synthesized, the

(45) histones move into the cell nucleus, where section of

DNA wrap around them to form a structure that resem-

bles beads, or knots, on a string. The beads are DNA

segments wrapped around the histones; the string is the

intervening DNA. And it is the structure of these beaded

(50) DNA strings that guides the fate of the cells in which

they are located.

25. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are

(A)   located in the nucleus of the embryo cells

(B)   evenly distributed unless the embryo is not developing normally

(C)   inactive until the embryo cells become irreversibly committed to their final function

(D)  identical to those that were already present in the unfertilized egg（E）
            

(E)   present in larger quantities than is necessary for the development of a single individual

请问上文中划线部分的翻译，另外，对第25题有疑问
            

25.

E is the best answer.

The second and third paragraphs of the passage indicate that morphogenetic determinants are

substances in the embryo that are activated after the egg has been fertilized and that “tell a cell

what to become” (lines 21-23). If, as the author asserts in the first paragraph, biologists have

succeeded in dividing an embryo into two parts, each of which survives and develops into a

normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early

embryo is greater than that required for the development of a single individual.

请问The second and third paragraphs of the passage indicate that morphogenetic determinants are

substances in the embryo that are activated after the egg has been fertilized and that “tell a cell

what to become” (lines 21-23).和后面的If, as the author asserts in the first paragraph, biologists have

succeeded in dividing an embryo into two parts, each of which survives and develops into a

normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early

embryo is greater than that required for the development of a single individual.有什么关系啊？这道题虽然我认为前四个选项都不对，但是，总感觉得有点牵强,看来我是没懂ETS的意思,请多指教!

 

[此贴子已经被作者于2007-4-1 0:20:37编辑过]

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25.

E is the best answer.

The second and third paragraphs of the passage indicate that morphogenetic determinants are

substances in the embryo that are activated after the egg has been fertilized and that “tell a cell

what to become” (lines 21-23). If, as the author asserts in the first paragraph, biologists have

succeeded in dividing an embryo into two parts, each of which survives and develops into a

normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early

embryo is greater than that required for the development of a single individual.

请问The second and third paragraphs of the passage indicate that morphogenetic determinants are

substances in the embryo that are activated after the egg has been fertilized and that “tell a cell

what to become” (lines 21-23).和后面的If, as the author asserts in the first paragraph, biologists have

succeeded in dividing an embryo into two parts, each of which survives and develops into a

normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early

embryo is greater than that required for the development of a single individual.有什么关系啊？这道题虽然我认为前四个选项都不对，但是，总感觉得有点牵强,看来我是没懂ETS的意思,请多指教!

咦？25题的题目为什么贴不上呢？？？编辑贴的时候还有，怎么发表以后就没有了？我把原文的格式去掉了呀？

[此贴子已经被作者于2007-4-1 0:23:49编辑过]

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