[求助]OG-5-27 好像没人问过

ring_cheng

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 [ring1] 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.

27. According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?

(A) They did not attempt to replicate [ring10] the original experiment of separating an embryo into two parts.

(B) They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment.

(C) They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments.

(D) They assumed that it was crucial to perform the separation experiment at an early stage in the embryo’s life.

(E) They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned.

同意答案是E。但好像也挑不出B的不对呢？因为即然这些科学家认为这些细胞都是undetermined，那么他们自然也就不会意识到在cell determination 和separation experiment有联系了。请指教！

xealot

....between the issue of cell determination and the outcome of the separation experiment

其实偶现在也没看出来在cell determination和the OUTCOME of the separation experiment之间有什么联系...

ring_cheng

抱歉，能不能再解释一下。B：“早期的科学家没有意识到cell determination与outcome of experiment的联系”  是错误的吗？

我的感觉是早期科学家认为细胞undetermined，所以不管怎么切，切后的细胞都会发育成一个个的个体；因此他们没有意识到the issue of cell determination与outcome of experiment有联系. 只是后面的科学家通过不同切法的实验才意识到有可能有cell determinant 物质存在。

怎么看不出来呢？郁！！！再请指点！

BiteGmat

仅通过“…… the cells in the early embryo are undetermined……” 就能推出“They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment”太勉强了，完全是凭感觉。认为早期胚胎未被决定，就能说没有认识到决定物质的存在？“早期胚胎未被决定”和“决定物质的存在”并不矛盾啊。胚胎发育到一定阶段后，决定物质才开始发挥作用的。在此之前“决定物质的存在”而且“早期胚胎未被决定”，此时如果合理分割，胚胎可以发育成两个单体。

我说这些话也是凭感觉，讲不出理论来，水平有限不好意思，还是请其他NN来帮忙吧

ring_cheng

还是不明白，我的理解是：

早期科学家认为胚胎是undetermined（也即不存在所谓的决定物质）, 所以不论怎么切，都不会影响到切割实验的结果（如果他们认为有某种物质能够决定胚胎发育，则胚胎就是determined，他们也就不会认为不论怎么切结果都一样了），也就说B：他们不认为胚胎决定的问题与切割实验的结果有什么关系。

请帮忙指点一下！

多谢了！

[此贴子已经被作者于2005-3-23 9:45:25编辑过]

EASYSUMMER

我也说说我得理解。

因为早期的生物学家根本没意识到“所谓的决定物质”，所以哪来的他们之间的联系之说。

B 答案的隐含前提应该是生物学家已经知道这么一种物质。。前提都不存在，哪来这个结论。  所以我理解B应是属于文章没有涉及，所以错误。。

胡说一通继续讨论。。。

而今迈步从头越

27. According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?

(A) They did not attempt to replicate [ring10] the original experiment of separating an embryo into two parts.

(B) They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment.

(C) They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments.

(D) They assumed that it was crucial to perform the separation experiment at an early stage in the embryo’s life.

(E) They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned.

同意答案是E。但好像也挑不出B的不对呢？因为即然这些科学家认为这些细胞都是undetermined，那么他们自然也就不会意识到在cell determination 和separation experiment有联系了。请指教！

个人意见！我觉得这道题B的connection过于含糊，什末样的connection呢？可以理解为他们意识到了，只不过他们的想法是错误的而已：他们认为outcome和cell determination没有关系！原文：This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. 我觉得这样也是一种connection，就好像错误的认识也是一种认识一样的！如果选项改为说They realized that there was no connection between the issue of cell determination and the outcome of the separation experiment. 我觉得B就对了！

关键在于否定的对象！是不是有点强词夺理？！

BiteGmat

同意easysummey的说法！

[此贴子已经被作者于2005-3-24 10:46:01编辑过]

ring_cheng

有道理！谢谢。看来我有些砖牛角尖了：）

swlfx

我的一点看法：

         题目中的outcome所指的应该是一个胚胎被切割后可以成为两个独立的完整胚胎；

         而cell determination指的应该是独立完整胚胎形成的决定因素；

         早期的家家们认为随便怎么分割都可以产生实验中的结果，这被后来的家家们证明是有问题的。

        所以B选项不对。

个人意见。