请教OG 5－27

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.

以前有人讨论过此题B与E的选择，我这里是想问C错在哪里。见文中highlight部分，我觉得这里重点是在说之后生物学家发现切割不同植物的胚胎，情况会不同。可E强调是不同的切割方法， 而不是切割不同类的植物。confusing!!

请望指点！

顺便问下A：They did not attempt to replicate the original experiment.

觉得也对啊，他们没有重复实验，所以没有发现问题。Later biologists之所以发现问题，是因为重复了实验，但没得到相同结果。