og-5-28

思谦

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.

28. It can be inferred from the passage that the initial production of histones after an egg is fertilized takes place

(A) in the cytoplasm

(B) in the maternal genes

(C) throughout the protoplasm

(D) in the beaded portions of the DNA strings

(E) in certain sections of the cell nucleus

A is the best answer. In the third paragraph, the author asserts that substances that function as

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morphogenetic determinants are located in the cytoplasm of the cell and become active after the

cell is fertilized. In the fourth paragraph we learn that these substances are “maternal messenger

RNA’s” and that they “direct, in large part, the synthesis of histones,” which, after being

synthesized, “move into the cell nucleus” (lines 59-68). Thus, it can be inferred that after the egg

is fertilized, the initial production of histones occurs in the cytoplasm.

L30这里不是有个protoplasm吗，为什么可以排除C呀？请指教！

思谦

谢谢Z520m!

[此贴子已经被作者于2005-8-6 22:37:10编辑过]