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Nearly a century ago, biologists found that if they

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Nearly a century ago, biologists found that if they [#permalink]

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New post 01 Feb 2013, 04:27
Complete discussion is given at the below mentioned link
nearly-a-century-ago-biologists-found-that-if-they-separate-96221.html


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 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 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 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 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 to show that, in a sense, cell determination begins even before an egg is fertilized.
Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell, i.e., in that part of the 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 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 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 histones move into the cell nucleus, where sections of DNA wrap around them to form a structure that resembles 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 DNA strings that guides the fate of the cells in which they are located.


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

1 located in the nucleus of the embryo cells

2 evenly distributed unless the embryo is not developing normally

3 inactive until the embryo cells become irreversibly committed to their final function

4 identical to those that were already present in the unfertilized egg

5 present in larger quantities than is necessary for the development of a single individual

E is the OA
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Re: Nearly a century ago, biologists found that if they separate [#permalink]

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New post 01 Feb 2013, 06:00
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a toughie...!
@carcass...you must be playing at 40+ when such a passage had appeared. Jealous!!!
Coming back to question.
Its given in the passage that if one cuts an early embryo in proper plane, then the resulting parts may yield two individual beings. This "yielding" is helped by morphogenetic elements. (Sorry for bad English).
For the production of a single individual being, one morphogenetic element is sufficient but since even if you cut an early embryo into two, the two parts become respective individuals then the entire process seems to tell that morphogenetic elements are more than what is required for one individual.
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Re: Nearly a century ago, biologists found that if they separate [#permalink]

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New post 02 Feb 2013, 03:49
C is indeed a close call, if not for a re-read this would have been the (incorrect) choice.
Md's are inactive until the egg fertilization not until the cell determination. Once the egg is fertilized, Md's become active and govern the fate of the cells.
D is just an assertion with no facts. A and B are incorrect choices derived from facts mentioned in the passage moreover either of the choices would not classify as 'inference'.
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Re: Nearly a century ago, biologists found that if they separate [#permalink]

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New post 02 Feb 2013, 10:37
Marcab wrote:
a toughie...!
@carcass...you must be playing at 40+ when such a passage had appeared. Jealous!!!
Coming back to question.
Its given in the passage that if one cuts an early embryo in proper plane, then the resulting parts may yield two individual beings. This "yielding" is helped by morphogenetic elements. (Sorry for bad English).
For the production of a single individual being, one morphogenetic element is sufficient but since even if you cut an early embryo into two, the two parts become respective individuals then the entire process seems to tell that morphogenetic elements are more than what is required for one individual.
+1E
Regards.



good job man :)
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Re: Nearly a century ago, biologists found that if they [#permalink]

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New post 13 Aug 2015, 05:55
Hello from the GMAT Club VerbalBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: Nearly a century ago, biologists found that if they   [#permalink] 13 Aug 2015, 05:55
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