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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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28 Mar 2013, 10:49
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.

According to the passage, the morphogenetic determinants present in the unfertilized egg cell are which of the following?

A. Proteins bound to the nucleus
B. Histones
C. Maternal messenger RNA's
D. Cytoplasm

[Reveal] Spoiler:
OA: C

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Last edited by egmat on 09 Apr 2013, 12:06, edited 1 time in total.
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Re: New official GMAT passage [#permalink]

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28 Mar 2013, 13:05
Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants.
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The substances that Gross studied are maternal messenger RNA's products of certain of the maternal genes
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03 Apr 2013, 21:49
option C is the correct answer.

wats OA
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Re: New official GMAT passage [#permalink]

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03 Apr 2013, 23:00
I would go with C. Here are the reasons

A. Proteins bound to the nucleus - Proteins bound to DNA not nucleus
B. Histones - Synthesis of Histones and not just Histones
C. Maternal messenger RNA's - Correct Referred in the beginning of last 2 paragraphs
D. Cytoplasm - describes the location
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Re: Nearly a century ago, biologists found that if they [#permalink]

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13 Apr 2013, 08:40
IMO C

The third para talks about the "substances" - morphogenetic determinants
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Re: Nearly a century ago, biologists found that if they [#permalink]

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14 Apr 2013, 21:09
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Option C is correct answer which can be directly inferred from the following lines:
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?
.........
Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development.
..............
Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants.
..................................
The substances that Gross studied are maternal messenger RNA's products of certain of the maternal genes.

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

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13 Jul 2016, 03:42
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Re: Nearly a century ago, biologists found that if they   [#permalink] 13 Jul 2016, 03:42
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# Nearly a century ago, biologists found that if they

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