Reading Comprehension Questions & Answers - 5 | English Language for UPSC CSE PDF Download

Based on the Passage, answer the following questions:
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 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.
Q1: It can be inferred from the passage that the morphogenetic determinants present in the early embryo are 
(a) located in the nucleus of the embryo cells 
(b) evenly distributed unless the embryo is not developing normally 
(c) inactive until the embryo cells become irreversibly committed to their final function
(d) present in larger quantities than is necessary for the development of a single individual 

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Ans: (d)
Sol: The second and third paragraphs of the passage indicate that morphogenetic determinants are substances in the embryo that are activated after the egg has been fertilized and that “tell a cell what to become”. If, as the author asserts in the first paragraph, biologists have succeeded in dividing an embryo into two parts, each of which survives and develops into a normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early embryo is greater than that required for the development of a single individual.

Q2: The main topic of the passage is 
(a) the early development of embryos of lower marine organisms 
(b) the main contribution of modern embryology to molecular biology 
(c) the role of molecular biology in disproving older theories of embryonic development 
(d) cell determination as an issue in the study of embryonic development 

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Ans: (d)
Sol: In identifying the main topic of the passage, you must consider the passage as a whole. In the first paragraph, the author provides a historical context for the debate described in the second paragraph, concerning when and how the determination of embryo cells takes place. The third and fourth paragraphs provide a specific example of the “Recent discoveries in molecular biology” that may lead to the resolution of that debate.

Q3: 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 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 different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned  

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Ans: (d)
Sol: According to the author, early investigators arrived at the conclusion that the cells of the embryo are undetermined because they “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”. However, later biologists discovered that when an embryo was cut in places different from the one used by the early investigators, it did not form two whole embryos. Because the earlier biologists apparently arrived at their conclusion without attempting to cut an embryo in different planes, it would appear that they assumed, erroneously, that different ways of separating the embryos would not affect the fate of the two embryo parts.

Q4: 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 

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Ans: (a)
Sol: In the third paragraph, the author asserts that substances that function as 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”. Thus, it can be inferred that after the egg is fertilized, the initial production of histones occurs in the cytoplasm.

Q5: It can be inferred from the passage that which of the following is dependent on the fertilization of an egg? 
(a) Copying of maternal genes to produce maternal messenger RNA’s 
(b) Synthesis of proteins called histones 
(c) Division of a cell into its nucleus and the cytoplasm 
(d) Determination of the egg cell’s potential for division 

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Ans: (b)
Sol: Lines in the passage indicate that substances that function as morphogenetic determinants are inactive in the unfertilized egg and that when the egg is fertilized, they “become active and, presumably, govern the behavior of the genes they interact with.” In the fourth paragraph, we learn that these substances exert their control over the fate of the cell by directing “the synthesis of histones.” Because these histones cannot be synthesized until the substances that function as morphogenetic determinants become active, and because these substances do not become active until the egg is fertilized, it can be inferred that the synthesis of the histones is dependent on the fertilization of the egg.

6. 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 

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Ans: (c)
Sol: In his study of sea urchins, Gross “found that an unfertilized egg contains substances that function as morphogenetic determinants.” The passage asserts that the “substances that Gross studied are maternal messenger RNA’s,” and we learn that these maternal messenger RNA’s can be found in “ a wide variety of organisms”.