Answer and explanation for question 30.
Question 31.
Based on the table, is the percentage of adenine in each organism’s D N A the same or does it vary, and which statement made by the authors is most consistent with that data?
A. The same; “Two of the possible bases—adenine and guanine—are purines, and the other two—thymine and cytosine—are pyrimidines.”
B. The same; “The important point is that only certain pairs of bases will fit into the structure.”
C. It varies; “Adenine for example, can occur on either chain; but when it does, its partner on the other chain must always be thymine.”
D. It varies; “It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of bases is the code which carries the genetical information.”
Answer and explanation for question 31.
Answers and explanations for questions 22 through 31 are provided in the next section of this document. You may skip directly to the beginning of the next passage if you do not want to review answers and explanations now.
Answers and Explanations for Questions 22 through 31
Explanation for question 22.
Choice B is the best answer. Sentence 2 of paragraph 1 of the passage describes D N A as “a very long chain, the backbone of which consists of a regular alternation of sugar and phosphate groups.” The backbone of D N A, in other words, is the main structure of a chain made up of repeating units of sugar and phosphate.
Choice A is incorrect because the passage describes D N A on the molecular level only and never mentions the spinal column of organisms. Choice C is incorrect because the passage describes the backbone of the molecule as having “a regular alternation” of sugar and phosphate, not one or the other. Choice D is incorrect because the nitrogenous bases are not the main structural unit of D N A; rather, they are attached only to the repeating units of sugar.
Explanation for question 23.
Choice D is the best answer. The authors explain that hydrogen bonds join together pairs of nitrogenous bases, and that these bases have a specific structure that leads to the pairing: “One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains” (sentence 5 of paragraph 3). Given the specific chemical properties of a nitrogenous base, it would be inaccurate to call the process random.
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Choice A is incorrect because sentence 3 of paragraph 1 describes how nitrogenous bases attach to sugar but not how those bases pair with one another. Choice B is incorrect because sentence 5 of paragraph 1 does not contradict the student’s claim. Choice C is incorrect because sentence 3 of paragraph 3 describes how the two molecules’ chains are linked, not what the specific pairing between nitrogenous bases is.
Explanation for question 24.
Choice D is the best answer. In sentence 1 of paragraph 2 the authors state: “The first feature of our structure which is of biological interest is that it consists not of one chain, but of two.”
Choices A and B are incorrect because sentence 1 of paragraph 2 explicitly states that it is the two chains of D N A that are of “biological interest,” not the chemical formula of D N A, nor the common fiber axis those two chains are wrapped around. Choice C is incorrect because, while the X-ray evidence did help Watson and Crick to discover that D N A consists of two chains, it was not claimed to be the feature of biological interest.
Explanation for question 25.
Choice C is the best answer. In sentence 1 of paragraph 2 the authors claim that D N A molecules appear to be comprised of two chains, even though “It has often been assumed …there would be only one” (sentence 3 of paragraph 2). The authors support this claim with evidence compiled from an X-ray: “the density, taken with the X-ray evidence, suggests very strongly that there are two [chains]” (sentence 4 of paragraph 2).
Choices A, B, and D are incorrect because the authors mention density and X-ray evidence to support a claim, not to establish that D N A carries genetic information, present a hypothesis about the composition of a nucleotide, or confirm a relationship between the density and chemical formula of D N A.
Explanation for question 26.
Choice B is the best answer. The authors explain that “only certain pairs of bases will fit into the structure” (sentence 4 of paragraph 3) of the D N A molecule. These pairs must contain “a purine and the other a pyrimidine in order to bridge between the two chains” (sentence 5 of paragraph 3), which implies that any other pairing would not “fit into the structure” of the D N A molecule. Therefore, a pair of purines would be larger than the required purine/pyrimidine pair and would not fit into the structure of the D N A molecule.
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Choice A is incorrect because this section is not discussing the distance between a sugar and phosphate group. Choice C is incorrect because the passage never makes clear the size of the pyrimidines or purines in relation to each other, only in relation to the space needed to bond the chains of the D N A molecule. Choice D is incorrect because the lines do not make an implication about the size of a pair of pyrimidines in relation to the size of a pair consisting of a purine and a pyrimidine.
Explanation for question 27.
Choice D is the best answer. The authors explain how the D N A molecule contains a “precise sequence of bases” (sentence 2 of paragraph 5), and that the authors can use the order of bases on one chain to determine the order of bases on the other chain: “If the actual order of the bases on one of the pair of chains were given, one could write down the exact order of the bases on the other one, because of the specific pairing. Thus one chain is, as it were, the complement of the other, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself” (sentences 3 and 4 of paragraph 5). The authors use the words “exact,” “specific,” and “complement” in these lines to suggest that the base pairings along a D N A chain is understood and predictable, and may explain how D N A “duplicate[s] itself” (sentence 4 of paragraph 5).
Choice A is incorrect because the passage does not suggest that most nucleotide sequences are known. Choice B is incorrect because these lines are not discussing the random nature of the base sequence along one chain of D N A. Choice C is incorrect because the authors are describing the bases attached only to the sugar, not to the sugar-phosphate backbone.
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Explanation for question 28.
Choice C is the best answer. Sentence 4 of paragraph 1 states that “Two of the possible bases—adenine and guanine—are purines,” and on the table the percentages of adenine and guanine in yeast D N A are listed as 31.3% and 18.7% respectively.
Choices A, B, and D are incorrect because they do not state the percentages of both purines, adenine and guanine, in yeast D N A.
Explanation for question 29.
Choice A is the best answer. The authors state “We believe that the bases will be present almost entirely in their most probable forms. If this is true, the conditions for forming hydrogen bonds are more restrictive, and the only pairs of bases possible are: adenine with thymine, and guanine with cytosine” (sentences 1 and 2 of paragraph 4). The table shows that the pairs adenine/thymine and guanine/cytosine have notably similar percentages in D N A for all organisms listed.
Choice B is incorrect. Although the choice of “Yes” is correct, the explanation for that choice misrepresents the data in the table. Choices C and D are incorrect because the table does support the authors’ proposed pairing of nitrogenous bases in D N A molecules.
Explanation for question 30.
Choice A is the best answer as it gives the percentage of cystosine (17.3%) in sea urchin D N A and the percentage of guanine (17.7%) in sea urchin D N A. Their near similarity pairing supports the authors’ proposal that possible pairings of nitrogenous bases are “adenine with thymine, and guanine with cytosine” (sentence 2 of paragraph 4).
Choices B, C, and D do not provide the best evidence for the answer to question 29. Choice B (cytosine and thymine), Choice C (cytosine and adenine), and Choice D (guanine and adenine) are incorrect because they show pairings of nitrogenous bases that do not compose a similar percentage of the bases in sea urchin D N A.
Explanation for question 31.
Choice D is the best answer. The table clearly shows that the percentage of adenine in each organism’s D N A is different, ranging from 24.7% in E.coli to 33.2% in the octopus. That such a variability would exist is predicted in sentence 2 of paragraph 5, which states that “in a long molecule many different permutations are possible.”
Choices A and B are incorrect because the table shows that the percentage of adenine varies between 24.7% and 33.2% in different organisms. Choice C is incorrect because sentence 3 of paragraph 4 states that adenine pairs with thymine but does not mention the variability of the base composition of D N A.
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