End skippable figure description.
Question 22.
The authors use the word “backbone” in sentence 2 of paragraph 1 and sentence 1 of paragraph 5 to indicate that
A. only very long chains of D N A can be taken from an organism with a spinal column.
B. the main structure of a chain in a D N A molecule is composed of repeating units.
C. a chain in a D N A molecule consists entirely of phosphate groups or of sugars.
D. nitrogenous bases form the main structural unit of D N A.
Answer and explanation for question 22.
Question 23.
A student claims that nitrogenous bases pair randomly with one another. Which of the following statements in the passage contradicts the student’s claim?
A. “To each sugar is attached a nitrogenous base, which can be of four different types.”
B. “So far as is known, the sequence of bases along the chain is irregular.”
C. “The bases are joined together in pairs, a single base from one chain being hydrogen-bonded to a single base from the other”
D. “One member of a pair must be a purine and the other a pyrimidine in order to bridge between the two chains”
Answer and explanation for question 23.
Question 24.
In the second paragraph, what do the authors claim to be a feature of biological interest?
A. The chemical formula of D N A
B. The common fiber axis
C. The X‑ray evidence
D. D N A consisting of two chains
Answer and explanation for question 24.
Question 25.
The authors’ main purpose of including the information about X‑ray evidence and density is to
A. establish that D N A is the molecule that carries the genetic information.
B. present an alternate hypothesis about the composition of a nucleotide.
C. provide support for the authors’ claim about the number of chains in a molecule of D N A.
D. confirm the relationship between the density of D N A and the known chemical formula of D N A.
Answer and explanation for question 25.
Question 26.
Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it” (sentence 6 of paragraph 3) implies that a pair
A. of purines would be larger than the space between a sugar and a phosphate group.
B. of purines would be larger than a pair consisting of a purine and a pyrimidine.
C. of pyrimidines would be larger than a pair of purines.
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D. consisting of a purine and a pyrimidine would be larger than a pair of pyrimidines.
Answer and explanation for question 26.
Question 27.
The authors’ use of the words “exact,” “specific,” and “complement” in sentences 3 and 4 of the final paragraph functions mainly to
A. confirm that the nucleotide sequences are known for most molecules of D N A.
B. counter the claim that the sequences of bases along a chain can occur in any order.
C. support the claim that the phosphate‑sugar backbone of the authors’ model is completely regular.
D. emphasize how one chain of D N A may serve as a template to be copied during D N A replication.
Answer and explanation for question 27.
Question 28.
Based on the table and passage, which choice gives the correct percentages of the purines in yeast D N A?
A. 17.1% and 18.7%
B. 17.1% and 32.9%
C. 18.7% and 31.3%
D. 31.3% and 32.9%
Answer and explanation for question 28.
Question 29.
Do the data in the table support the authors’ proposed pairing of bases in D N A?
A. Yes, because for each given organism, the percentage of adenine is closest to the percentage of thymine, and the percentage of guanine is closest to the percentage of cytosine.
B. Yes, because for each given organism, the percentage of adenine is closest to the percentage of guanine, and the percentage of cytosine is closest to the percentage of thymine.
C. No, because for each given organism, the percentage of adenine is closest to the percentage of thymine, and the percentage of guanine is closest to the percentage of cytosine.
D. No, because for each given organism, the percentage of adenine is closest to the percentage of guanine, and the percentage of cytosine is closest to the percentage of thymine.
Answer and explanation for question 29.
Question 30.
According to the table, which of the following pairs of base percentages in sea urchin D N A provides evidence in support of the answer to question 29?
A. 17.3% and 17.7%
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B. 17.3% and 32.1%
C. 17.3% and 32.8%
D. 17.7% and 32.8%
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