Sat 2015 Practice Test #1 Answer Explanations

Choice C is the best answer.
In lines 12-14 the authors claim that 
DNA molecules appear to be comprised of two chains, even though 
“it has often been assumed . . . there would be only one” (lines 15-17). 
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]” (lines 18-19). 
Choices A, B, and D are incorrect because the authors mention 
density and X-ray evidence to support a claim, not to establish that 
DNA carries genetic information, present a hypothesis about the 
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density and chemical formula of DNA. 
QUESTION 26 
Choice B is the best answer.
The authors explain that “only certain pairs 
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These pairs must contain “a purine and the other a pyrimidine in order 
to bridge between the two chains” (lines 27-29), which implies that any 
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Therefore, a pair of purines would be larger than the required purine/ 
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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 DNA 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. 
QUESTION 27 
Choice D is the best answer.
The authors explain how the DNA 
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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 

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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” (lines 45-51). The authors use the words 
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base pairings along a DNA chain is understood and predictable, and 
may explain how DNA “duplicate[s] itself” (line 51). 
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 DNA. Choice C is incorrect because the authors 
are describing the bases attached only to the sugar, not to the sugar-
phosphate backbone. 
QUESTION 28