Sat 2015 Practice Test #1 Answer Explanations

Choice B is the best answer.
Lines 2-4 of the passage describe DNA 
as “a very long chain, the backbone of which consists of a regular 
alternation of sugar and phosphate groups.” The backbone of DNA, 
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 DNA 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 DNA; rather, they 
are attached only to the repeating units of sugar. 
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 
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be a purine and the other a pyrimidine in order to bridge between the 
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nitrogenous base, it would be inaccurate to call the process random. 
Choice A is incorrect because lines 5-6 describe how nitrogenous 
bases attach to sugar but not how those bases pair with one another. 
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nitrogenous bases is. 

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Choice D is the best answer.
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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 lines 12-14 explicitly state that 
it is the two chains of DNA that are of “biological interest,” not the 
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are wrapped around. Choice C is incorrect because, while the X-ray 
evidence did help Watson and Crick to discover that DNA consists of 
two chains, it was not claimed to be the feature of biological interest. 
QUESTION 25