This passage is adapted from J. D. Watson and F. H. C. Crick

The authors use the word “backbone” in lines 3 and 39 to indicate that

• only very long chains of DNA can be taken from an organism with a spinal column

• the main structure of a chain in a DNA molecule is composed of repeating units.

• a chain in a DNA molecule consists entirely of phosphate groups or of sugars.

• nitrogenous bases form the main structural unit of DNA.

A student claims that nitrogenous bases pair randomly with one another. Which of the following statements in the passage contradicts the student’s claim?

• Lines 5-6 (“To each... types”)

• Lines 9-10 (“So far... irregular”)

• Lines 23-25 (“The bases... other”)

• Lines 27-29 (“One member... chains”)

In the second paragraph (lines 12-19), what do the authors claim to be a feature of biological interest?

• The chemical formula of DNA

• The common fiber axis

• The X-ray evidence

• DNA consisting of two chains

The authors’ main purpose of including the information about X-ray evidence and density is to

• establish that DNA is the molecule that carries the genetic information

• present an alternate hypothesis about the composition of a nucleotide.

• provide support for the authors’ claim about the number of chains in a molecule of DNA.

• confirm the relationship between the density of DNA and the known chemical formula of DNA.

26 Based on the passage, the authors’ statement “If a pair consisted of two purines, for example, there would not be room for it” (lines 29-30) implies that a pair

• of purines would be larger than the space between a sugar and a phosphate group.

• of purines would be larger than a pair consisting of a purine and a pyrimidine.

• of pyrimidines would be larger than a pair of purines

• consisting of a purine and a pyrimidine would be larger than a pair of pyrimidines.

The authors’ use of the words “exact,” “specific,” and “complement” in lines 47-49 in the final paragraph functions mainly to

• confirm that the nucleotide sequences are known for most molecules of DNA.

• counter the claim that the sequences of bases along a chain can occur in any order

• support the claim that the phosphate-sugar backbone of the authors’ model is completely regular.

• emphasize how one chain of DNA may serve as a template to be copied during DNA replication.

Based on the table and passage, which choice gives the correct percentages of the purines in yeast DNA?

• 17.1% and 18.7%

• 17.1% and 32.9%

• 18.7% and 31.3%

• 31.3% and 32.9%

Do the data in the table support the authors’ proposed pairing of bases in DNA?

• 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

• 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.

• 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

• 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.

According to the table, which of the following pairs of base percentages in sea urchin DNA provides evidence in support of the answer to the previous question?

• 17.3% and 17.7%

• 17.3% and 32.1%

• 17.3% and 32.8%

• 17.7% and 32.8%

Based on the table, is the percentage of adenine in each organism’s DNA the same or does it vary, and which statement made by the authors is most consistent with that data?

• The same; “Two of... pyrimidines” (lines 6-8)

• The same; “The important... structure” (lines 25-26)

• It varies; “Adenine... thymine” (lines 36-38)

• It varies; “It follows... information” (lines 41-45)