Erwin Chargaff investigated the nucleotide composition of DNA. He analyzed DNA from various organisms and measured the relative amounts of adenine (A), guanine (G), cytosine (C), and thymine (T) present in the DNA of each organism. Table 1 contains a selected data set of his results.

Table 1. Nucleotide composition of selected organismsample DNA from
Organism %A %G %C %T %(G+C) %(A+T)
Octopus 33.2 17.6 17.6 31.6 35.2 64.8
Chicken 28.0 22.0 21.6 28.4 43.7 56.4
Rat 28.6 21.4 20.5 28.4 42.9 56.0
Grasshopper 29.3 20.5 20.7 29.3 41.2 58.6
Wheat 27.3 22.7 22.8 27.1 45.5 54.4

Which of the following statements best explains the data set?

Question

Erwin Chargaff investigated the nucleotide composition of DNA. He analyzed DNA from various organisms and measured the relative amounts of adenine (A), guanine (G), cytosine (C), and thymine (T) present in the DNA of each organism. Table 1 contains a selected data set of his results.

Table 1. Nucleotide composition of selected organismsample DNA from
Organism	%A	%G	%C	%T	%(G+C)	%(A+T)
Octopus	33.2	17.6	17.6	31.6	35.2	64.8
Chicken	28.0	22.0	21.6	28.4	43.7	56.4
Rat	28.6	21.4	20.5	28.4	42.9	56.0
Grasshopper	29.3	20.5	20.7	29.3	41.2	58.6
Wheat	27.3	22.7	22.8	27.1	45.5	54.4

Which of the following statements best explains the data set?

BlackTom AI · Accepted Answer

Rereading the question, we’re looking at Chargaff’s data showing the relative percentages of A, G, C, and T across several organisms, with (G+C) and (A+T) also given. This sets up a test of Chargaff’s rules about base pairing and DNA composition.
Option 1: 'Since the A and the G add up to approximately 50 percent in each sample, adenine and guanine molecules must pair up in a double-stranded DNA molecule.' This claim is incorrect because A and G are purines and do not pair with each other in DNA. In double-stranded DNA, A pairs with T and G pairs with C, not A with G. The sum A+G being near 50% has nothing to do with a direct pairing relationship between A and G.
Option 2: 'Since the A and the T are approximately the same in each sample, adenine and thymine molecules must pair up in a double-stranded DNA molecule.' This statement aligns with Chargaff’s rules: A roughly equals T within each organism, and G roughly equals C. The pairing rule in the DNA double helix is that A pairs with T, and G pairs with C, which explains why A and T are similar in amount across samples.
Option 3: 'Since the %(A+T) is greater than the %(G+C) in each sample, DNA molecules must have a poly-A tail at one end.' The observation that A+T exceeds G+C says something about overall base composition, but it does not imply the existence of a poly-A tail or any structural feature like ends of DNA. This is a non sequitur and not supported by Chargaff’s data.
Option 4: 'Since the C and the T add up to approximately 50 percent in each sample, cytosine and thymine molecules must both contain a single ring.' This is incorrect on multiple counts: first, C and T do not sum to 50% in these samples; their sums with G and A vary. Second, the statement about ring structure (single ring) is unrelated to base-pairing rules and is not derived from these percentages. It confuses is structural chemistry with base-pairing patterns.
In summary, the reasoning shows that the most consistent interpretation with Chargaff’s data is the pairing rule where A ≈ T and G ≈ C in each organism, which is best captured by Option 2. The other options rely on misapplied or incorrect inferences about base pairing, total composition, or molecular structure that do not follow from the presented data.

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