Erwin Chargaff analysed the chemical composition of DNA from different species. What crucial fact relevant to the DNA story did he discover?

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When evaluating Chargaff’s findings on DNA composition, we need to test each proposed equality against what his data showed about base pairing.
Option 1: 'All species had equal numbers of the bases Adenine, Cytosine, Guanine and Thymine.' This would imply a uniform 1:1:1:1 distribution among all four bases across species, which is not what Chargaff observed. In reality, A pairs with T and C pairs with G, so A and T are equal to each other, and C and G are equal to each other, but the four bases are not necessarily present in equal overall amounts.
Option 2: 'All species had equal numbers of the bases Adenine and Guanine.' This mixes A with G, which is not the pairing rule Chargaff identified. A pairs with T, not with G, so stating A equals G across species misrepresents the specific base-pair relationships he uncovered.
Option 3: 'All species had equal numbers of the bases Adenine and Thymine.' This aligns with Chargaff’s observation that A and T are present in equal amounts within a given species’ DNA, reflecting the complementary pairing A with T. This statement captures the key implication of his rule and is consistent with the well-known Chargaff rules (A = T and C = G).
Option 4: 'All species had equal numbers of the bases Adenine and Cytosine.' This would assert a direct equality between A and C, which contradicts Chargaff’s findings. The pairing rule does not link A with C, so this option is inaccurate with respect to the established base-pairing relationships.
In summary, option 3 correctly reflects the crucial insight from Chargaff: within each species, adenine and thymine occur in equal amounts, mirroring the A–T pairing, while cytosine and guanine are also equal to each other in the same way. The other options either imply incorrect cross-base equalities or ignore the fundamental A–T and C–G pairing pattern.

My LMS Subjects Quiz Topic 9: Behind the DNA Helix

Erwin Chargaff analysed the chemical composition of DNA from different species. What crucial fact relevant to the DNA story did he discover?

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