What Was Chargaff's Rule and How Did It Disprove the Tetranucleotide Hypothesis? (AHL)

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Chargaff analysed DNA from different organisms and found two important patterns:

For example, a DNA sample might contain $30\%$ adenine, $30\%$ thymine, $20\%$ guanine and $20\%$ cytosine. Chargaff’s rule is satisfied because $A=T$ and $G=C$, but the tetranucleotide prediction is not satisfied because the four percentages are unequal.

A common misconception is that Chargaff discovered complementary base pairing itself. He established the quantitative base relationships; Watson and Crick later used these data, together with structural evidence, to develop the double-helix model with complementary base pairs.

Exam Technique

This is AHL content in A1.2 Nucleic acids. For an “explain” question, state both the tetranucleotide prediction and Chargaff’s contradictory evidence. Do not merely write “A equals T and G equals C”; also explain that species-specific, unequal base proportions showed DNA was sufficiently variable to store genetic information.