The big idea: Chargaff measured the bases in DNA. They were not in equal amounts, but A always matched T and G matched C.
The points to remember
- The tetranucleotide hypothesis (1910) said DNA has equal amounts of A, G, C and T, in a repeating sequence.
- A short repeat of four bases could not carry much information, so DNA looked too simple to be the genetic material.
- Around 1950 Chargaff measured the percentage of each base in DNA from many species.
- The amounts were not equal, and they differed between species.
- But in every species %A ≈ %T and %G ≈ %C, so purines ≈ pyrimidines (a ratio of about 1).
Remember it as: Chargaff's rule: A = T, G = C.
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The big idea: No number of agreeing results can prove a hypothesis, but one clear disagreeing result can disprove it. Chargaff's data did that to the tetranucleotide hypothesis.
The points to remember
- Induction: however many results fit a hypothesis, they can never prove it; the next result might not fit.
- Falsification: one clear result that contradicts a hypothesis is enough to reject it.
- Chargaff's unequal amounts contradicted the tetranucleotide hypothesis, so it was falsified.
- His A = T and G = C ratios were later explained by complementary base pairing in the double helix (1953).
Evaluating a hypothesis with data: Look at every row. Say whether the data support the hypothesis, then name any value that does not fit (an anomaly) and what might explain it, such as measurement error.
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How it is asked: Paper 1A: fill in a missing percentage using A = T and G = C. Paper 1B: a table of ratios, then explain how it falsifies the tetranucleotide hypothesis, or evaluate a hypothesis about the ratios.
A table shows base ratios in DNA from several organisms. Explain how these results falsify the tetranucleotide hypothesis.
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The one-mark evaluation that often follows:
Using the data, evaluate the hypothesis that the ratio of purines to pyrimidines in DNA is 1.00 in all organisms.
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See the answer broken into clear points, with guidance on how each point earns marks.