IB Biology HLDNA, RNA & the Genetic CodePaper 1 & 2Nature of Science~8 min read
Chargaff’s Data
Erwin Chargaff did something unglamorous: he counted bases in DNA from lots of different organisms. The pattern he found destroyed the leading theory of DNA structure and handed Watson and Crick the clue they needed.
📘 What you need to know
Erwin Chargaff analysed the DNA composition of different organisms during the 1930s and 1940s.
He found that the number of purine bases equals the number of pyrimidine bases.
He also found that adenine = thymine and guanine = cytosine.
This means a purine can only pair with a pyrimidine, since they have different sizes — the foundation of complementary base pairing.
Phoebus Levene had proposed the tetranucleotide hypothesis: that DNA was a repeating tetramer unit.
Chargaff’s data falsified that hypothesis in the late 1940s by showing DNA composition is organism-specific.
NOS: the problem of induction means a hypothesis can never be proved absolutely true, but it can be falsified. Karl Popper argued that science advances through falsification.
What Chargaff found
During the 1930s and 1940s, Erwin Chargaff analysed the DNA composition of many different organisms. Two clear patterns emerged, and they hold true for every organism tested:
The exact percentages change from organism to organism. The relationships between them never do.
What the pattern means
If A always equals T and G always equals C, the bases must be pairing up. And since purines are larger (two rings) and pyrimidines are smaller (one ring), a purine can only pair with a pyrimidine between the sugar–phosphate backbones — anything else would make the molecule an uneven width.
This forms the foundation of complementary base pairing in DNA.
Notice the order of discovery. Chargaff did not set out to discover base pairing. He measured amounts, published the numbers, and the structural explanation came later from other people. Data can be more useful than the person who collected it realises.
Nature of Science: the problem of induction
The inductive scientific method works like this: a scientist makes observations and collects raw data, analyses it, forms a hypothesis, and then tests that hypothesis with a suitably designed investigation. General conclusions get drawn from specific observations.
There is a hidden weakness in that process. Using data gathered in the past to make general predictions about the future assumes that the future will behave the same way as when you gathered the data.
So it is impossible to prove a hypothesis absolutely true by inductive reasoning — you can never be sure your past observations will hold true in future. This is the problem of induction, and it is the main reason most scientific theories are considered tentative.
The asymmetry at the heart of science
No number of confirmations can prove a hypothesis
A single contradicting result can falsify it
Even if several investigations support a hypothesis, it can still be proven incorrect (falsified) later as new discoveries are made. For this reason the philosopher Karl Popper argued that new scientific knowledge is gained not by inductive steps but by the falsification of existing hypotheses.
🧠
The classic way to picture it
You could see a million white swans and still not have proved that all swans are white. But one black swan settles the matter instantly. Confirmation piles up slowly and proves nothing; falsification happens once and is decisive.
Falsifying the tetranucleotide hypothesis
Now apply that to a real example.
In the early 1900s the biochemist Phoebus Levene discovered the pentose sugars of DNA and RNA. He then suggested that the structure of nucleic acid was a repeating tetramer unit, which he called a nucleotide. This became known as the tetranucleotide hypothesis.
In effect it claimed that DNA was a monotonous repeating block of the four bases in fixed equal amounts — which would make DNA far too simple and repetitive to carry genetic information.
Tetranucleotide hypothesis
What Chargaff’s data showed
Base amounts
All four bases present in equal amounts
A = T and G = C, but A does not equal G
Between species
The same in every organism
Composition is organism-specific
Information capacity
A repeating unit carries no information
Varied sequences can carry huge amounts of information
Why had nobody spotted this earlier? At the time of Levene’s research there were limitations to the analytic techniques available, which made it difficult to determine the relative amounts of nucleotides in nucleic acids. Better methods produced better data.
The tetranucleotide hypothesis was falsified by Chargaff’s data in the late 1940s, which showed the organism-specificity of nucleic acids. When the structure of DNA was determined in the 1950s, it further proved that Levene’s repeating tetramer unit would not be suitable for carrying genetic information from one generation to the next.
This is why the IB pairs Chargaff with Popper. The tetranucleotide hypothesis was never disproved by argument — it was disproved by a measurement that contradicted it. That is falsification working exactly as Popper described.
Worked examples
WE 1
Explain how Chargaff’s data supports complementary base pairing
Explain how Chargaff’s data provides evidence for complementary base pairing in DNA. (3 marks)
Point 1: state the findings
Chargaff found that the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.
Point 2: draw the conclusion
Equal amounts suggest that these bases must pair together in fixed pairs.
Point 3: bring in size
He also found purines equal pyrimidines, and since purines are larger than pyrimidines, each pair must be one purine with one pyrimidine.
Equal amounts + different sizes = A pairs with T, C pairs with Gboth findings are needed — the equal amounts and the purine/pyrimidine sizes
WE 2
Explain falsification using the tetranucleotide hypothesis
Using the tetranucleotide hypothesis as an example, explain why falsification is important in science. (4 marks)
Point 1: the problem with induction
Hypotheses formed by inductive reasoning can never be proven absolutely true, because we cannot be sure past observations will hold in future.
Point 2: what can be done insteadA hypothesis can be falsified by a single result that contradicts it, which is why Popper argued science advances through falsification.
Point 3: the hypothesis in question
Levene proposed that DNA was a repeating tetramer with equal amounts of all four bases.
Point 4: how it was falsified
Chargaff’s data showed base composition is organism-specific and that A = T and G = C rather than all four being equal, so the hypothesis was rejected.
Data that contradicts a hypothesis removes it, and that is how the field moved forwardname Popper and Levene — NOS questions reward using the actual example
💡 Exam tips
Learn Chargaff’s rules as two statements: purines = pyrimidines and A = T, G = C.
Remember the names and roles: Chargaff counted bases, Levene proposed the tetranucleotide hypothesis, Popper argued for falsification.
Use the word falsified, not “disproved by argument” or “shown to be silly”.
For data questions, apply the rules directly: if A = 28%, then T = 28% and the rest is split between G and C.
Mention that limitations in analytic techniques explain why Levene got it wrong — that is a fair, scientific reason.
Say scientific theories are tentative. It is the exact word the syllabus uses.
⚠ Common mistakes
Saying all four bases are present in equal amounts. A = T and G = C, but A does not equal G.
Saying Chargaff discovered the double helix. He provided the data; Watson and Crick built the structure.
Claiming a hypothesis can be proved true. Because of the problem of induction, it can only be supported or falsified.
Confusing Levene with Chargaff. Levene proposed the idea that Chargaff’s data overturned.
Writing that Levene was careless. The analytic techniques of the time were genuinely limited.
Treating falsification as a failure of science. It is how science makes progress.
That completes DNA, RNA & the Genetic Code. Look back across the seven notes and you will see one story: a simple repeating unit, joined into strands, paired by a fixed rule, packaged into a nucleus — and a century of experiments working out that this is where the instructions live.
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