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Today it seems obvious that genes make proteins and many proteins are enzymes. In the 1930s nobody could show how a gene produced its effect. George Beadle and Edward Tatum found a way by choosing the right organism and turning the usual question around. This post covers their experiment, a worked example of the reasoning they used, and the habits behind it.
What was the question?
Beadle's Nobel lecture traces it to Archibald Garrod, an Oxford physician who studied inherited "inborn errors of metabolism" such as alcaptonuria, in which urine darkens in air. Garrod argued that affected people lack one enzyme, and in 1923 he wrote: "We may further conceive that the splitting of the benzene ring of homogentisic acid in normal metabolism is the work of a special enzyme, that in congenital alcaptonuria this enzyme is wanting." Beadle notes that geneticists of the time largely ignored this.
Beadle came to the same idea through fruit flies. With Boris Ephrussi he transplanted eye tissue between Drosophila larvae and found that two eye-colour mutants, vermilion and cinnabar, each blocked a different step in making eye pigment. In the lecture he writes: "What was later called the 'one gene-one enzyme' concept was clearly in our minds at this time although as I remember, we did not so designate it."
Why switch to bread mould?
Identifying the chemicals behind fly eye colour was slow. So, Beadle explains, they decided "to reverse the procedure and look for gene mutations that influence known chemical reactions." They needed an organism with simple food needs that was easy to study genetically. The bread mould Neurospora crassa fitted. Tatum, the biochemist of the pair, found that apart from salts and sugar its only growth requirement was the vitamin biotin, as both his Nobel biography and Beadle's lecture record.
How did the experiment work?
Beadle's lecture sets out the plan:
Irradiate the mould's spores to cause mutations.
Cross them and collect single spores.
Grow each on a "complete" medium with added vitamins and amino acids.
Transfer each strain to a minimal medium with no supplements. A strain that grows on the complete medium but not the minimal one has lost the ability to make something.
Test it on media with single added substances to find out what it needs.
They prepared more than a thousand single-spore cultures before testing, to avoid being discouraged by early failures. "The 299th spore isolated gave a mutant strain requiring vitamin B6 and the 1,085th one required B1." They vowed to keep going until they had ten mutants, and soon had dozens. Crosses with the original strain gave four mutant and four normal spores in each spore sac, showing that each defect was a change in a single gene. They reported the first results in Genetic Control of Biochemical Reactions in Neurospora, in PNAS in November 1941.
The Nobel Prize summary states the conclusion: "each enzyme corresponds to a particular gene." Beadle and Tatum shared the 1958 prize with Joshua Lederberg, "for their discovery that genes act by regulating definite chemical events."
A worked example: ordering a pathway from mutants
This is the logic behind many genetics and biochemistry questions. The numbers here are sample data, not from the 1941 paper.
A mould makes compound Z, which it needs to grow, through intermediates X and Y. Three mutant strains are tested on minimal medium with one added compound each:
Mutant 1 grows only if Z is added.
Mutant 2 grows if Y or Z is added.
Mutant 3 grows if X, Y or Z is added.
Step 1. A compound rescues a mutant only if it comes after the blocked step.
Step 2. Z rescues all three, so Z is last. Y rescues two, so Y comes before Z. X rescues only one, so X comes first. The order is precursor → X → Y → Z.
Step 3. Place the blocks. Mutant 3 is blocked before X, mutant 2 between X and Y, and mutant 1 between Y and Z.
Step 4. Predict what builds up. A mutant blocked just before a step tends to pile up the compound in front of the block, so mutant 1 should accumulate Y.
The common mistake is to reverse the order: the compound that rescues the most mutants comes last, not first. The same reasoning explains Beadle's fly result: a vermilion eye grown in a cinnabar host became normal, because cinnabar is blocked at the later step and so supplies the earlier substance that vermilion lacks.
If pathways and gene expression still feel abstract, Fermi's AI tutor can work out what you already know and build a plan for the session, so you start from the step you are missing. For the next part of the story, how genes are read into proteins, see our post on the molecular basis of inheritance.
What can a student take from Beadle and Tatum?
Choose a problem you can actually test. Neurospora made the question answerable.
Plan for failure. Preparing a thousand cultures before looking kept them going.
Give credit where it is due. Beadle used his Nobel lecture to say that Garrod had seen the idea decades earlier.
Make up your own four-step pathway tonight, invent three mutants, and ask a friend to order it from your growth data.
