What did George Beadle and Edward Tatum do?

Answer 1

Beadle and Tatum did a lot of things. The thing they are now famous for is their mold experiment where they obstructed various genes to determine a one gene to one enzyme hypothesis. While this hypothesis was proven wrong, it was a breakthrough experiment that highlighted the relationship between enzymes and genes.

Their experiment subject was a mold called Neurospora crassa, a bread mold. They bombarded it with X-rays, a treatment known to cause genetic changes, and then looked amongst the surviving mold for mutants whose nutritional needs were different from the control type, or wild type, which had very little needs because it was able to produce most of the other molecules it needed. Beadle and Tatum grew the various mutants and wild types in a minimal medium made up of agar, inorganic salts, glucose, and vitamin biotin.

They then identified the mutants that couldn't survive on this minimal medium due to their inability to synthesize the molecules they needed to survive. Tatum and Beadle placed these mutants in a complete growth medium which consisted of the minimal medium with the addition of all 20 amino acids and other nutrients. The mutants survived.

Thing began to get exciting. To figure out the metabolic defects in each nutritional mutant, Beadle and Tatum took samples of the various mutants and placed them in vials containing minimal medium and another single nutrient. If the mutant could survive, the metabolic defect would be related to the single nutrient. They continued this with minimal medium (MM) and various types of single nutrients. They noted that one of the vials that contained surviving mutants was the one with added arginine.

They went on the further narrow down the metabolic defect. They did further tests with more specificity. They went on to distinguish three classes of arginine-requiring mutants because the arginine-synthesizing pathway had three steps and each mutant required different sets of compounds to produce it.

Their results look a little like this:

............................Wild type..Class I Mutants..Class II Mutants..Class III Mutants

MM........................Survived.............Died..................Died.....................Died

MM + ornithine......Survived..........Survived..............Died.....................Died

MM + citruline........Survived..........Survived...........Survived.................Died

MM + arginine.......Survived...........Survived...........Survived..............Survived

where the minimum nutrients are needed to make ornithine through Enzyme A (not the actual name), ornithine is needed to make citruline through Enzyme B, and citruline is needed to make arginine through Enzyme C.

Tatum and Beadle could deduce from their results that each class must be blocked by a different step in this pathway because the mutants in that specific class lacked the enzyme required to catalyze the step.

Therefore Class I must have lacked enzyme A which would have produced ornithine because when it was added, Class I survived. Class II must have lacked enzyme B which would have produced citruline from ornithine, and Class III lacked enzyme C which would have produced arginine from citruline.

Because each of the mutants had a single defected gene, Tatum and Beadle's experiment was strong proof for the one gene-one enzyme hypothesis. They were rewarded with a Nobel Prize in 1958 for, in the words of the Nobel committee, "their discovery that genes act by regulating definite chemical events".

Today we know that one gene doesn't actually code for a single protein or even a single polypeptide (subunits which may make up a larger protein like hemoglobin which is made up of two polypeptides). Because of RNA processing, or post-transcriptional modification, a single gene can code for many (as in hundreds, maybe thousands) of proteins.