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mutations citrulline-* arginine

The addition of ornithine allows the growth of group I mutants but not group II or group III mutants; thus, muta-

It is important to note that this procedure does not necessarily detect all steps in a pathway; rather, it detects only the steps producing the compounds tested.

Using mutations and this type of reasoning, Beadle and Tatum were able to identify genes that control several biosynthetic pathways in Neurospora. They established that each step in a pathway is controlled by a different enzyme, as shown in Figure 15.3 for the arginine pathway. The results of genetic crosses and mapping studies demonstrated that mutations affecting any one step in a pathway always map to the same chromosomal location. Beadle and Tatum reasoned that mutations affecting a particular biochemical step occurred at a single locus that encoded a particular enzyme. This idea became known as the one gene, one enzyme hypothesis: genes function by encoding enzymes, and each gene encodes a separate enzyme. When research showed that some proteins are composed of more than one polypeptide chain and that different polypeptide chains are encoded by separate genes, this model was modified to become the one gene, one polypeptide hypothesis. __

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