Inference of a biochemical pathway

    This sketch shows a general model of a biochemical pathway. Substances X, Y and Z are intermediates in a metabolic pathway.  Three enzymes A, B, & C convert some precursor to X, X to Y, and Y to Z, respectively. If all enzymes are present, the pathway is complete, and growth occurs (+). If any of the enzymes is missing or non-functional, growth cannot occur (-), irrespective of the presence of the other enzymes.  

    In the arginine synthesis pathway of Neurospora, the intermediate substances are the amino acids  arginine (arg), citrulline (cit), and ornithine (orn).  Their relative order in the pathway is unknown. Given data showing the growth response of a set of mutant strains to added amino acids, the problem is (1) to sort out the correspondence of arg, cit, and orn to X, Y & Z, and (2) to sort out the correspondence of the mutant strains to defects in enzymes A, B, and C that interconvert these substances.

    Beadle & Tatum did their experiments before the chemical nature of genes and gene mutations was understood. Their insight was to connect the absence of a particular enzymatic function with a defect in a particular "gene": "One Gene, One Enzyme."   We now understand the molecular process by which genes direct the synthesis of enzymes  (DNA makes RNA makes Protein), and that enzyme defects are due to alterations of the DNA sequence (mutations).

    It is important to recognize that enzyme defects, and the altered phenotypes they produce, are consequences of DNA mutations, not mutations per se. Proteins do not mutate!