A biosynthesis (in this case the biosynthesis of the amino acid arginine) does usually occur in several steps. In the wild type, the necessary genes (5 in the case of arginine) are present, the enzymes E1 - E5 are produced and accordingly also the product arginine (Arg). The mutants 1 - 4 have a genetic block that causes an interruption of the biosynthesis pathway. The intermediate products (D, C, B or A) accumulate within the cells. The cells can keep growing only if arginine is supplied among the nutriments (= auxothrophic mutants).
