Each reaction of intermediary metabolism occurs as stepwise conversions of one substance into another. All the steps which transform a precursor substance to its end product which is ultimately expressed in to a structural or functional phenotypic trait, constitute a biosynthetic pathway. Each step of a biosynthetic pathway is catalyzed by a specific enzyme which in its turn being produced by a specific gene. It has become clear now that ( 1) DNA of genes itself, does not have an enzymatic character, therefore, does .not involve directly in a biosynthetic pathway of cell; (2) between the gene and final product or between the gene and enzyme there is a long, tortuous path; (3) the immediate or primary gene product is a kind of RNA called mRNA (messenger ribonucleic acid) which is complementary to the DNA of the genes and which controls the assemblage of the amino acids to form enzymes, at the surface of cytoplasmic ribosomes. Thus, to produce a particular phenotypic trait DNA transcribes mRNA which translates into an enzymatic or structural protein which ultimately produces a phenotypic trait by the process of phenogenesis. The relationship between genes and enzymes had been suspected very early in the history of biochemical or physiological genetics.
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