"Mendelian genetics was a rather abstract subject, since no one knew what genes were actually made of, or how they operated. The first great leap forward came when biochemists demonstrated that each step in a biochemical pathway was determined by a single gene. Each biosynthetic reaction is carried out by a specific protein known as an enzyme. Each enzyme has the ability to mediate one particular chemical reaction and so the one gene—one enzyme model of genetics (Fig. 1.02) was put forward by G. W. Beadle and E. L. Tatum, who won a Nobel prize for this scheme in 1958. Since then, a variety of exceptions to this simple scheme have been found. For example,some complex enzymes consist of multiple subunits, each of which requires a separate gene."