Beadle and Tatum linked genes to biochemistry by proposing there was one gene for each enzyme.

Much of modern molecular biology deals with how genes are regulated. (See Chapters 9, 10 and 11.)

Genes Determine Each Step in Biochemical Pathways

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.

A gene determining whether flowers are red or white would be responsible for a step in the biosynthetic pathway for red pigment. If this gene were defective, no red pigment would be made and the flowers would take the default coloration—white. It is easy to visualize characters such as the color of flowers, pea pods or seeds in terms of a biosynthetic pathway that makes a pigment. But what about tall versus dwarf plants and round versus wrinkled seeds? It is difficult to interpret these in terms of a single pathway and gene product. Indeed, these properties are affected by the action enzyme A protein that carries out a chemical reaction protein A polymer made from amino acids;proteins make up most of the structures in the cell and also do most of the work

FIGURE 1.03 Wild-type and Mutant Genes

If red flowers are found normally in the wild, the "red" version of the gene is called the wild-type allele. Mutation of the wild-type gene may alter the function of the enzyme so ultimately affecting a visible characteristic. Here, no pigment is made and the flower is no longer red.

Wild-type gene

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