Suppose you want to isolate E. coli mutants incapable of synthesizing the amino acid arginine. You grow mutagenized bacteria on a master plate of complete defined media and make a replica plate (by pressing a velvet block onto the master plate and then pressing it onto the replica plate—bacteria will be transferred) on media lacking arginine. Colonies that grew on the master plate but not the replica plate are arginine-synthesizing mutants. In this example, the replica plate