FIGURE 5-13 Gene organization in prokaryotes and in eukaryotes. (a) The tryptophan (trp) operon is a continuous segment of the E. coli chromosome containing five genes (blue) that encode the enzymes necessary for the stepwise synthesis of tryptophan. The entire operon is transcribed from one promoter into one long continuous trp mRNA (red). Translation of this mRNA begins at five different start sites, yielding five proteins (green). The order of the genes in the bacterial genome parallels the sequential function of the encoded proteins in the tryptophan synthesis pathway. (b) The five genes encoding the enzymes required for tryptophan synthesis in baker’s yeast (Saccharomyces cerevisiae) are carried on four different chromosomes. Each gene is transcribed from its own promoter to yield a primary transcript that is processed into a functional mRNA encoding a single protein. The lengths of the various chromosomes are given in kilobases (10³ bases).