Chapter 9, Problem 19P

The lacZ gene from E. coli encodes the enzyme β-galactosidase, which can catalyze the conversion of a colorless compound called X-gal into a blue product. Molecular biologists have taken advantage of this property by constructing plasmid vectors that contain the lacZ gene with an EcoRI site in its middle (see figure that follows). After cutting this vector with the EcoRI enzyme, scientists ligate it together with EcoRI-digested human genomic DNA, transform the resultant molecules into ampicillin-sensitive E. coli cells, and plate these cells on petri plates containing ampicillin and X-gal. Some of the colonies growing on this plate are white in color, while others are blue. Why?