Suppose a scientist wants to clone the gene for human insulin. He uses a plasmid that contains the lacZ gene, an origin of replication, and an ampicillin resistant gene. He inserts the human insulin gene into a restriction site located within the lacZ gene. Then he transforms this plasmid into bacteria without a functional lacZ gene and spreads the appropriate dilution onto bacterial plates. When the product of the lacZ gene, beta‑galactosidase, interacts with the subtrate X‑Gal on LB agar, colonies turn blue. Which plate indicates that the human insulin gene was successfully inserted in the plasmid?

Insertion inactivation is the process when a gene of interest is inserted into the sequence of another gene and thereby inactivating the expression on this gene. If the gene sequence into which the gene of interest is cloned produces a visible product, the process can be used to differentiate the recombinant colonies from the non-recombinants.

In the given example, the human insulin gene was inserted into the lac Z gene. This would stop the expression of the lac Z gene. The transformed cells that have successfully taken up the recombinant plasmids would not produce the blue colonies. Since the plasmid also carried the ampicillin resistance gene, the transformed cells would be able to grow in the presence of ampicillin while the non-transformed cells would not be able to do so. Therefore, the colorless colonies growing on the X gal+ampicillin rich medium are the recombinant colonies.