EXPRESSION AND SCREENING
Once the recombinant DNA is assembled, it is ready to be introduced into bacterial cells, in this case E. coli. The process by which plasmids enter bacterial cells is called transformation. This can happen in nature, leading to the spread of antibiotic resistance genes in a population, or it can be forced in laboratories to produce recombinant proteins.

In the lab, we place the transformed bacteria on nutrient agar containing an antibiotic and a chemical that turns the colonies blue if they have a functional lacZ gene. The presence of the antibiotic stops the growth of bacteria that did not take up a plasmid. The chemical additive shows which colonies have our gene of interest, in this case insulin.

When insulin is correctly inserted into the plasmid, it knocks out expression of lacZ, so the colonies remain white. If the bacterial cells can grow into a blue colony on antibiotic-laced agar, they must have a plasmid, but the plasmid does not have insulin inserted in it.

White colonies, with the insulin gene, can then be purified, and human insulin can be mass-produced in bacteria. This recombinant insulin can be made cheaply, and it is more effective in the treatment of diabetes than animal insulin ever was.