The plasmid is introduced into a bacterium by transformation so that more copies of plasmid (and the gene inserted into the plasmid, too) can be made. Usually each bacterial cell picks up one plasmid. Typically Escherichia coli, an intestinal bacteria, is used as the "friendly workhorse." The new recombinant bacteria has a new gene, the DNA encoding a protein - that now instructs the bacteria to make a new mRNA which in turn makes a new protein. Our goal is to make a bacterium that can make human insulin.

Because a total mRNA extract from pancreas was used, you actually have all the active genes in pancreas. To find the specific recombinant bacterium with the target insulin mRNA/cDNA, you need to "screen". In this process, a probe that recognizes either the insulin sequence of DNA (a hybridization probe that will match up with the insulin gene sequence) or the insulin protein (an antibody that will stick to insulin made by the recombinant bacteria).