How do scientists use recombinant DNA technology to produce the drug insulin?

First, scientists isolate the gene that controls insulin production. Then, they take that gene and insert into a plasmid (a circular ring of DNA found in bacteria). When the bacterium with that plasmid reproduces, it carries that insulin-producing gene. Then they can either mass-reproduce that bacteria or transfer the plasmid to another rapidly-reproducing bacteria. After some time, they harvest the insulin from the bacteria.