In the production of human insulin by bacteria the human insulin gene is incorporated into the genetic material of these microorganisms. The mutant bacteria multiply forming lineages of insulin-producing bacteria.

In bacteria there are circular strands of DNA called plasmids, minichromosomes which act as an accessory to the main DNA. To create a mutant bacteria capable of producing insulin a plasmid is submitted to the action of restriction

enzymes (restriction endonucleases) specialized in cutting DNA fragments. The once circular plasmid is open by the restriction enzyme. The same enzyme is used to cut a human DNA molecule containing the insulin gene. The piece of human DNA containing the insulin gene then has its extremities bound to the plasmid with the help of DNA ligases.
The recombinant plasmid containing the human insulin gene is then inserted into the bacteria.

Another human hormone already produced by recombinant bacteria is GH (somatotropin, or growth hormone).

The insertion of DNA molecules into cells of an individual is also the method of the gene therapy, a promising treatment for genetic diseases. In gene therapy cells from an organism deficient in the production of a given protein receive (by means of vectors, e.g., virus) pieces of DNA containing the protein gene and they then begin to synthesize the protein.   _{Your Comment}