In 1979, Stanley Norman Cohen and Herbert Wayne Boyer filed for and received the U.S. patent for the "Process for Producing Biologically Functional Molecular Chimeras." The patent explains how the two men and their coworkers created functional recombinant molecules that produced a novel product they wanted.

Plasmids are small, extra-chromosomal, circular deoxyribonucleic acid (DNA) molecules that are found in many bacteria and that are capable of self-replication (copying). Intact genes put into these plasmids will be copied and expressed just as any other "natural" gene found on the plasmid. To produce a recombinant plasmid according to the methods developed by Cohen and Boyer, one first takes the chosen gene and removes it from whatever DNA strand it inhabits by cutting it out with a restriction enzyme (typically a restriction enzyme that creates "sticky ends"). Then, the bacterial plasmid is treated with the same restriction enzyme. The two DNAs are mixed together and DNA ligase is added. The sticky ends match up, and the ligase "patches" the strands together to reconstitute a complete plasmid, only now the plasmid is a recombinant.

Host cells, such as Escherichia coli (E. coli), when exposed to calcium chloride (a simple salt), become more permeable to molecules outside their cell walls. When these salt-treated cells are mixed with DNA under the correct conditions, the DNA enters the cells. Once the cells have recovered from this transformation procedure, they begin to manufacture the protein products specified by the recombinant plasmid. To separate these bacterial "factories" from bacteria that did not take up recombinant DNA during transformation, some sort of selection mechanism is used.

In Cohen's system, the recombinant plasmids all carry an antibiotic resistance gene, such as that coding for tetracycline resistance. After transformation, exposure to tetracycline in the growth media will kill off all cells that did not take up recombinant DNA. In this way, only the correctly transformed cells are kept, grown, and their recombinant products recovered.

This technique allows a researcher to express the product for any gene he or she cares to study. Insertion of the gene for human insulin into a bacterial plasmid allowed Genentech, the company founded by Boyer, to mass-produce human insulin for the treatment of diabetes. Engineered human insulin works better in patients than animal-produced insulin, thus providing better treatment outcomes for patients. Human insulin was only the first of a long list of synthetic medicines produced by recombinant DNA technology.

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