Recombinant DNA

Recombinant DNA is DNA that has been deliberately assembled from more than one source. It rests on two tools. The first is restriction enzymes — molecular scissors that cut DNA only where a specific short sequence appears, and cut in a staggered way that leaves short single-stranded "sticky ends." The second is the plasmid: a small ring of DNA that bacteria carry and copy. Cut a plasmid and a piece of foreign DNA with the *same* enzyme, and their sticky ends are complementary; they base-pair together, and an enzyme seals the joint into one continuous ring. Put that ring back into a bacterium, and as the bacterium divides it copies the foreign gene along with its own — millions of identical copies, a "clone."

This is the moment biology became an engineering discipline. For the whole history of life, genes had moved only down family lines, parent to offspring within a species. Recombinant DNA let a scientist take a gene from any organism and install it in another — across the gulf between animal and bacterium, between kingdoms — because the genetic code is universal and every cell's DNA is the same chemical. It turned the living cell into a programmable factory: give a bacterium the human gene for a protein, and it will manufacture that protein by the vatful.

Stanley Cohen at Stanford had learned to move plasmids into bacteria; Herbert Boyer at UC San Francisco worked with the restriction enzyme EcoRI, which leaves clean sticky ends. Recognizing over that late Hawaiian dinner that the pieces fit, they combined them in 1973: cutting Cohen's plasmid pSC101 with EcoRI, splicing in DNA cut by the same enzyme, and returning the recombinant ring to *E. coli*, which faithfully replicated it. The following year they went further and inserted ribosomal genes from an African clawed frog, *Xenopus* — and the bacterium not only copied the frog DNA but transcribed it. A gene had crossed a billion years of evolutionary distance and still worked.

Recombinant DNA is the foundation of the entire biotechnology industry. Within a few years it was used to make human insulin in bacteria, ending reliance on insulin harvested from animal pancreases, and then human growth hormone, clotting factors, and vaccines. It gave rise to gene cloning, genetically modified crops, and the routine "molecular cloning" that underlies most laboratory biology. Its power also prompted the scientists themselves to call the 1975 Asilomar conference, where they debated and set the first safety rules for their own new craft.