Illustration of bacterium containing plasmids with foreign DNA. By the end of the 1960s, DNA could be cut and pasted. But scientists needed a mechanism to copy it in order to maintain a large enough sample to work with. That breakthrough came in 1971, when scientists found an efficient way to introduce tiny loops of DNA called plasmids into E. coli. These loops of plasmid DNA can carry a package of foreign DNA into the host cell, and confer a benefit such as drug resistance. Scientists can import plasmids containing a selected piece of DNA into a bacterium. Normal bacterial reproduction produces huge quantities of the desired DNA.
Stanley Cohen and Herbert Boyer inserted the recombinant DNA molecule they created into E. coli bacteria by means of a plasmid, thereby inducing the uptake and expression of a foreign DNA sequence known as "transformation."
In 1974, scientists in the field of recombinant DNA drafted a letter calling upon "scientists throughout the world" to suspend certain types of studies until hazards could be assessed. Paul Berg talks about the "Moratorium Letter."