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 1944, Avery and MacLeod, and McCarty purified the transforming principle of Fred Griffith's earlier experiment with pneumocossus bacteria. By a process of elimination, these scientists showed that DNA was the factor that caused genetic transformation.
DNA transformation is a naturally occuring but rare event in which DNA can be transferred into bacteria. In 1970, Morton Mandel and Akiko Higa discovered a way to make E. coli more "competent" for transforming foreign DNA. Their calcium chloride method is