The discovery of transformation by Frederick Griffith in Streptococcus pneumonia has played an important role in how we are now able to introduce plasmid DNA molecules into cells. Transformation is the uptake of DNA molecules released from the donor cell by the recipient cell. It is one of the three ways bacteria are able to exchange genetic material. In Griffith’s experiment he introduced mice to two different forms of S. pneumonia, one smooth, pathogenic and encapsulated and the other rough, nonpathogenic and noncapsulated (Snustad, 193). The mice were injected with live rough strain and heat killed smooth strain. The deaths of the mice lead Griffith to conclude that some genes of the killed smooth strain were transformed to the rough strain and the bacteria became encapsulated and pathogenic, therefore leading to the death of the mice (Snustad, 193).
Plasmids are small circular DNA molecules. They are not essential for survival of the host bacteria. Some carry genes that allow resistance to antibiotics (Anderson). Plasmid pUC18 is a circular DNA molecule. It contains portions of the E. coli Lac Z gene, which encodes for the first 146 amino acids of β - galactosidase. E. coli contains the Lac Z gene, which encodes β - galactosidase . The E. coli Lac operon digests lactose. Once E. coli is transformed with pUC18, complementation occurs. E. coli produces active β –galactoidase. The active β –galactoidase hydrolyzes the substrate, X gal , which is located on the agar plates.