Stanley Norman Cohen

Stanley Norman Cohen is an American geneticist.

Originally from Perth Amboy, New Jersey, Cohen is a graduate of Rutgers University, and received his doctoral degree from the University of Pennsylvania School of Medicine in 1960. Following subsequent training at various institutions, including the National Institutes of Health, he joined the faculty of Stanford University in 1968.

It was there that he began to explore the field of bacterial plasmids. He wanted to understand how the genes of plasmids could make bacteria resistant to antibiotics. In 1972, Cohen's investigations, combined with those of Herbert Boyer, led to the development of methods to combine and transplant genes. This discovery signalled the birth of genetic engineering, and he received National Medal of Science(1988) in his honor. Today, Cohen is a professor of genetics and medicine at Stanford, where he works on a variety of scientific problems including cell growth and development.

Experiment
Stanley Cohen and Herbert Boyer made what would be one of the first genetic engineering experiments, in 1973. They demonstrated that the gene for frog ribosomal RNA could be transferred into bacterial cells and expressed by them. First they constructed a plasmid, which would be the vector, called pDC101. This plasmid contained a single site for the restriction enzyme EcoR1 and a gene for tetracycline resistance. The restriction enzyme EcoR1 was used to cleave the frog DNA into small segments. Next, the frog DNA fragments were combined with the plasmid, which had also been cleaved with EcoR1. The sticky ends of the DNA segments aligned themselves and were afterwards joined together using DNA ligase. The plasmids were then transferred into a strain of E. coli and plated onto a growth medium containing tetracycline. The cells that incorporated the plasmid carrying the tetracycline gene grew and formed a colony of bacteria. Some of these colonies consisted of cells that carried the frog ribosomal RNA gene. The scientists then tested the colonies that formed after growth for the presence of frog ribosomal DNA.