GNN - Genetics and Genomics Timeline

Paul Berg assembled the first DNA molecules that combined genes from different organisms. Results of his experiments, published in 1972, represented crucial steps in the subsequent development of recombinant genetic engineering. By stepwise methods such as he devised, individual genes could be isolated and inserted into mammalian cells or into such rapidly growing organisms as bacteria. The genes themselves could then be studied, and their protein products expressed and even manufactured in quantity.

Paul Berg

The prospect of recombinant DNA emerged from a series of advances in biochemistry—most especially, from discoveries of new enzymes. Particularly important were the restriction enzymes that act as "scissors" to cut molecules of DNA at specific points. Similarly, ligases are enzymes that forge covalent bonds. The discovery of DNA ligase provided a kind of chemical soldering that could restore DNA after a foreign gene was spliced into it. These and other enzymes, captured from nature, could be used as tools in genetic engineering.

In creating hybrid DNA molecules, Berg employed the much-studied SV40 monkey virus and a bacterial virus known as the l (or lambda) bacteriophage. The SV40 virus has few genes, lacks a protein coat, and is is convenient to work with. The l bacteriophage normally invades a type of E. coli, where it replicates according to the nutritional environment. The DNA of both viruses takes the form of closed loops. Berg's original idea was to open the SV40 DNA, and splice into it genes snipped out of the bacteriophage. The virus could then replicate in cells, as in nature, and the products of the bacteriophage genes could also be expressed.

In Berg's cut-and-splice method he created, in the DNA of both viruses, what came to be known as "sticky ends." Restriction enzymes were first used to open the circular units of DNA of phage and virus. In separate operations, types of terminal transferase (another enzyme) were used to add complementary DNA bases (adenine and thymine) to the ends of the molecules. When both kinds of DNA were incubated together, the ends would anneal naturally. Addition of DNA ligase would seal the plasmid. In succeeding with a series of enzymatic reactions, Berg wrote that his methods "are general and offer an approach for covalently joining any two DNA molecules together."

Potential dangers of recombinant genetic engineering emerged even before Berg published his landmark paper. Although the SV40 virus was thought to be innocuous in humans, the prospect of an altered form of the virus spreading through such a common bacterial agent as E. coli caused Berg to defer part of his research program. He did not insert the recombinant virus into bacterial cells as he originally planned. (With bacterial and animal genes, Herbert Boyer and Stanley Cohen took this step shortly.) A professor at Stanford University, in 1974 Berg published a widely discussed letter on the potential dangers of recombinant DNA research. Subsequently, a moratorium on research in 1975 provided time for regulations to be devised and put into effect in 1976.

In 1980 Paul Berg shared the Nobel Prize in Chemistry with Walter Gilbert and Frederick Sanger, for "his fundamental studies of the biochemistry of nucleic acids, with particular regard to recombinant DNA."

Paul Berg winner of the
1980 Albert Lasker Award for Basic Medical Research

Jackson, D.A. et al. Biochemical method for inserting new genetic information into DNA of simian virus 40: Circular SV40 DNA molecules containing lambda phage genes and the galactose operon of Escherichia coli. Proc Nat Acad Sci USA 69, 2904-2909 (October 1972).

Back to GNN Home Page