Excerpted from BIO. "What is Biotechnology?" Washington, D.C.: Biotechnology Industry Organization, 1989. Obtained from Genentech's Access Excellence
In the early 1970s, biochemists at Stanford University showed that genetic traits could indeed be transferred from one organism to another. In this experiment, the DNA of one microorganism recombined with the inserted DNA sequence of another, and thus had been edited to exhibit a very specific modification.
The actual editing, or insertion process, is painstaking, for it involves manipulating incredibly tiny pieces of incredibly tiny organisms. But the process can be explained in terms of editing a written text: scissors and "glue" are used to "cut" and "paste."
The methods used in rDNA technology are fairly simple. We take, for example, the sentence (gene) for insulin production in humans and paste it into the DNA of Escherichia coli, a bacterium that inhabits the human digestive tract. The bacterial cells divide very rapidly making billions of copies of themselves, and each bacterium carries in its DNA a faithful replica of the gene for insulin production. Each new E. coli cell has inherited the human insulin gene sentence.
How do we transfer the gene embodying the instruction for insulin production? One approach would be to cut the appropriate gene from human DNA and paste, or splice, it into plasmid DNA, a special kind of DNA that takes a circular form and can be used as a vehicle for this editing job. Our "scissors" are the class of enzymes called restriction enzymes. There are well over a hundred restriction enzymes, each cutting in a very precise way a specific base sequence of the DNA molecule. With these scissors used singly or in various combinations, the segment of the human DNA molecule that specifies insulin production can be isolated.
This segment is "glued" into place using an enzyme called DNA ligase. The result is an edited, or recombinant, DNA molecule. When this recombinant plasmid DNA is inserted into E. coli, the cell will be able to process the instructions to assemble the amino acids for insulin production. More importantly, the new instructions are passed along to the next generation of E. coli cells in the process known as gene cloning.
This highly simplified description of rDNA technology does not fully convey the enormous complexity and awesome economy and efficiency of genetic processes. But we can begin to understand how, by using rDNA, it is possible to produce substances of medical and economic value.