The basic process of recombinant DNA technology revolves around DNA activity in the synthesis of protein. During this synthesis, DNA provides the genetic code for the placement of amino acids in proteins. By intervening in this process, scientists can change the nature of the DNA, thereby changing the nature of the protein expressed by that DNA. By inserting genes into the genome of an organism, the scientist can induce the organism to produce a protein it does not normally produce.
The technology of recombinant DNA has been made possible in part by extensive research on microorganisms during the last half-century. One important microorganism in recombinant DNA research is Escherichia coli, commonly referred to as E. coli. The biochemistry and genetics of E. coli are well known, and its DNA has been isolated and made to accept new genes. The DNA can then be forced into fresh E. coli cells and the bacteria will begin to produce the proteins specified by the foreign genes. Such altered bacteria are said to have been transformed.
Knowledge about viruses has also aided the development of DNA technology.Viruses are fragments of nucleic acid surrounded by a protein coat. In some cases, viruses attack cells and replicate within the cells, thereby destroying them. In other cases, the viruses enter cells, and their nucleic acid joins with the nucleic acid in the cell nucleus. By attaching DNA to viruses, scientists use viruses to transport foreign DNA into cells and to connect it with the nucleic acid of the cells.
Another common method for inserting DNA into cells is to use plasmids, which are small loops of DNA in the cytoplasm of bacterial cells. Working with a plasmid is much easier than working with a chromosome, so plasmids are often the carriers, orvectors, of DNA. Plasmids can be isolated, recombined with foreign DNA, then inserted into cells where they multiply as the cells multiply.
Interest in recombinant DNA and biotechnology heightened considerably during the 1960s and 1970s with the discovery of restriction enzymes. These enzymes catalyze the opening of a DNA molecule at a “restricted” point, regardless of the source of the DNA. Figure 1 shows that a human DNA molecule is opened at a certain site by the restriction enzyme EcoRl (upper left), and the desired DNA fragment is isolated (lower left). Plasmid DNA is treated with the same enzyme and opened. The DNA fragment is spliced into the plasmid to produce the recombinant DNA molecule.