changed in such a way as to make possible the production of new substances or new functions. As an example, biologists have now learned how to transplant the gene that produces light in a firefly into tobacco plants. The function of that gene—the production of light—has been added to the normal list of functions of the tobacco plants.
Furthermore, the instructions stored in a DNA molecule can easily be passed on from generation to generation. When a cell divides (reproduces), the DNA within it also divides. Each DNA molecule separates into two identical parts. Each of the two parts then makes a copy of itself. Where once only one DNA molecule existed, now two identical copies of the molecule exist. That process is repeated over and over again, every time a cell divides.
This discovery gave a chemical meaning to the term gene. According to our current understanding, a specific arrangement of nitrogen bases forms a code, or set of instructions, for a cell to make a specific protein. The protein might be the protein needed to make red hair, blue eyes, or wrinkled skin (to simplify the possibilities). The sequence of bases, then, holds the code for some genetic trait.
Genetic engineering procedures
Genetic engineering requires three elements: the gene to be transferred, a host cell into which the gene is inserted, and a vector to bring about the transfer. Suppose, for example, that one wishes to insert the gene for making insulin into a bacterial cell. Insulin is a naturally occurring protein made by cells in the pancreas in humans and other mammals. It controls the breakdown of complex carbohydrates in the blood to glucose. People whose bodies have lost the ability to make insulin become diabetic.
The first step in the genetic engineering procedure is to obtain a copy of the insulin gene. This copy can be obtained from a natural source
The second step in the process is to insert the insulin gene into the vector. The term vector means any organism that will carry the gene from one place to another. The most common vector used in genetic engineering is a circular form of DNA known as a plasmid. Endonucleases are used to cut the plasmid molecule open at almost any point chosen by the scientist. Once the plasmid has been cut open, it is mixed with the insulin gene and a ligase enzyme. The goal is to make sure that the insulin gene attaches itself to the plasmid before the plasmid is reclosed.
The hybrid plasmid now contains the gene whose product (insulin) is desired. It can be inserted into the host cell, where it begins to function just like all the other genes that make up the cell. In this case, however, in addition to normal bacterial functions, the host cell also is producing insulin, as directed by the inserted gene.
Notice that the process described here involves nothing more in concept than taking DNA molecules apart and recombining them in a different arrangement. For that reason, the process also is referred to as recombinant DNA (rDNA) research.
Applications of genetic engineering
The possible applications of genetic engineering are virtually limitless. For example, rDNA methods now enable scientists to produce a number of products that were previously available only in limited quantities. Until the 1980s, for example, the only source of insulin available to diabetics was from animals slaughtered for meat and other purposes. The supply was never large enough to provide a sufficient amount of affordable insulin for everyone who needed insulin. In 1982, however, the U.S. Food and Drug Administration approved insulin produced by genetically altered organisms, the first such product to become available.
Since 1982, the number of additional products produced by rDNA techniques has greatly expanded. Among these products are human growth hormone (for children whose growth is insufficient because of genetic problems), alpha interferon (for the treatment of diseases), interleukin-2 (for the treatment of cancer), factor VIII (needed by hemophiliacs for blood clotting), erythropoietin (for the treatment of anemia), tumor necrosis factor (for the treatment of tumors), and tissue plasminogen activator (used to dissolve blood clots).
Genetic engineering also promises a revolution in agriculture. Recombinant DNA techniques enable scientists to produce plants that are resistant to herbicides and freezing temperatures, that will take longer to ripen, and that will manufacture a resistance to pests, among other characteristics.
Today, scientists have tested more than two dozen kinds of plants engineered to have special properties such as these. As with other aspects of genetic engineering, however, these advances have been controversial. The development of herbicide-resistant plants, for example, means that farmers are likely to use still larger quantities of herbicides. This trend is not a particularly desirable one, according to some critics. How sure can we be, others ask, about the risk to the environment posed by the introduction of "unnatural," engineered plants?
The science and art of animal breeding also are likely to be revolutionized by genetic engineering. For example, scientists have discovered that a gene in domestic cows is responsible for the production of milk. Genetic engineering makes it possible to extract that gene from cows who produce large volumes of milk or to manufacture that gene in the laboratory. The gene can then be inserted into other cows whose milk production may increase by dramatic amounts because of the presence of the new gene.
Human gene therapy
One of the most exciting potential applications of genetic engineering involves the treatment of human genetic disorders. Medical scientists know of about 3,000 disorders that arise because of errors in an individual's DNA. Conditions such as sickle-cell anemia, Tay-Sachs disease, Duchenne muscular dystrophy, Huntington's chorea, cystic fibrosis, and Lesch-Nyhan syndrome result from the loss, mistaken insertion, or change of a single nitrogen base in a DNA molecule. Genetic engineering enables scientists to provide individuals lacking a particular gene with correct copies of that gene. If and when the correct gene begins functioning, the genetic disorder may be cured. This procedure is known as human gene therapy (HGT).
The first approved trials of HGT with human patients began in the 1980s. One of the most promising sets of experiments involved a condition known as severe combined immune deficiency (SCID). Individuals with SCID have no immune systems. Exposure to microorganisms that would be harmless to the vast majority of people will result in diseases that can cause death. Untreated infants born with SCID who are not kept in a sterile bubble become ill within months and die before their first birthday.
In 1990, a research team at the National Institutes of Health (NIH) attempted HGT on a four-year-old SCID patient. The patient received about one billion cells containing a genetically engineered copy of the gene that his body lacked. Another instance of HGT was a procedure, approved in 1993 by NIH, to introduce normal genes into the airways of cystic fibrosis patients. By the end of the 1990s, according to the NIH, more than 390 gene therapy studies had been initiated. These studies involved more than 4,000 people and more than a dozen medical conditions.
In 2000, doctors in France claimed they had used HGT to treat three babies who suffered from SCID. Just ten months after being treated, the babies exhibited normal immune systems. This marked the first time that HGT had unequivocally succeeded.
The commercialization of genetic engineering
The commercial potential of genetically engineered products was not lost on entrepreneurs in the 1970s. A few individuals believed that the impact of rDNA on American technology would be comparable to that of computers in the 1950s. In many cases, the first genetic engineering firms were founded by scientists involved in fundamental research. The American biologist Herbert Boyer, for example, teamed up with the venture capitalist Robert Swanson in 1976 to form Genentech (Genetic Engineering Technology). Other early firms like Cetus, Biogen, and Genex were formed similarly through the collaboration of scientists and businesspeople.
The structure of genetic engineering (biotechnology) firms has, in fact, long been a source of controversy. Many observers have questioned the right of a scientist to make a personal profit by running companies that benefit from research that had been carried out at publicly funded universities. The early 1990s saw the creation of formalized working relations between universities, individual researchers, and the corporations founded by these individuals. Despite these arrangements, however, many ethical issues remain unresolved.