A chimera is a three-headed mythological beast consisting of a goat, a lion and serpent, which was supposed to rouse terror and awe in the beholder. When Dr. Stanley Cohen of Stanford University, a pioneer in recombinant-DNA technology, first implanted foreign genes in bacteria to produce a unique life form, he called his new bacteria "DNA chimeras." They too have given man pause.
Almost all living cells, no matter where they are located in an organism, contain the genetic "blueprint” for the entire organism. In humans, for example, even though a bone cell only functions as part of the bone, its genes carry the information required for the creation and operation of every cell in the human body. All genes in cells, whether human or bacterial, are composed of a nucleic acid, DNA, arranged in two strands, and each strand includes thousands of nitrogen "bases” strung together like beads along it. There are four kinds of “base” molecules: adenine, cytosine, guanine and thymine—A, C, G and T for short (any genetics researcher worth his salt will keep his blackboard constantly filled with these four letters in every conceivable combination). The two strands of DNA are held together in a specific pattern commonly called the double helix, which looks like a spiral ladder: opposing bases on each of the two strands or sides of the ladder bond together to form the rungs or “base pairs." This is the essential chemical structure of DNA.
A gene is a certain section of the DNA strand that tells a cell to perform a specific function. These sections can be anywhere from a dozen to several thousand bases long. The total number of bases and their particular arrangement within each section define the “genetic code,” which in turn determines the structure and function of that gene. The information in the code is “read off" and utilized by “factories" in the cell to create proteins and enzymes necessary for its life processes.
That science has managed to find out what little it knows about genes at all is astonishing considering the minute scale involved. Most bacteria have a double strand of DNA one millimetre long and less than two-millionths of a centimetre wide containing three million bases: this is folded into a space less than 1/1,000th of a millimetre across. Human DNA includes three billion bases packed into a space not much larger, and if unravelled it would stretch for several metres.
Although the phrase “gene-splicing" suggests that molecular biologists actually perform the operation with tiny scalpels, given the microscopic entities they're working with this isn’t possible—yet. Socalled "restriction enzymes" cut the DNA strand at specific points; other enzymes, called DNA ligase, reunite them. Gene-splicing is therefore accomplished by reacting the host DNA and the gene they wish to splice into it with these enzymes in a solution. The researcher knows that those bacteria with successfully spliced DNA will react differently to a particular antibiotic than the original bacteria; when this antibiotic is added, the desired bacteria can then be separated from those bacteria whose DNA is unchanged.
There are two ways researchers can obtain the gene they wish to implant. The most common technique is to isolate the particular sequence of nitrogen bases on the DNA strands comprising the desired gene and snip it out with the restriction enzymes. The second is to synthesize the gene artificially from readily available lab chemicals once its sequence of bases has been determined. In both techniques the genetic material is then spliced (the DNA is “recombined”) using restriction enzymes and DNA ligase at an appropriate place on the DNA of the host bacteria. These later replicate to produce “clones” with a new genetic makeup.
Saran Narang, a molecular biologist at Canada’s National Research Council, is an acknowledged master at the art of artificially synthesizing genes. He prefers this method because there is a hit-and-miss element to natural isolation—-the scientist is never absolutely sure he has obtained the gene he wants, no more and no less. Not only might he then not succeed in inducing the desired properties in the host bacteria but there is more danger of introducing undesirable and potentially harmful properties than if the gene is synthesized and hence completely under human control. Narang’s recently constructed gene for human insulin has precisely these advantages: although it’s not the first synthesized insulin gene, he feels it’s the closest approximation yet to that gene in its natural state. Gene synthesis is still a lengthy procedure demanding highly skilled technicians, but Robert Bender’s team at BIO LOGICALS' Montreal lab has just developed a machine that will efficiently automate some of the steps involved.
Once a gene has been spliced into a host, two problems remain: will it program cells just as it would in its natural environment and, most important from a commercial standpoint, can the host bacteria's genes be further adjusted or recombined so that the desired substances now being produced by the cell appear in sufficient quantities? Narang has solved the first problem for his insulin gene by synthesizing “linker” genes which attach the insulin gene to the DNA of the host bacteria, and their successful functioning was confirmed by his partner, Ray Wu of Cornell University. Now it’s up to Connaught Laboratories of Toronto to adapt the new life forms to the mass production of insulin.
Narang is eager to try out his new techniques on the synthesis of interferon, the much-publicized hormone that might defeat cancer. One major difficulty is that, although interferon has already been successfully produced using natural isolation techniques, no one has yet “cracked” its genetic code to determine the natural sequence of its base pairs so that it can be synthesized. This will be Narang’s initial task, but he is cautiously optimistic: “The big breakthrough with interferon could come at any time—and anytime could be tomorrow or 10 years away.”
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