The first wave of genetic revolution involved study of the structure of the deoxyribonucleic acid (DNA) molecule. This new insight into the mysteries of heredity quickly led to another revolution. In the early 1970s, scientists discovered unique ways to manipulate genes. This technology, known as ligation, has generated scores of studies. The methods involve copying, dissecting, modifying, and even recombining portions of DNA from different sources. More importantly, these foreign genes can be made to become an integral part of the bacterium. They will replicate along with the original genes just as if they had always been a part of the bacterial cell. Thus, when the genetically altered bacteria replicate, the spliced genes are transmitted from one bacteria to the next. The foreign genes can also be functional within their bacterial host. They can be introduced to make their normal gene products. The substances secreted by altered bacteria can often be produced more efficiently than by conventional methods.
Scientists were eager to use this general knowledge to solve practical problems. Presently, gene-splicing has many applications, especially in medicine and agriculture. Recombinant vaccines for hepatitis and therapeutics such as insulin have already been produced. A new type of tomato that has a longer shelf life was made using gene-splicing techniques. This variety of tomato is expected to save farmers and shippers millions of dollars each year. Efforts are also focused on cloning bacterial genes for nitrogen fixation into crop plants, thus eliminating the need for most fertilizers. Private gene-splicing companies are emerging to help meet the need for these new products, fostering the rapid growth of the biotechnology industry. Among the recently developed foods derived from new plant varieties created through this technology are an herbicide-tolerant soybean and a virus- and/or insect-protected tomato, potato, corn, squash, and papaya.
The stage for such growth was set in 1953, when James Watson and Francis Crick discovered the molecular structure of DNA. DNA ligases had been discovered and purified in five separate laboratories in 1967. These enzymatic compounds are important because they enable certain "sticky ends" of the broken DNA to be fused together. They can also be fused together with any other segment of DNA that has been cleaved by the same restriction enzyme. Hugh Smith, a molecular geneticist, described the first restriction endonuclease in 1970. This enzyme protects the bacteria cells against viral infection and is also able to break foreign DNA at specific sites. Herbert Boyer, a bacteriologist and molecular geneticist, is credited with isolating and describing a specific restriction endonuclease called EcoRI. This enzyme has proved to be especially useful in the development of cloning methods. Thus, the two main tools required for gene-splicing, the "glue" (DNA ligase) and the "scissors" (restriction endonuclease) were now available. It wasn 't long before Paul Berg, a biochemist, and his colleagues were able to successfully recombine different DNA molecules in a test tube.
The recombinant DNA revolution gained momentum in the fall of 1973 at Stanford University and at the University of California at San Francisco (UCSF). Stanley Cohen (Stanford) and Herbert Boyer (UCSF) were the scientists credited with pioneering the techniques that now allow scientists to insert DNA from virtually any source into bacteria cells and detect the expression of foreign genes in the bacteria. Cohen achieved the first successful transformation experiments in the common bacteria Escherichia coli. Cohen and Boyer later published a landmark report that detailed the mixing and recombining of DNA from two separate plasmids in E. coli. A plasmid is simply a small, circular extrachromosomal DNA molecule that replicates independently from the other bacterial DNA. Later experiments showed that it was possible to go one step further and combine DNA from an unrelated bacterium, Staphylococcus aureus, with E. coli. A year later they reported on the first successful cloning of animal genes in E. coli. In this experiment, specific genes from the African clawed toad, Xenopus laevis, were spliced into the E. coli bacterium.
In addition to the practical applications of ligation, scientists hope that recombinant DNA methods will help answer some of the basic questions of cell biology. They would also like to gain a better understanding of the processes that control human heredity and development.
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