Genetic engineering, recombinant DNA technology, genetic modification/manipulation (GM) and gene splicing are terms that apply to the direct manipulation of an organism's genes. Genetic engineering is different from traditional breeding, where the organism's genes are manipulated indirectly. Genetic engineering uses the techniques of molecular cloning and transformation to alter the structure and characteristics of genes directly. Genetic engineering techniques have found some successes in numerous applications. Some examples are in improving crop technology, the manufacture of synthetic human insulin through the use of modified bacteria, the manufacture of erythropoietin in hamster ovary cells, and the production of new types of experimental mice such as the oncomouse (cancer mouse) for research.
The term "genetic engineering" was coined in Jack Williamson's science fiction novel Dragon's Island, published in 1951, two years before James Watson and Francis Crick showed that DNA could be the medium of transmission of genetic information.
Engineering
There are a number of ways through which genetic engineering is accomplished. Essentially, the process has five main steps:
1. Isolation of the genes of interest
2. Insertion of the genes into a transfer vector
3. Transfer of the vector to the organism to be modified
4. Transformation of the cells of the organism
5. Selection of the genetically modified organism (GMO) from those that have not been successfully modified.
Isolation is achieved by identifying the gene of interest that the scientist wishes to insert into the organism, usually using existing knowledge of the various functions of genes. DNA information can be obtained from cDNA or gDNA libraries, and amplified using PCR techniques. If necessary, i.e. for insertion of eukaryotic genomic DNA into prokaryotes, further modification may be carried out such as removal of introns or ligating prokaryotic promoters.
Insertion of a gene into a vector such as a plasmid can be done once the gene of interest is isolated. Other vectors can also be used, such as viral vectors, bacterial conjugation, liposomes, or even direct insertion using a gene gun. Restriction enzymes and ligases are of great use in this crucial step if it is being inserted into prokaryotic or viral vectors. Daniel Nathans and Hamilton Smith received the 1978 Nobel Prize in Physiology or Medicine for their isolation of restriction endonucleases.
Once the vector is obtained, it can be used to transform the target organism. Depending on the vector used, it can be complex or simple. For example, using raw DNA with gene guns is a fairly straightforward process but with low success rates, where the DNA is coated with molecules such as gold and fired directly into a cell. Other more complex methods, such as bacterial transformation or using viruses as vectors have higher success rates.
After transformation, the GMO can be selected from those that have failed to take up the vector in various ways. One method is screening with DNA probes that can stick to the gene of interest that was supposed to have been transplanted. Another is to package genes conferring resistance to certain chemicals such as antibiotics or herbicides into the vector. This chemical is then applied ensuring that only those cells that have taken up the vector will survive.
Applications
The first genetically engineered medicine was synthetic human insulin, approved by the United States Food and Drug Administration in 1982. Another early application of genetic engineering was to create human growth hormone as replacement for a compound that was previously extracted from human cadavers. In 1987 the FDA approved the first genetically engineered vaccine for humans, for hepatitis B. Since these early uses of the technology in medicine, the use of GM has gradually expanded to supply a number of other drugs and vaccines.
One of the best-known applications of genetic engineering is the creation of GMOs for food use (genetically modified foods); such foods resist insect pests, bacterial or fungal infection, resist herbicides to improve yield, have longer freshness than otherwise, or have superior nutritional value.
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Agrobacterium Mechanism
