Transgenic Techniques
Transgenics describes the process of introducing foreign deoxyribonucleic acid (DNA) into a host organism’s genome. The foreign DNA, or “trans­gene,” that is transferred to the recipient can be from other individuals of the same species or even from unrelated species. In multicellular organisms, this is often done through experimental manipulation of gametes or early embryos. Usually the transgene is incorporated at a very early stage in em­bryonic development so that cells of the entire organism contain the trans­gene. A wide range of species can be made transgenic including plants, insects, worms, and vertebrates. The most commonly genetically manipu­lated vertebrate animal is the mouse because a variety of techniques exist to produce transgenic mice.
Transgenic techniques have been used for a number of goals: to deter­mine an unknown gene’s function; to analyze the malfunction of a mutated gene; to model human disease; and to provide better agricultural and phar­maceutical products by making transgenic plants and animals. For example, insect-resistant transgenic plants have been engineered. While the benefits of modified plants and animals are far-reaching, there is debate about the ethics of genetically altering plants and animals, and the impact these al­terations may have on the environment.
There are several ways to introduce a transgene into the organism. Microinjection is one of these techniques. As its name suggests, microin­jection is the process of injecting the transgene into the nucleus of a cell where it is randomly inserted into the host genome. This technique, ini­tiated in 1981, is most commonly used to generate transgenic mice. DNA is injected into the nucleus of a fertilized egg, which is then transferred to a foster mother. If the introduced DNA becomes integrated into the developing embryo’s genome, the offspring will carry the transgene. Two other techniques for random insertion are retroviral and transposable el­ement insertion.
An important application of transgenic technology, introduced in the 1990s, is gene targeting, or the production of “knock-out” organisms. The term “knock out” refers to the ability to disrupt a specific gene, so that it no longer encodes a complete protein. Genes are knocked out by being replaced by a transgene that has been disrupted in vitro, either by the addition of some sequence into the gene itself or by the deletion of part of the gene. Gene replacement occurs when the disrupted transgene is introduced into a cell. Here, it recombines with the recipient’s copy of that gene, inserting itself into the chromosome by homologous re­combination.
In mice, transgenes can also be introduced with cultured embryonic stem (ES) cells, using selection techniques to recover the transformed cells. The altered ES cells are then injected into early mouse embryos, and result in a mosaic embryo with normal and transgenic cells. If the altered cells con­tribute to the germ cells of the mouse, progeny in a subsequent mating will inherit the knocked out gene. These mice can then be mated to produce mice that are homozygous for both copies of the altered gene (both copies of the gene are knocked out). These mice can then be carefully examined to determine what happens when the specific gene is absent. The knock­out technique is most commonly applied to mice, insects, and yeast.
Extensions of gene targeting are the “knock-in” approach and condi­tional mutation. The knock-in approach involves inserting a mutated gene or a similar gene in place of the gene of interest. The newly added gene is expressed at the same time and location as the replaced gene. This method allows scientists to study the effects of mutations in genes as well as discover if certain genes have redundant functions. Conditional mutation is a way of either turning on or turning off the gene of interest, and can be done ei­ther in specific tissues or at specific time points.
References
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Pub­lishing, 2000.
Drlica, Karl. Understanding DNA: A Guide for the Curious. New York: John Wiley & Sons, 1997.
Strachan, Tom, and Andrew Read. Human Molecular Genetics, 2nd ed. New York: Wiley-Liss, 1999.