The term generally does not cover organisms whose genetic makeup has been altered by conventional cross breeding or by "mutagenesis" breeding, as these methods predate the discovery of the recombinant DNA techniques. Technically speaking, however, such techniques are, by definition, genetic modification.
Genetic modification involves genetic engineering, also known as gene splicing, a technique to splice together DNA fragments from more than one organism and thus preparing a "recombinant" DNA molecule in a test tube, producing a single piece of genetic material containing the original information from multiple fragments which can then be inserted into another organism. This is achieved by cutting up DNA molecules with restriction enzymes and splicing these fragments together using DNA ligase. A transgenic organism that contains such DNA sequences from a foreign organism integrated into its own genome, the term "transgenic" literally means across gene. A mouse or fish engineered to express the green fluorescence protein, for example, would be considered a transgenic organism, since the gene coding for the protein originated from a species of jellyfish.
With current technology, transgenic organisms can be produced with only a very small proportion of extraneous DNA. For example, the genome of most mammals contains three billion basepairs of DNA, while it becomes relatively difficult to insert more than 10,000 to 20,000 basepairs of foreign DNA. More sophisticated techniques using yeast artificial chromosomes and bacterial artificial chromosomes allow insertions of up to 320,000 basepairs [3] - approximately 0.01% of the total genome. In concept, multiple rounds of transgenesis or interbreeding of transgenics could lead to organisms with a higher proportion of foreign DNA, but cost and time considerations prevent this.
In order to introduce new DNA into the receiving host, vectors are used. Vectors range from small circular pieces of DNA such as plasmids, to various viruses that can carry and transmit genetic information. Three processes are known by which the genetic composition of bacteria can be altered.
Transformation is a process by which some bacteria are naturally capable of taking up DNA to acquire new genetic traits. This phenomenon was discovered by Frederick Griffith in 1928, although the fact that it was specifically DNA molecules that carried the genetic information was not proven until 1944. Bacteria that are competent to undergo transformation are frequently used in molecular biology. The foreign DNA uptake is facilitated by the presence of certain cations, such as Ca2+, or by the use of electric current (electroporation). Transformation does not normally integrate new DNA into the bacterial chromosome. Instead, it remains on a plasmid.
In conjugation, DNA is transferred from one bacterium to another via a temporary connecting tube of protein called a pilus (a process analogous to but biologically distinct from mating). A plasmid is transferred through the pilus. Conjugation is not widely used for the artificial genetic modification of bacteria, but happens often in nature.
Transduction refers to the introduction of new DNA into a bacterial cell by a bacteriophage, a virus that infects bacteria.
In order to gain knowledge about a particular gene's function, researchers often use knock out organisms. These organisms have a specific gene that has been functionally destroyed or "knocked out." They are used extensively in disease research with model organisms. For example, when investigating the cause of cystic fibrosis, researchers identified the CFTR gene as a likely candidate for the disease, found the mouse equivalent, bred a mouse with this gene "knocked out", and noted that the knockout mouse also had cystic fibrosis.
Genetic modification of plants
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Genetic modification of animals
Like bacteria and plants, animals can be genetically modified by viral infection. However, the genetic modification occurs only in those cells that become infected, and in most cases these cells are eventually eliminated by the immune system. In some cases it is possible to use the gene-transferring ability of viruses for gene therapy, i.e. to correct diseases caused by a defective gene by supplying a normal copy of the gene. Permanent genetic modification of entire animals can be accomplished in mice. The process begins by first genetically modifying a mouse embryonic stem cell. This is normally done by physically introducing into the cell a plasmid that can integrate into the genome by a process known as transfection [4]. During transfection the DNA integrates into the animal genome via non-homologous recombination. This altered cell is implanted into a blastocyst (an early embryo), which is then implanted into the uterus of a female mouse. A pup born from this blastocyst will be a chimera containing some cells derived from the unmodified cells of the blastocyst and some derived from the modified stem cell. By selecting mice whose germ cells (sperm- or egg-producing cells) developed from the modified cell and interbreeding them, pups that contain the genetic modification in all of their cells will be born. Baylor College of Medicine currently has one of the largest transgenic mice facilities in the country.
There has also been the genetically manipulated bull Herman with 55 offspring. A human gene was built into his genetic code while in an early embryonic stage in 1990. As a result, milk from his female descendants contained the human protein lactoferrine, that can be used as medicine, but it was present at such low levels that it was not profitable to extract them.
Insects can be genetically modified by injecting them with artificial transposons and a source of the enzyme transposase. The transposon, which can include new genes, is then integrated into the genome. Such insertions are unstable and can 'jump-out' in the presence of transposase.
Transgenic fish are often created by microinjection. First generation is mosaic but several lines have been produced with the transgene incorporated into the germ line and transgenic fish can then be produced "the natural way" by crossing male and female gametes. Although many types of transgenic fish exists (e.g. for increased cold tolerance, antibiotic production, ornamental Glofish etc) the main focus had been on so called growth hormone transgenic fish, mainly salmonids, tilapias and carps. These fish have an over-production of growth hormone which results in increased growth rate from a few percent up to 30-40 times that of wild-types. In some species, final size is increased as well as growth rate providing an incentive for commercial breeders to farm such fish. However, ecological concerns over potential negative effects of transgenic fish in nature largely prevent the commencement of commercial production. A large and important portion of the research on transgenic fish today is therefore focusing on environmental risk-assessment of GH-transgenic fish.
Controversy
See also Genetically modified food and Transgenic plants
Genetic modification (GM) is the subject of controversy in its own right [5]. Some see the science itself as intolerable meddling with "natural" order, despite many known examples of natural genetic crossings occurring throughout history (see for example horizontal gene transfer). While some would like to see it banned, others push simply for required labeling of genetically modified food. Other controversies include the definition of patent and property pertaining to products of genetic engineering and the possibility of unforeseen global side effects as a result of modified organisms proliferating. The basic ethical issues involved in genetic research are discussed in the article on genetic engineering.
In 2004, Mendocino County, California became the first county in the United States to ban the production of GMOs. The measure passed with a 57% majority. In 2005, a standing committee of the government of Prince Edward Island in Canada began work to assess a proposal to ban the production of GMOs in the province. This is a largely symbolic and empty gesture as PEI has already banned GMO potatoes, which account for most of its crop. In California, the Trinity and Marin counties have also imposed bans on GM crops, while ordinances to do so were unsuccessful in Butte, San Luis Obispo, Humboldt and Sonoma counties. Supervisors in the ag-rich counties of Fresno, Kern, Kings, Solano, Sutter and Tulare have passed resolutions supporting the practice [6].
Currently, there is little international consensus regarding the acceptability and effective role of modified "complete" organisms such as plants or animals. A great deal of the modern research that is illuminating complex biochemical processes and disease mechanisms makes vast use of genetic engineering.
The practice of genetic modification as a scientific technique is not restricted in the United States. Individual genetically modified crops (such as soybeans) are subject to intense study before being brought to market and are common in the United States, but estimates of their market saturation vary widely. Some countries in Europe have taken the opposite position, stating that genetic modification has not been proven safe, and therefore that they will not accept genetically modified food from the United States or any other country. This issue has been brought before the World Trade Organization, which determined that not allowing modified food into the country creates an unnecessary obstacle to international trade. Consequently, genetic modification within agriculture is an issue of some strong debate in the United States, the European Union, and some other countries.
Some critics have raised the concern that conventionally bred crop plants can be cross-pollinated (bred) from the pollen of modified plants. Pollen can be dispersed over large areas by wind, animals, and insects. Recent research with creeping bentgrass has lent support to the concern when modified genes were found in normal grass up to 21 km (13 miles) away from the source, and also within close relatives of the same genus Agrostis [7]. GM proponents point out that outcrossing, as this process is known as, is not new. The same thing happens with any new open-pollinated crop variety—newly introduced traits can potentially cross out into neighbouring crop plants of the same species and, in some cases, to closely related wild relatives. Defenders of GM technology point out that each GM crop is assessed on a case by case basis to determine if there is any risk associated with the outcrossing of the GM trait into wild plant populations. The fact that a GM plant may outcross with a related wild relative is not, in itself, a risk unless such an occurrence has consequences. If, for example, a herbicide resistance trait was to cross into a wild relative of a crop plant it can be predicted that this would not have any consequences except in areas where herbicides are sprayed, such as a farm. In such a setting the farmer can manage this risk by rotating herbicides. If patented genes are outcrossed, even accidentally, to other commercial fields and a person deliberately selects the outcrossed plants for subsequent planting then the patent holder has the right to control the use of those crops. This was supported in Canadian law in the case of Monsanto Canada Inc. v. Schmeiser.
An often cited controversy is a hypothetical Technology Protection technology (dubbed terminator by NGOs). This yet to be commercialised technology would allow the production of first generation crops that would not generate seeds in the second generation because the plants yield sterile seeds. The patent for this so-called "terminator" gene technology is owned by Delta and Pine Land and the USDA despite it often being mis-associated with Monsanto. In addition to the commercial protection of proprietary technology in selfpollinating crops such as soybean (a generally contentious issue) another purpose of the terminator gene is to prevent the escape of genetically modified traits from crosspollinating crops into wild-type species by sterilizing any resultant hybrids. The terminator gene technology created a backlash amongst those who felt the technology would prevent re-use of seed by farmers growing such terminator varieties in the developing world and was ostensibly a means to exercise patent claims. Use of the terminator technology would also prevent "volunteers", or crops that grow from unharvested seed, a major concern that arose during the Starlink debacle.
Transgenics featured in fiction
Genetically modified characters, whether as heroes, villains, or backdrop, feature prominently in many works of fiction, in particular science fiction and cyberpunk, where it is used as a plot device to explain differences in a character or setting, such as explaining increased longevity or eradication of disease in a fictional civilization.
In the Spiderman movie, Peter Parker was bitten by a super-spider, enhanced with the genes of many different spiders. The abilities of all these spiders were then transferred from the super-spider, into Peter, turning him into Spiderman.
The videogame character Shadow the Hedgehog was originally a science experiment who was fused with the DNA of Black Doom, causing him to have the genes of aliens as well as hedgehogs. This however, was not revealed until the game Shadow the Hedgehog.
Dark Angel deals with transgenics escaped from a secret gouvernment facility. The main character Max, a human / cat transgenic engineered to be a supersoldier in future warefare, tries to lead a normal life while searching for others of her kind.
In the Maximum Ride books by James Patterson, the main characters are human/bird transgenics.
See also
External links