A genetically modified organism is an organism whose genetic material has been deliberately altered. Examples are diverse, and include commercial strains of wheat that have been modified by irradiation since the 1950s, transgenic experimental animals such mice, or various microscopic organisms altered for the purposes of genetic research.
"Genetically modified organism" does not necessarily imply transgenic substitution of genes from another species, although research is actively being conducted in this field. For example, genes for fluorescent proteins can be co-expressed with complex proteins in cultured cells to facilitate study by biologists, and modified organisms are of great use in researching the mechanisms of cancer and other diseases
Methods of genetic modification
Genetic modification of bacteria
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. 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 strand of DNA called a pilus (a process analogous to but biologically distinct from mating). Conjugation is not widely used for the artificial genetic modification of bacteria.
- Transduction refers to the introduction of new DNA into a bacterial cell by a bacteriophage (a virus that infects bacteria).
Genetic modification of plants
- See main article Transgenic plants.
The principal technique for the genetic modification of plants is based on a natural ability of the bacterium Agrobacterium tumefaciens. This bacterium infects plants and causes a tumor-like growth termed a crown gall. A. tumefaciens contains a plasmid (a circular piece of DNA) that transfers from the bacteria into the infected plant and integrates into the plant's genome. The transferred genes cause the plant to form the gall, which houses the bacteria and produces nutrients that support the bacteria's growth. A number of scientists contributed to this discovery throughout the late 1960s and the 1970s, with key discoveries by Jeff Schell, Marc Van Montagu, Georges Morel and Jacques Tempé. By 1983 biotechnology had reached the point where it was possible to insert additional genes of interest into A. tumefaciens and thus transfer those genes into plants. This process is most commonly used to create transgenic crop plants for agricultural purposes.
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 defective genes by supplying a normal copy of the genes. Permanent genetic modification of whole 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 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.
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.  (http://www.gene.ch/genet/2004/Apr/msg00024.html),  (http://www.leidenuniv.nl/mare/2004/27/englishpages.html)
Insects can be genetically modified by injecting them with artificial transposons and a source of 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.
Controversies over genetic modification
Genetic modification (GM) is the subject of controversy in its own right  (http://www.csa.com/hottopics/gmfood/overview.html). Some see the science itself as intolerable meddling with "natural" order, and 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.
Recently, Mendocino County, California became the first county in the United States to ban the production of GMOs. The measure passed with a 57% majority.
There is currently little international consensus regarding the acceptability and 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 great 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 soya) 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.
There is some question as to whether genetically modified crops that confer pest resistance might be harmful to humans as well. Current pest-resistant strains use a relatively harmless organic toxin derived from the bacterium Bacillus thuringiensis. However, this is an area of controversy not only among the general public, but also among scientists. For example, the widespread use of an endogenous toxin such as Bt in a transgenic population will eventually result in resistant plant pests. As the bacillus itself is used as a pesticide by organic farmers, this reduces the efficacy of one of the few tools available to them. In Australia, the Agriculture department noticed that the parasite of the cotton plant, which was supposed to be killed by the GMO cotton variety Ingard, was proliferating.
Concern over the spread of genetically modified plant pollens has arisen, the claim being that natural plants can be cross-pollenated by the pollen modified plants. Pollen can be dispersed over large areas by wind, animals, and insects. Recent research has lent support to the concern when modified genes were found in normal plants up to 21 km (13 miles) away from the source, and also within close relatives of the original plants.