What is a Transgenic Species?

Fig.1 - Crown Gall

A transgenic species is an organism that has had part of another organism's DNA (often known as a 'transgene') transferred into it using recombinant DNA technology.

The resulting organism is often known as a genetically modified organism (GMO). Examples include transgenic crops that have had genes inserted into them to improve features such as resistance to insects or disease, shelf life or nutritional value, and genetically modified bacteria which can produce human proteins on a large scale.

Other uses for GMOs include medical research and the manufacture of pharmaceuticals. In the pharmaceutical industry, GMOs such as goats, rabbits, pigs and bacteria can be used to manufacture insulin, vaccines, growth hormones, human protein C, anticoagulant and haemoglobin.

Inserting Genes Into Bacteria

Bacteria can be transformed when plasmids (loops of bacterial DNA) containing the desired gene are introduced into them. The genes on the plasmid are then expressed, and can be passed on to all subsequent bacterial generations.This process begins by using a specific restriction enzyme to remove the desired gene from human DNA. The same enzyme is then used to cut a bacterial plasmid in one place. The human gene is inserted into the gap in the plasmid, and an enzyme called DNA ligase then re-joins the 'sticky' ends that were formed by the restriction enzyme.

The recombinant plasmid is then introduced into a bacterial cell in the presence of cold calcium chloride, which makes the cell walls of the bacterium more porous. Exposure to an an electric potential difference, also known as ‘electroporation’, may also be used to successfully introduce the plasmid into the host cell. Recombinant bacteria are currently used to produce large amounts of human insulin and human growth hormone for pharmaceutical use.

Inserting Genes Into Animals

DNA microinjection is the most widely used transgenesis method in animals and involves the microinjection of a gene into the nucleus of a fertilised ovum. This is a random process and may not always lead to expression of the desired gene. As a result, transgenic animals are mated to ensure that their offspring acquire the transgene.

Embryonic stem cell-mediated gene transfer is another method used to introduce genes into animal cells. Here, the modified DNA is incorporated into embryonic stem cells of the chosen animal. These stem cells are then introduced into an embryo, resulting in a chimeric organism (that is, one where only some cells contain the desired gene).

Retrovirus mediated gene transfer is also used in some instances. Retroviruses, viruses capable of inserting their genetic material into a host cell, are modified to transfer the desired gene into the animal. Not all cells of the animal will have the gene incorporated into them, so the animal will again be chimeric. The transgene will only be genetically transmitted if the retrovirus invades the animal's sex cells. Alternatively, chimeric animals are inbred for up to 20 generations until the desired gene is present in every cell.

Transgenic animals currently being researched include transgenic cattle, sheep and pigs that can produce milk that contains human insulin, human collagen, human fertility hormones and monoclonal antibodies.

Inserting Genes Into Plants

Agrobacterium tumefaciens (crown gall bacteria) is often used as a vector to carry the desired transgene into plant cells, as its normal infection process involves inserting a circular DNA plasmid into the host cell. The plasmid is initially modified by cutting it in two places with a restriction enzyme and inserting the desired gene. It is then incorporated into the targeted plant cell via the crown gall bacteria (see fig. 1) or is introduced manually.

Examples of GM plants produced using this method include Bt cotton (fig.2), which has had a gene inserted into it to make it toxic to pests such as bollworms, and GM soybeans, which incorporate a gene from the Salmonella bacterium to make them resistant to 'roundup' insecticide.

Fig.2 - BT Cotton

Most crop plants, including the cereals and grasses, are monocotyledonous: that is, their leaves have parallel veins and they don't have a tap root system. Inserting genes into this type of plant can prove to be more difficult than with higher plants.Transgenic monocotyledons can instead be produced using a particle gun. This fires a gold bullet, coated with the desired DNA, into the nucleus of the target plant. Examples of GM monocotyledonous crops include herbicide resistant GM Canola (rapeseed), corn and sugar cane, and insect resistant Bt corn.

Ethical and Environmental Concerns Linked to GMOs

One concern associated with GM plants and animals has always been the risk of horizontal gene transfer; that is, the transgene may become incorporated into related, or even unrelated species. Herbicide resistance could, for example, spread to weeds growing alongside GM Canola with the result that they can no longer be eradicated by farmers.

Transgenic organisms may also escape to the wild and compete with native species for resources - the superior 'Sumo Salmon', a transgenic fish containing the gene for human growth hormone, is an example of this. In addition, social inequity could result from the use of GMOs. For instance, large corporations may end up owning the rights to the most productive crops, so restricting access to those most in need of them.

Thorough testing of GM0s and their products is also needed to avoid any risk to public health - the need for this was recently highlighted by a study linking Monsanto's GM corn to organ damage in rats.

Despite these concerns, the use of transgenic species in agriculture, medicine and the pharmaceutical industry has already revolutionised our lives in countless ways.

References

Goldstein, K, 2010, ‘Monsanto’s GMO Corn Linked to Organ Failure, Study Reveals’, Huffington Post, huffingtonpost.com, accessed 21/3/2010

Jigar Nare, S, 2008, 'Transgenic Animals- A Boon by Biotechnology', pharmainfo.net, accessed 23/3/2010

Pighin, J, 2003, 'Transgenic Crops: How Genetics is Providing New Ways to Envision Agriculture', scq.ubc.ca, accessed 21/3/2010