Creating Transgenic Organisms

An organism can acquire a new trait by having a new gene introduced into its DNA. By changing the genetic makeup (genotype) of the organism, the characteristics it displays—or its phenotype—can also be altered. Under appropriate conditions, the new gene can be inserted into the DNA of a cell; this gene will be transcribed and translated into protein along with all the other genes being expressed in the cell.
How does a multicellular organism such as an animal, which has many, many cells, acquire a new trait encoded by a gene from a different organism? Several different methods have been developed. The first step in all of them involves insolating the gene of interest and then linking it to another piece of DNA that contains sequences that enable the gene to be expressed in the appropriate tissues of the recipient organism. This constructed segment of DNA is then inserted into the animals using one of the techniques described below.
Microinjection
In this method, eggs are isolated from animals and fertilized in vitro, and then the constructed
DNA containing the foreign gene is injected—using a very fine needle—into the nucleus of the egg. The foreign DNA is inserted at random locations into the DNA of the fertilized egg. The egg is then implanted into the oviduct of a surrogate animal, where the egg then develops. This method has been used to create many different kinds of transgenic animals, from mice to large animals such as cattle. However, its efficiency in producing transgenic animals is low; only a small percentage of the implanted eggs develop into transgenic animals, and only a small proportion of these animals express the inserted gene efficiently because of the random insertion into the organism’s genome.
Retroviral Vectors
The gene of interest is inserted into the genome of a retrovirus and then this virus is used to infect embryonic cells, which then develop into organisms carrying the gene of interest. However, like mi­ croinjection, this method is very inefficient. The gene is inserted randomly into different sites in the DNA of different embryonic cells. Not only might the DNA be expressed at low levels or not at all, as in microinjection, but it may be expressed only in certain cells.

 

Embryonic Stem Cell Transfer
This method allows for the insertion of the genes of interest into very specific sites in the genome of the recipient organism. Embryonic stem cells are isolated from the recipient organism and grown in tissue culture flasks. These cells are then modified by inserting DNA containing the gene of interest and sequences that enable the DNA to be inserted into specific sites in the genome. These modified embryonic stem cells are then injected into the blastocyst stage of a developing recipient organism, and this blastocyst containing the gene of interest is implanted into a surrogate mother. The resulting organisms express the gene more efficiently. This method has only been used to develop transgenic mice.

Source: European Initiative for Biotechnology Education. 1998. Transgenic animals—unit 11. Retrieved August 18, 2008, from http://www.ipn.uni-kiel.de/eibe/UNIT11EN.PDF.