In order to genetically modify an organism, the new gene construct (known as a transgene) must be inserted into the DNA of the host cell. If the new gene codes for a particular protein, then this protein may be expressed in the animal - this depends on the site at which the DNA integrates and whether the gene is 'switched on'. The transgene, or gene construct, consists of the gene coding for the protein of interest plus a promoter sequence which is intended to regulate the expression of the gene product by 'switching on' and 'switching off' the gene at the appropriate time. Sometimes the purpose of modification is to 'knock out' a gene, or disrupt its function, rather than to add a new gene.
Providing the integration happens early enough in an organism's development, preferably when it is at the single cell stage, the new DNA will be copied along with the original DNA as cells divide and every cell of the organism should contain the new gene. Approximately half of the germ cells should contain the new gene so that the genetic modification will be passed to some offspring. This is called 'germ line transmission'. It is unlikely that all germ cells will contain the new DNA as there is a non-replicative division when sperm or eggs are formed.
The following methods have been used to genetically modify animals:
- Microinjection - also called pronuclear injection. DNA is injected into the nucleus of a single cell embryo using a very fine needle. Typically 200 - 500 copies of the gene construct are injected into each embryo. The injected DNA is incorporated randomly into the genome of some of the embryos.
- Viruses - particularly retroviruses - are often used as 'vectors' to introduce new genetic material into cells because they are naturally well equipped to infiltrate them. Retroviruses are a type of virus which replicates by integrating itself into the host DNA and is then copied with the host genetic material as the cell divides.
- Embryonic stem (ES) cell culture and modification allows a much more targeted approach to genetic modification. However, despite many attempts to obtain ES cells from rats and farm animals, ES cells have so far only ever been isolated from some strains of mice. The importance of the technique is that the ability to culture the ES cells allows for much more selective modification techniques with some control over the integration site. For example, modification can be targeted so that a transgene replaces the equivalent native gene or so that genes are 'knocked out' - made ineffective by removal or disruption.
- Sperm mediated transfer - using genetically modified sperm as a vector for introducing foreign DNA into the egg has obvious attractions as artificial insemination of livestock and poultry is routine.