This section only gives a very short introduction of the topic. Only the main themes are summarized.
Methods for gene transfere: Micro injection of DNA into the paternal pro nucleus
right after fertilization of the oocyte.
The success rate is low, a few per thousand. There is always the risk of insertional
mutageneses, i.e. the
introduced gene inserts itself into a functional part of another gene and thus ruins its functions.
Micro injection or other forms of gene transfere into embryonal stem cells or other types of pluripotent cells
by use of homologous recombination. After selecting for the transgene cells, which
is possible in a cell culture. Selection for the recombinated cells can be done by means
of a linked which, is part of the construction. Cultivation methods for
foetal stem cells are only fully developed for mice.
It is still on a trial basis for all of our domestic animal species.
After transgenesis in ES, or other foetal cell, the transgenic cells are injected into a blastocyst. Then a chimeric animal is formed, some of these transgenic would form gametes containing the inserted gene construction.
Homologous recombination. The targeting of genes becomes possible by means of homologous recombination. This is of great significance to study the function of an unknown gene (with known DNA-sequence). By means of homologous recombination so called 'knock out' mice can be created with a destructed gene function. By investigating the offspring of the 'knock out' mice it is possible to identify the function of a gene, which is completely unknown. So the 'knock out' mice have become the modern test tubes for identification of the functions of a gene.
Gene constructions.
A transgene is normally composed of a promotor and a structural gene. The promotor
decides when and where
the expression of the gene occurs. If the gene is to be expressed in the mammary gland the casein promoter
is often utilized, as the casein is one of the important milk proteins. The structural gene
is either with or without introns.
If the gene originates from a bacteria, it is always without introns. Genes
originating from eukaryotee can be cDNA or
genomic. The genomic genes are normally very large, which is a problem. The larger
the construction the more
difficult it is to make a functional transgenic animal.
Motives for transgenesis.
Other motives for transgenesis
Gene farming - production of drugs
Organ donation -xeno transplantation
Many medical companies have started the experimentation and production of drugs by means of transgenic animals.
Normally, a single animal can produce sufficient drugs for the world market, so the topic is
of little importance for this
section. The same applies for organ donation, an example is the use of a swine heart
for a human. In order for this technique to
work it is necessary to knock out most of the strong ordinary antigenic systems
in swine. This is difficult, but what is worse is that the
xeno transplantation is dangerous. It is well known that modern pests in humans, AIDS etc.,
stem from
the animal world. It is a well known fact that swine carry a lot of endogenous retro viruses. They
could be a large potential
hazard to mankind if extensive xeno transplantation is used.
To get an overview of the topic see the following chapter The Science of Transgenic Animals for Food Production