Genetic Modification and Uses of Transgenic Animals
Understanding Transgenic Animals
Transgenic animals are animals with a modified genome. A foreign gene is inserted into the animal’s genome to alter its DNA. This method is performed to improve the genetic traits of the target animal.
Initially, the improvement of genetic traits was achieved through selective breeding methods. In this process, animals with desired genetic characteristics were mated to produce offspring with improved traits. Since this technique was time-consuming and expensive, it was later replaced by recombinant DNA technology.
What is Transgenesis?
Transgenesis is the phenomenon in which a foreign gene with desired characteristics is introduced into the genome of the target animal. The foreign gene introduced is known as the transgene, and the animal whose genome is altered is known as transgenic. These genes are passed on to successive generations.
Transgenic animals are genetically engineered and are also known as genetically modified organisms (GMOs). The first genetically modified organism was engineered in 1980.
Methods of Gene Transfer
Physical Transfection
In this method, the gene of interest is directly injected into the pronucleus of a fertilized ovum. It was the very first method that proved effective in mammals and was applicable to a wide variety of species. Other methods of physical transfection include particle bombardment, ultrasound, and electroporation.
Chemical Transfection
One chemical method of gene transfection includes transformation. In this method, the target DNA is taken up in the presence of calcium phosphate. The DNA and calcium phosphate co-precipitate, which facilitates DNA uptake. Mammalian cells possess the ability to take up foreign DNA from the culture medium.
Retrovirus-Mediated Gene Transfer
To increase the chances of expression, the gene is transferred by means of a vector. Since retroviruses have the ability to infect the host cell, they are used as vectors to transfect the gene of interest into the target genome.
Viral Vectors
Viruses are used to transfect recombinant DNA (rDNA) into the animal cell. Viruses possess the ability to infect the host cell, express well, and replicate efficiently.
Bactofection
It is the process by which the gene of interest is transferred into the target genome with the help of bacteria.
Notable Transgenic Animals and Clones
Dolly the Sheep
Dolly the sheep was the first mammal to be cloned from an adult cell. In this process, udder cells from a 6-year-old Finn Dorset white sheep were injected into an unfertilized egg from a Scottish Blackface ewe, which had its nucleus removed. The cell was made to fuse by electrical pulses. After the fusion of the nucleus of the cell with the egg, the resultant embryo was cultured for six to seven days. It was then implanted into another Scottish Blackface ewe, which gave birth to Dolly.
Transgenic Mice
Transgenic mice are developed by injecting DNA into the oocytes or 1-2 celled embryos taken from female mice. After injecting the DNA, the embryo is implanted into the uterus of receptive females.
Applications of Transgenic Animals
Normal Physiology and Development Studies
In transgenic animals, a foreign gene is introduced, which may alter growth factors. Hence, these animals facilitate the study of gene regulation and their effect on the everyday functions of the body.
Disease Modeling and Research
Transgenic animals are specially designed to study the role of genes in the development of certain diseases. Moreover, in order to devise a cure for these diseases, transgenic animals are used as model organisms. These transgenic models are used in research for the development of medicines. For example, we have transgenic models for diseases such as Alzheimer’s and cancer.
Biological Products and Pharmaceuticals
A number of biological products, such as medicines and nutritional supplements, are obtained from transgenic animals. Research for the manufacture of medicines to treat diseases such as phenylketonuria (PKU) and hereditary emphysema is ongoing. The first transgenic cow, Rosie (1997), produced milk containing human protein (2.4 grams per liter). This milk contains the human gene alpha-lactalbumin and could be given to babies as an alternative to natural cow milk.
Vaccine Safety Testing
Transgenic animals are used as model organisms for testing the safety of vaccines before they are injected into humans. This testing was conventionally done on monkeys.