15.4: Transgenic Organisms
Transgenic organisms are genetically engineered to carry transgenes—genes from a different species—as part of their genome. The transgene may either be a different version of one of the organism’s genes or a gene that does not exist in their genome. Transgenes are usually generated by recombinant DNA and DNA cloning techniques. Transgenic bacteria, plants, and animals allow scientists to address biological queries and design practical solutions.
Creating a Transgenic Organism
Scientists begin the process of transgenesis—introducing a transgene into an organism’s genome—by selecting an appropriate technique. There are several biological, chemical, and physical methods of transgenesis. A common biological method involves the virus-mediated introduction of foreign DNA into a host cell genome, called transduction. A popular chemical method uses calcium phosphate (Ca3(PO4)2). The method is based on the formation of a Ca3(PO4)2/DNA precipitate to facilitate DNA binding to and entering cells. Physical methods such as microinjection—a technique that uses a thin, glass needle to manually insert genetic material into cells—artificially introduce DNA by force.
Once inside the cell, a transgene can either integrate randomly or at a specific site in the genome with the help of DNA repair enzymes (i.e., recombination). These transgenic cells then multiply and replicate the transgene as part of their genome, stably expressing the researcher’s gene of interest. A transgene may not integrate into the genome, and hence induce only transient expression of the researcher’s gene of interest. Usually, a selectable marker (e.g., antibiotic resistance gene) or a reporter gene (e.g., GFP) are included along with the gene of interest, so that cells with successful transgene integration can be identified.
Transgenes Can Be Introduced into Animals and Plants
In animals, the transgene is typically inserted into an early-stage, fertilized egg by microinjection. The hope is that the transgene will integrate into the germ cells—reproductive precursor cells that become gametes (i.e., egg or sperm)—so that it will express in all of the developing organism’s cells. Furthermore, germline integration is heritable, meaning the transgene can be passed down through generations by breeding. The transgenic animals are backcrossed—the offspring are mated with the parent—to create lines of animals that are homozygous for the transgene.
Plant transgenesis routinely uses a biological method, such as bacterial vector delivery, to introduce foreign DNA into cells. Rhizobium radiobacter (formerly known as Agrobacterium tumefaciens) is a soil-dwelling, pathogenic bacterium that can infect plants and integrate its plasmid DNA into the plant’s genome. Scientists have modified R. radiobacter so that the plasmid DNA can carry a transgene. Plant tissue samples are cultured with R. radiobacter to allow for infection and the integration of the transgene. These tissues are further cultured on selective media that induce shoot and root growth until the nascent plant can be transferred to soil. These transgenic plants are backcrossed to create lines of high yield, transgenic plants.
Practical Uses of Transgenic Organisms
Transgenic organisms have many applications in agriculture, science, industry, and medicine. For example, transgenic plants have been produced that are insect-resistant to increase yield and reduce the use of pesticides (e.g., Bt corn); bacteria have been engineered for use in biomedical research and to produce biofuels; and transgenic animals have been used to manufacture medicines—such as human proteins—and to create models of human disease. Scientists leverage the power of transgenic plants, bacteria, and animals to research gene expression, create desired gene products, or promote valuable traits.