A donor gene is a specific segment of DNA selected from one organism for insertion into another. The goal is to introduce a new capability or characteristic into the recipient, or host, organism. This precise transfer of a single functional unit of DNA distinguishes genetic engineering from traditional selective breeding, which mixes the entire set of genes from two organisms.
The introduction of this foreign DNA, which carries the code for a desired trait, results in a genetically modified organism (GMO). This technology fundamentally alters an organism’s genetic makeup to achieve a specific outcome, such as enhancing its nutritional value or making it resistant to disease.
Sources of Donor Genes
Scientists obtain donor genes from two primary origins: natural living organisms and artificial laboratory synthesis. Genes can be identified and isolated from a vast array of natural sources, including bacteria, fungi, plants, and animals. For example, a gene from a soil bacterium can be selected for its ability to produce a protein that is toxic to crop-destroying insects. This process involves identifying the specific gene responsible for a desirable trait within the donor organism’s entire genome.
The second source is synthetic creation. Using laboratory equipment called DNA synthesizers, scientists can construct genes from scratch. This capability allows for the creation of entirely novel genes or the modification of existing ones to enhance their function. This artificial synthesis provides a high level of control, enabling researchers to design genes with optimized expression or unique properties tailored to the goals of a project.
The Role of Donor Genes in Genetic Modification
A donor gene provides the host cell’s machinery with the instructions needed to produce a new protein, which in turn manifests as the desired trait. This is a direct method for altering an organism’s characteristics in a predictable way.
The traits imparted by donor genes are diverse. A gene might provide a plant with resistance to a particular herbicide, allowing farmers to control weeds without harming their crops. In another context, a donor gene could enable a microorganism to produce a valuable pharmaceutical compound. The modification is not always about adding a function; sometimes the goal is to switch off an existing gene that causes an undesirable quality.
To ensure the donor gene functions correctly, it is inserted along with additional genetic elements. A “promoter” sequence acts as a switch, controlling when and where the gene is expressed within the host organism. A “marker gene” is also frequently included, which allows scientists to easily identify which cells have successfully incorporated the new genetic material. This entire package is called a construct, and it is designed for efficient integration.
The Gene Transfer Process
The delivery of a donor gene into a host cell relies on a delivery vehicle known as a vector. Vectors carry the donor gene and its associated elements into the host’s cells and facilitate its integration into the host’s genome. The most common vectors are plasmids, which are small, circular DNA molecules from bacteria, and modified viruses.
The process begins with “molecular scissors” called restriction enzymes. These enzymes cut the desired donor gene from its source DNA and cut open the circular plasmid vector. Using the same enzyme for both cuts creates compatible “sticky ends” on the donor gene and the plasmid, allowing the gene to fit precisely into the opening.
Once the donor gene is positioned, an enzyme called DNA ligase acts as “molecular glue.” It forms strong chemical bonds, permanently splicing the donor gene into the vector. This newly formed, combined DNA molecule is known as recombinant DNA. The recombinant plasmid is now ready to be introduced into the host organism.
The final step is getting the vector into the host cells, a process called transformation. For bacteria, this can be achieved by applying a brief heat shock. For plant and animal cells, methods like microinjection or using a “gene gun” can be employed. When using viral vectors, scientists take advantage of the virus’s natural ability to infect cells and deliver its genetic payload, after first removing the viral genes that cause disease.
Applications in Science and Medicine
In medicine, one of the first uses of donor genes was the production of human insulin for treating diabetes. Scientists isolated the human gene for insulin and inserted it into E. coli bacteria using a plasmid vector. This transformed the bacteria into microscopic factories that could produce vast quantities of pure human insulin, which was first commercialized in 1982.
In agriculture, donor genes have created crops with built-in protection against pests. A well-known example is Bt corn, which incorporates a gene from the soil bacterium Bacillus thuringiensis (Bt). This gene produces a protein that is toxic to the larvae of certain insects but is harmless to humans. This allows the corn plants to defend themselves, reducing the need for chemical insecticide sprays.
Scientific research has also been transformed by using donor genes as markers. The gene for Green Fluorescent Protein (GFP), originally from a jellyfish, produces a protein that glows green under ultraviolet light. Researchers can attach the GFP gene to another gene of interest and insert them together into an organism. If the transfer is successful, the cells will glow green, providing a clear visual signal that the donor gene has been incorporated and is being expressed.