Definition
Recombinant DNA is a molecule of DNA that has been modified to include genes from multiple sources, either through genetic recombination or through laboratory techniques. In the lab, bacteria can be transformed with recombinant DNA. Genetic recombination occurs during meiosis in a process known as crossing over.
Overview
Recombinant DNA in eukaryotes is responsible for increasing genetic diversity. Alleles of genes that were previously linked on a chromosome can be completely redistributed to create new combinations of traits. This process happens regularly during meiosis to mix and match genes from paternal and maternal sources.
In genetic engineering, scientists use recombinant DNA created in the laboratory or extracted from an organism to add to the genome of another organism. Because of the universal design of DNA, the recombinant DNA does not have to stay in the same species. This means that scientists can easily add genes from one species into bacteria to produce a product.
For example, insulin is regularly produced by means of recombinant DNA within bacteria. A human insulin gene is introduced into a plasmid, which is then introduced to a bacterial cell. The bacteria will then use its cellular machinery to produce the protein insulin, which can be collected and distributed to patients.
Recombinant DNA Examples
Meiosis in Eukaryotes
Eukaryotic organisms that go through sexual reproduction must also go through the process of meiosis, which reduces the genetic material leading to fertilization. During meiosis, the chromosomes of eukaryotes are condensed, and pair with their homologous chromosome. Each pair of homologous chromosomes represents the same sequence of DNA, from different parental origins. When the homologs are connected during meiosis, they can exchange similar sequences of DNA is the process of crossing-over.
While each organism has tens of thousands of genes, the number of chromosomes is much smaller. This necessitates that there be more than one gene per chromosome, hundreds usually. If genetic recombination did not take place, the variety between these genes would be limited.
For example, pretend that there are only two alleles for coat color in a population, black and white. There are also two alleles for eye color, brown and blue. If the gene for eye color and the gene for coat color exist on the same chromosome, they are called linked genes. Without recombinant DNA, an organism could only pass on the combination of alleles that was passed from its parents.
Insect Resistant Crops
Genetic engineering and recombinant DNA are widely used in modern agriculture. For centuries, farmers have been trying to make their crops resistant to both insects, and the herbicides used on weeds. With the advent of genetic engineering, scientists are able to identify and segregate genes of interest and place them in crop species.
To increase insect resistance, for instance, scientists have placed genes from bacteria into the DNA of corn, cotton, and other crops. The genes they selected produce the protein Bt. This protein is lethal to insect larvae that eat it. Scientists create recombinant DNA from the genomes of these bacteria. The new DNA is then inserted into the genome of the crop being protected. When the new plants start to grow, their cells express the bacterial DNA and the protein Bt is produced.
Farmers that do not grow genetically modified crops must spray their crops with pesticides, which are very expensive. Crops that produce Bt protect themselves as they grow. This is important, considering that crop-eating insects do over a billion dollars of damage annually. With genetic engineering, this loss could be averted.
Gene Therapy
Sickle-cell disease is an inherited blood disorder that affects many millions of people worldwide. The condition actually increased in prevalence because in it milder forms it confers resistance to malaria. Like many genetic disorders, there is currently no cure. Patients with sickle-cell disease must undergo a variety of dangerous procedures to extend their life.
However, gene therapy is an emerging medical technique that uses recombinant DNA to restore function to cells stricken by genetic disorders. Sickle-cell anemia was one of the first diseases to be reversed by gene therapy. Mice with the sickle-cell traits were treated by Harvard researchers by delivering the DNA for proper blood cell formation through an altered HIV virus. Since HIV has a proclivity for the immune system, it readily deposited the recombinant DNA into stem cells taken from the host.
The same concept has been used in humans as early as 1990, although mass treatment is still not available. The use of viruses with recombinant DNA is a contentious subject, as a virus could reproduce in the environment with unintended consequences. While the full consequences of these actions are unknown, their many benefits continue to pressure policy-makers and the public into accepting them. With the proper guidelines, recombinant DNA technology is sure to revolutionize the world in a positive way.
Recombinant DNA Process
Scientists regularly use recombinant DNA to add traits to certain species of bacteria or produce organisms which have additional traits. There is a basic process for getting recombinant DNA into cells, though the exact method varies depending on the specific organism.
In general, the first part of the process includes creating a plasmid which contains the sequence of DNA which will be added to an organism. The simplest organism to add recombinant DNA to is bacteria. Bacterial cells reproduce quickly, which allows many chances for the recombinant DNA to enter a cell and proliferate.
After creating a plasmid containing the recombinant DNA, it must be added to the cells. To do this, the cells are commonly heated to the point that their cell membranes become more permeable. Some cells will die, but the plasmid will successfully work its way into some of the bacterial cells present.
The final process of creating organisms with recombinant DNA is to allow the cells to cool and grow. Often, the plasmid introduced also has a gene which enables the bacteria to survive antibiotic treatments. When growing the transformed bacteria, an antibiotic is introduced. Any bacteria that survive are ones that have been transformed with recombinant DNA. They now have the plasmid, which includes both the recombinant DNA and a gene for antibiotic resistance.
Uses of Recombinant DNA
Scientists can use this feature of DNA for many purposes. First, any gene of interest can be easily replicated by inserting the gene into a bacterial plasmid and letting the bacteria reproduce normally. Plasmids are small rings of DNA. If the exact sequence of the plasmid is known, a scientist can cut the ring open using special proteins called restriction enzymes.
Once the plasmid is opened, the gene of choice can be inserted. If all the right sequences are present, the bacteria that absorbs the plasmid will produce the protein encoded for by the recombinant DNA. Further, when the bacteria reproduce, the gene will also reproduce. Bacteria can double their population in less than an hour, which can lead to large bacterial populations producing large amounts of a product for scientific, medical, or industrial purposes.
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