Recombinant DNA Definition
Recombinant DNA is a molecule of DNA that has been modified, either through genetic recombination or through laboratory techniques. In eukaryotic organisms, genetic recombination occurs during meiosis in a process known as crossing over. 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. 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.
Scientist 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 reproduces, 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. The DNA of viruses can also be recombined, to deliver specific genes and proteins to cells in an organism.
Examples of Recombinant DNA
Example #1: 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.
While each organisms 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 phenotypic variety produced by organisms could not take place. 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.
Example #2: 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, scientist have placed genes from a 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. Scientist create recombinant DNA from the genomes of these bacteria. The recombinant DNA is then inserted into the genome of crop being protected. When the new plants start to grow, their cells express the bacterial DNA and the protein Bt is produced. While there is some concern about recombinant DNA used to create human food, it is nonetheless used in more than one-third of the commercially grown corn in the U.S.!
This is due to the recombinant DNA protecting the crops from insects, which damage crops without the gene. 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. Genetically engineered crops containing recombinant DNA are being used in a wide variety of applications globally, and are increasing at an exponential rate. Someday, scientist may be able to produce a crop through genetic engineering that solve the human hunger crisis.
Example #3: 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. One drug, hydroxyurea, stimulates the production of more hemoglobin, but has toxic side-effects. Alternatively, a patient can undergo a painful and dangerous bone marrow transplant. But, before a transplant can even begin the tissues of the patient must be matched precisely with a correct donor. Without perfect matching, the patient can reject the new bone marrow, leading to inflammation, pain, and even death.
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 recombinant DNA for proper blood cell formation through an altered HIV virus. Since HIV has a proclivity for the human immune system, it readily deposited the recombinant DNA into stem cells taken from the host. To stop the previous bone marrow from competing with the transformed cells, the marrow is broken down in the patient with X-rays. The transformed cells with recombinant DNA were then reinserted into the mice, curing their sickle-cell disease.
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 can self-replicate and could reproduce in the environment. While the full consequences of recombinant DNA 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.
Related Biology Terms
- Genetic Recombination – A biological process in which parts of homologous chromosomes are exchanged.
- Genetically Modified Organism – An organism that contains recombinant DNA created in a laboratory.
- Genetic Engineering – The process of artificially modifying an organism’s genome.
1. A bacteria is dividing through binary fission. While duplicating its genome, a mistake is made. While the first daughter cell is identical to the original cell, the second contains a mutation. The mutation causes a specific protein to function faster, and produce more product. Oddly enough, this gives the bacteria a resistance to a certain bacterial virus. Does the second cell contain recombinant DNA?
C. Only if the bacteria contains a plasmid
2. A researcher extracts the gene that produces insulin, and places it within a plasmid. The researcher inserts the plasmid into a bacterial cell. The cell reproduces many times, creating many bacteria. Do all of these bacteria have recombinant DNA?
A. Only the ones with strands of the original recombinant DNA
B. None of the cells do once the first cell has divided and died
C. All of the cells contain recombinant DNA
3. Which of the following is NOT an example of recombinant DNA?
A. In a horse, crossing-over occurs, which redistributes genes onto different chromosomes
B. Scientist insert genes from a flower into cows, causing them to smell nice
C. A virus infects a cell, and hijacks cellular mechanisms to reproduce