Recombinant DNA Methodology
Recombinant DNA is a relatively new science that allows scientists to manipulate organisms and introduce new genes to allow them to do things they would never normally do. Scientists have created bacteria that produce human insulin, plants that are resistant to herbicides, and even helped change human genes. Regardless of the task, the methods biomedical engineers use to perform these recombinant DNA experiments are fairly simple.
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Structure of DNA
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DNA's structure is most commonly referred to as a double helix. At its simplest, DNA is a repeating chain of units called nucleotides. Each nucleotide contains a phosphate group, a sugar and a nitrogenous base. A molecule of DNA has two strands of phosphates and sugars running parallel to each other. Each sugar pair has two nitrogenous bases sticking together in between. There are four nitrogenous bases in DNA called guanine, adenine, thymine and cytosine but they are usually represented by the letters GATC. Guanine and cytosine always stick together, and adenine and thymine always stick together. The order of these nitrogenous bases determines the function of the DNA. It is this order that makes humans different from a gorilla or any other organism on the planet.
Preparing the Host DNA
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In order to insert a new gene into an organism, you need to know the sequence of the host's DNA. This is because you are going to splice open the DNA leaving some exposed nucleotides using chemicals called restriction enzymes. You can purchase a pre-made restriction enzyme for common applications involving bacteria and some human purposes, but you can also create your own restriction enzymes.
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Creating the New Gene
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When the restriction enzyme cuts open the DNA, the exposed nucleotides are known as sticky ends. If you know one sticky end has the sequence ATCG left open, you know the other end of your new gene will have to have TAGC at the end to fit in place. Once you know what sticky ends are going to be left behind by the restriction enzymes, you can begin to sequence the new gene. The gene in the middle usually comes from another source, so the sequence should be readily available. For instance, when creating insulin producing bacteria, researchers already know the sequence of DNA that produces insulin, so they simply add the appropriate sticky ends for the application and the gene is ready.
Introducing the New Gene
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Getting the new gene to the organism can occur in a number of different ways. Introduction to bacteria can be as simple as adding the gene to a solution with the bacteria, and the bacteria will take the DNA into the cell. Another method involves using crippled viruses called phages. A phage is a virus with the desired DNA product within. When it attacks the host cell, it injects the DNA inside. Since it is weakened, however, the virus doesn't infect the cell. Alternatively, scientists can directly inject the DNA into the nucleus of a cell using precise microtechnology.
Sealing It All Together
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At this point, the new DNA is sequenced and in place, and the restriction enzyme has cut open the original DNA and left the sticky ends waiting to grab the new gene. The final part of the process is done by an enzyme called DNA ligase. DNA ligase acts as a zipper on the cut up DNA, resealing all of the sticky ends together to produce one final strand of DNA. Once the DNA is sealed back together, with the new gene in place, the cell can begin using that new information to code for the desired protein.
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