How Is DNA Extracted & Examined?
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Extraction Techniques
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DNA extraction is a process involving many steps, and the exact procedure differs on what type of DNA is being extracted and for what purpose it is being extracted. A popular technique for identifying bacteria, for instance, is Fluorescence In Situ Hybridization, or FISH, which uses a fluorescent dye to detect the presence of DNA. Other techniques include Terminal Restriction Fragment Length Polymorphism and Sequencing, the latter of which is used to compare new DNA samples with existent samples on public record for criminal cases.
DNA is located in proteins inside cells, and the cells must be broken to be accessed. There are several procedures to open cells, including sonicating, bead-beating and vortexing. These use sound, force, heat and chemicals to deteriorate the cell barriers until they break. An additional chemical is often included to help flush out or deter other parts of the cell so the proteins can be examined more closely.
The DNA proteins are then broken down with some type of acidic solution and precipitated by being washed with cold ethanol or isopropanol, which washes away the extra substances previously used while preserving the DNA, which is not harmed by the alcohol mixture. This process forms a microscopic clump of DNA that is then recombined with chemicals to form a solution that can be tested for DNA properties.
This entire process requires several very specific instruments, including a bead beater and a centrifuge. A bead beater breaks apart cells by vibrating them against glass beads, and while there are other ways to lyse cells, a bead beater is one of the cheapest and most common. A centrifuge spins solutions at very fast speeds to separate DNA from various substances.
Comparing DNA
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Once the DNA has been isolated from the cells it was once contained in, a number of tests can be run on it. The most famous test is the 13-loci test that forensic scientists run to compare different DNA samples to see if they come from the same person. This is done by examining 13 regions of the DNA which differ slightly from person to person. While there may be people in whom these 13 regions are identical, this is extremely rare and usually only thought of as a theoretical possibility, since the loci act very much like fingerprints, providing a unique signature that can identify people.
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Uses
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DNA testing is done for a number of reasons. With humans, DNA is compared to identify suspects in crimes, identify victims of disasters who may not be recognizable and establish proper relationships in paternity cases. DNA is also used to detect bacteria, help find recipients for organ donors, establish proper pedigree for certain breeds of animals and even identify the origin of rare wines.
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