Life
Deoxyribonucleic Acid (DNA) reveals an individuals unique genetic code, something very helpful to reveal clues in various crimes around the world. By recovering DNA from a crime scene, technicians can perform various tests to compare and analyze the DNA sequence. Among these tests are Gel Electrophoresis, the Sanger Method, and Southern Blotting. DNA can be extracted from almost any human tissue. Sources of DNA found at a crime scene might include blood, seamen, tissue from deceased victim, cells in a hair follicle, and even saliva. DNA extracted from items of evidence is compared to DNA extracted from reference samples from known individuals, normally from blood. Extracted DNA is treated with restriction endonuclease, which is an enzyme that will cut double stranded DNA whenever a specific DNA sequence occurs. The enzyme most commonly used in forensic DNA analysis is HaeIII, which cutes DNA at the sequence 5'-GGCC-3'. Following DNA digestion, the resulting DNA fragments are separated by size via electrophoresis in agarose gels (The Biology Project 1). Gel electrophoresis is used to distinguish between samples of genetic material. Because DNA is a charged molecule, it will move when an electrical field is applied to a
The techniques available such as southern blotting, the Sanger method, and gel electrophoresis are very accurate, fairly quick, and relatively inexpensive (Trudinger 1). The laser detects the color and the computer records the identity of each fragment (DNA Techniques 2). Part of this pattern comes from the size of the DNA; part of it comes from the sequence of the DNA of a specific size (Fairfiled 1). Supported by glass plates, the gel is bathed in an aqueous or polyacrylamide solution (depending on the size of molecules to be separated) (DNA Techniques 1). If human genomic DNA (all three billion baspairs of it) is digested with Hind II and the resulting mixture of fragments run out on agarose gel, it would be impossible to detect the prescience or absence of one particular fragment. With the ladders side by side the different bases can be read out sequentially, allowing the DNA sequence to be determined in a straightforward way (Preuss 2). The DNA molecules then start moving across the solution from some initial small volume, that is, from essentially the same starting point, then the molecules can move at perceptibly different speeds. If the molecules fall into only a few discreet sizes, then bands (little rectangles) of DNA will appear in the gel. Invented by Robert Southern, Southern blotting is a two-step process: first the DNA fragments must be permanently transferred to a stable matrix, then fragments must be probed with a labeled DNA fragment that is complementary to the fragment of interest within the gel. Developed by British biochemist Frederick Sanger, this method is the first to combine the use of restriction enzymes, DNA polymerases, gel electrophoresis, and radioactive labeling to read the sequences of DNA fragments up to 500 base pairs long. The transfer is effected through capillary action, using a "sandwich" in which the gel and membrane are held between stacks of filter paper. Since its first use in an Australian court in 1989, the technology has moved from the controversial to the mainstream. In automated capillary electrophoresis machines four different fluorescent colors are used to mark ddNTPs. After a while, the molecules are separated by size.
Common topics in this essay:
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Frederick Sanger,
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dna techniques 1,
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