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

Another method of DNA analysis is the Sanger Method. 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. In the basic technique, numerous copies are generated from a single template strand of the DNA to be sequenced (Preuss 1). The method is based on the random incorporation of a modified nucleotide, ddNTP (dideoxynucleotides with 3'H instead of 3'OH on sugar) which cannot make phosphodiester bond with the next dNTP thus blocking further DNA synthesis. A series of DNA fragments is synthesized that reflects all the positions of the modified nucleotides, and thus ultimately the sequence of the DNA (DNA Techniques 2). These copies are produced using four special reagents that randomly stop the formation of complementary strands at different bases, depending on which reagent is introduced. For example, one chemical stops the reaction whenever it encounters a cytosine base in the sequence. The low concentration of this chemical ensures that somewhere among the fragments of many different lengths a fragment terminus will represent every cytosine in the sequence. The same process is repeated for all four bases. The four sets of DNA fragments, or "ladders", are then drawn through a porous gel by an electric field; the smaller pieces move readily, while the larger pieces may hardly move at all, and this whole process may take hours. 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). To determine the DNA sequence a technician reads the order of the bands from bottom to top. In automated capillary electrophoresis machines four different fluorescent colors are used to mark ddNTPs. The laser detects the color and the computer records the identity of each fragment (DNA Techniques 2).

While forensic scientist use d to use blood typing

Some common words found in the essay are:
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