How Does DNA Forensics Work?
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Using DNA, forensic scientists can accurately identify individuals that are suspects or victims of a crime. They use DNA sequencing technology to compare DNA samples by profiling 13 regions of the genome. Samples of a person's DNA can be obtained from blood, hair, bone and other parts of the body.
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First, the DNA must be extracted from an individual, such as in a crime scene, before scientists can further analyze it. Forensics experts then use sophisticated laboratory equipment to compare DNA samples. Markers are found in a sample by scientists that create DNA pieces, called probes, that bind to another DNA sequence inside the sample. These DNA probes form a series that forms a distinctive pattern for a particular individual. Therefore, forensic scientists can compare the profiles to determine if a suspect's sample matches the evidence. The more probes that are used, the more accurate the identification will be, but if 2 samples are alike in 4 or 5 regions, then it is a likely match.
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This is the general way in which DNA is analyzed. However, there are a few different technologies that are used in forensic investigations. One is called Restriction Fragment Length Polymorphism, which involves analyzing variable lengths of DNA fragments resulting from digesting a DNA sample with an enzyme. The enzyme cuts the DNA into a pattern of recognition sites, the presence or absence which generates variable length fragments that are compared with another sequence in the sample.
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Another technique in DNA forensics is known as PCR analysis. A small sample of DNA must be well preserved, but millions of exact copies of DNA can be made to be analyzed. Other types include mitochondrial DNA analysis and Y-chromosome analysis. A very accurate and often-used technique is STR analysis, which locates specific regions within nuclear DNA. The FBI uses 13 specific regions of DNA for a software program called CODIS, or Combined DNA Index System, which operates local, state, and national databases of DNA from convicted offenders, unsolved crimes and missing persons. There is a one in a billion chance that the 13 regions of DNA in any 2 individuals will be identical.
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The National DNA Databank, maintained by the CODIS software, contains over 5 million DNA profiles in its index. There are also 188,000 DNA profiles collected from crime scenes but not from any particular person. Any agency can take DNA evidence from a crime scene and use the software to compare it with profiles in the database. There are standing ethical, legal and social concerns about DNA databanking, because DNA provides so much information about a person outside of convicting or exonerating them. The accuracy of DNA in matching someone to a crime though certainly cannot be ignored.
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