DNA testing is the process of using DNA to assist in the identification of individuals. While 99.9 percent of human DNA sequences are the same, scientists are able to use repeat sequences in the DNA to distinguish individuals from each other since this information is always different. DNA testing is also known as DNA profiling, DNA typing or genetic fingerprinting and can be used to link suspects to crimes, establish paternity or maternity, prove blood relationship or determine genealogical roots.
- The fundamental key of DNA testing is in finding the differences in specific DNA sequences. Two sets of DNA are needed for comparison. In forensics, for example, these are often the DNA samples from the crime scene compared with the DNA from a suspect. The first step in testing may vary depending on different techniques (described in the following sections) but is basically the isolation of a part of a DNA sequence. The cut DNA fragments are then sorted by size using gel electrophoresis. A blot of the gel is made to record the gel's results and a probe is released onto the blot to find and bind onto a similar DNA sequence as the one associated with the probe.
- Restriction fragment length polymorphism (RFLP) is a technique of DNA isolation that is often used in forensics. Generally, RFLP requires large amounts of undegraded DNA, so it is ideal for crime scenes that are new with a lot of evidence. Once sufficient samples are obtained, restriction enzymes are used on the DNA to isolate a sequence known as AATT. The fragments are then run through a gel electrophoresis, made into a blot and probed (as mentioned above). The sizes of the DNA fragments found by the probe are analyzed and the results of each sample of DNA are compared. A match may be declared if the bands are all within 5 percent of one another in size.
- Polymerase chain reaction (PCR) is another technique of DNA amplification commonly used in forensics. Unlike RFLP, PCR has the ability to use information from very small and even degraded samples by greatly amplifying specific regions of DNA. PCR uses two short pieces of DNA known as oligonucleotide primers and a thermostable DNA polymerase to isolate the region of DNA to be copied. The samples of isolated DNA are then run under gel electrophoresis and blotted just as with RFLP for analysis and comparison. It should be noted that contamination of the DNA sample is of particular importance in PCR analysis. If any type of other DNA from either other people or things such as bacteria are present, many copies of the isolated DNA fragments may be made from the wrong source and give false results.
- Short tandem repeats (STR) are small regions of short repeated sequences in DNA which vary greatly between unrelated individuals. Only 5 to 20 percent of individuals share the same repeated sequences, so the identification of this particular region is very useful in identifying individuals. The more regions that are isolated and tested, the more accurate the results will be. STR is often isolated using PCR due to PCR's amplifying effects.
- Amplified fragment length polymorphism (AmpFLP) is a technique that relies on variable number tandem repeats (VNTR) to distinguish alleles, which are different forms of a gene. PCR is used to amplify DNA samples that are then run on a polyacrylamide gel to separate alleles instead of by size like other tests do. This particular test is highly automated and low-cost, so it is fairly popular in lower-income countries.
- Mutliplex STR is a commonly encountered STR test that utilizes several PCRs in one reaction to test various fragments at the same time. This form of testing is designed to isolate STR with different sizes from STR with overlapping sizes by applying different dyes to distinguish them from each other. The test also utilizes capillary tubes and electroferograms instead of gel electrophoresis to separate the DNA fragments. The benefit of this kind of testing is the quick results compared to other kinds of testing. However, multiplex STR is fairly new and can be subject to overinterpretation when partial profiles are used.
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