<P> A Short Tandem Repeat (STR) analysis is one of the most useful methods in molecular biology which is used to compare specific loci on DNA from two or more samples . A short tandem repeat is a microsatellite, consisting of a unit of two to thirteen nucleotides repeated hundreds of times in a row on the DNA strand . STR analysis measures the exact number of repeating units . This method differs from restriction fragment length polymorphism analysis (RFLP) since STR analysis does not cut the DNA with restriction enzymes . Instead, probes are attached to desired regions on the DNA, and a polymerase chain reaction (PCR) is employed to discover the lengths of the short tandem repeats . </P> <P> STR analysis is a tool in forensic analysis that evaluates specific STR regions found on nuclear DNA . The variable (polymorphic) nature of the STR regions that are analyzed for forensic testing intensifies the discrimination between one DNA profile and another . Forensic science takes advantage of the population's variability in STR lengths, enabling scientists to distinguish one DNA sample from another . The system of DNA profiling used today is based on PCR and uses simple sequences or short tandem repeats (STR). This method uses highly polymorphic regions that have short repeated sequences of DNA (the most common is 4 bases repeated, but there are other lengths in use, including 3 and 5 bases). Because unrelated people almost certainly have different numbers of repeat units, STRs can be used to discriminate between unrelated individuals . These STR loci (locations on a chromosome) are targeted with sequence - specific primers and amplified using PCR . The DNA fragments that result are then separated and detected using electrophoresis . There are two common methods of separation and detection, capillary electrophoresis (CE) and gel electrophoresis . </P> <P> Each STR is polymorphic, but the number of alleles is very small . Typically each STR allele will be shared by around 5 - 20% of individuals . The power of STR analysis comes from looking at multiple STR loci simultaneously . The pattern of alleles can identify an individual quite accurately . Thus STR analysis provides an excellent identification tool . The more STR regions that are tested in an individual the more discriminating the test becomes . </P>

What is it about short tandem repeat dna that makes it useful for dna fingerprinting
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