STR analysis

Source: Wikipedia, the free encyclopedia.
Short tandem repeat (STR) analysis on a simplified model using polymerase chain reaction (PCR): First, a DNA sample undergoes PCR with primers targeting certain STRs (which vary in lengths between individuals and their alleles). The resultant fragments are separated by size (such as electrophoresis).[1]
A partial human STR profile obtained using the Applied Biosystems Identifiler kit

Short tandem repeat (STR) analysis is a common

microsatellite with repeat units that are 2 to 7 base pairs in length, with the number of repeats varying among individuals, making STRs effective for human identification purposes.[2] This method differs from restriction fragment length polymorphism analysis (RFLP) since STR analysis does not cut the DNA with restriction enzymes. Instead, polymerase chain reaction
(PCR) is employed to discover the lengths of the short tandem repeats based on the length of the PCR product.

Forensic uses

Main article:
Short tandem repeats

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.[3] Scientific tools such as FBI approved STRmix incorporate this research technique.[4][5] 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[6] 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
.

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.[6] 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.[6] Given 10 loci, it can result in an error margin of 30%, or nearly one third of the time.[7]

From country to country, different STR-based DNA-profiling systems are in use. In North America, systems that amplify the

National DNA Database
) is in use. Whichever system is used, many of the STR regions used are the same. These DNA-profiling systems are based on multiplex reactions, whereby many STR regions will be tested at the same time.

The true power of STR analysis is in its statistical power of discrimination. Because the 13 loci that are currently used for discrimination in CODIS are

monozygotic twins
on Earth, the theoretical probability is not accurate.

In practice, the risk of contaminated-matching is much greater than matching a distant relative, such as contamination of a sample from nearby objects, or from left-over cells transferred from a prior test. The risk is greater for matching the most common person in the samples: Everything collected from, or in contact with, a victim is a major source of contamination for any other samples brought into a lab. For that reason, multiple control-samples are typically tested in order to ensure that they stayed clean, when prepared during the same period as the actual test samples. Unexpected matches (or variations) in several control-samples indicates a high probability of contamination for the actual test samples. In a relationship test, the full DNA profiles should differ (except for twins), to prove that a person was not matched as being related to their own DNA in another sample.[citation needed]

In biomedical research, STR profiles are used to authenticate cell lines.[9] Self-generated STR profiles can be compared with databases such as CLASTR (https://www.cellosaurus.org/cellosaurus-str-search/) or STRBase (https://strbase.nist.gov/). In addition, self-generated primary murine cell lines cultured before the first passaging can be matched with later passages, thus ensuring the identity of the cell line.

See also

References

  1. ^ Image by Mikael Häggström, MD, using following source image: Figure 1 - available via license: Creative Commons Attribution 4.0 International", from the following article:
    Roberta Sitnik, Margareth Afonso Torres, Nydia Strachman Bacal, João Renato Rebello Pinho (2006). "Using PCR for molecular monitoring of post-transplantation chimerism". Einstein (Sao Paulo). 4 (2).{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. ISBN 9780123745132
    .
  3. ^ National Commission on the Future of DNA Evidence (July 2002). "Using DNA to Solve Cold Cases" (PDF). U.S. Department of Justice. Retrieved 2006-08-08.
  4. ^ https://dfs.dc.gov/sites/default/files/dc/sites/dfs/page_content/attachments/STRmix%20Validation.pdf [bare URL PDF]
  5. PMID 28504203
    .
  6. ^
    PMID 2780284
    .
  7. PMID 17339205
    .
  8. ^ Felch, Jason; et al. (July 20, 2008). "FBI resists scrutiny of 'matches'". Los Angeles Times. pp. P8.
  9. PMID 33317212
    .
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