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Microsatellites or short tandem repeats (STRs) are sequences made up of a single sequence motif, no more than six base long, that is tandemly repeated, that is, arranged head-to-tail without interruption by any other base or motif but many microsatellite loci amplified by PCR in fact contain interruptions. Microsatellites have been detected within the genomes of every organism sofar analysed, and are often found at frequencies much higer than would be predicted purely on the basis of base composition. Microsatellites show high levels of polymorphism, and therefore the most useful for many purposes Microsatellites have assumed an increasingly important role as markers have been applied in fields as disparate as tumor biology , personal identificación population geneticanalysis, and the construction of human evolutionary trees).
Within the past decade microsatellites have developed into one of the most popular genetic markers. It had been known for some time that these sequences can differ in repeat number among individuals. Whit the advent of the PCR this property of microsatellite DNA was converted into a highly versatile genetic marker. PCR productsdifferent length can be amplified with primers flanking the variable microsatellite region. Due to the aviability of hig-througput capillary sequencers or mass spectrography the sizing of alleles is no longer a bottleneck in microsatellite analysis.
Microsatellites are associated with human disease, not only as markers of risk but also directly in disease aetiopathogenesis providing new insights intonon-Mendelian inheritance.

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A microsatellite consists of a specific sequence of DNA bases or nucleotides which contains mono, di, tri, or tetra tandem repeats. In the literature they can also be called simple sequence repeats (SSR), short tandem repeats (STR), or variable number tandem repeats (VNTR).  Alleles at a specific location (locus) can differ in the numberof repeats.  Microsastellites are inherited in a Mendelian fashion.
Because microsatellites are widely dispersed in eukaryotic genomes,  are highly variable, and are PCR based (requiring only minute amounts of starting template)  they have been used in many different areas of research such as:
Forensics- Microsatellite loci, generally known in forensic applications as Short Tandem Repeat (STR)loci, are widely used for forensic identification and relatedness testing, and are a predominant genetic marker in this area of application.  In forensic identification cases, the goal is typically to link a suspect with a sample of blood, semen or hair taken from a crime.  Alternatively, the goal may be to link a sample found on a suspect's clothing with a victim. Relatedness testing in criminalwork may involve investigating paternity in order to establish rape or incest.  Another application involves linking DNA samples with relatives of a missing person.  Because the lengths of microsatellites may vary from one person to the next, scientists have begun to use them to identify criminals and to determine paternity, a procedure known as DNA profiling or "fingerprinting". The features thathave made use of microsatellites attractive are due to their relative ease of use, accuracy of typing and high levels of polymorphism.  The ability to employ PCR to amplify small samples is particularly valuable in this setting, since in criminal casework only minute samples of DNA may be available. 
Diagnosis and Identification of Human Diseases- Because microsatellites change in length earlyin the development of some cancers, they are useful markers for early cancer detection.  Because they are polymorphic they are useful in linkage studies which attempt to locate genes responsible for various genetic disorders.
Population Studies- By looking at the variation of microsatellites in populations, inferences can be made about population structures and differences, genetic drift,...
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