Microsatellite - Forensic Analysis

Forensic Analysis

Microsatellite analysis is a relatively new technology in the field of forensics, having come into popularity in the mid-to-late 1990s. It is used for the genetic fingerprinting of individuals. The microsatellites in use today for forensic analysis are all tetra- or penta-nucleotide repeats (4 or 5 repeated nucleotides), as these give a high degree of error-free data while being robust enough to survive degradation in non-ideal conditions. Shorter repeat sequences tend to suffer from artifacts such as PCR stutter and preferential amplification, as well as the fact that several genetic diseases are associated with tri-nucleotide repeats such as Huntington's disease. Longer repeat sequences will suffer more highly from environmental degradation and do not amplify by PCR as well as shorter sequences.

The analysis is performed by extracting nuclear DNA from the cells of a forensic sample of interest, then amplifying specific polymorphic regions of the extracted DNA by means of the polymerase chain reaction. Once these sequences have been amplified, they are resolved either through gel electrophoresis or capillary electrophoresis, which will allow the analyst to determine how many repeats of the microsatellites sequence in question there are. If the DNA was resolved by gel electrophoresis, the DNA can be visualized either by silver staining (low sensitivity, safe, inexpensive), or an intercalating dye such as ethidium bromide (fairly sensitive, moderate health risks, inexpensive), or as most modern forensics labs use, fluorescent dyes (highly sensitive, safe, expensive). Instruments built to resolve microsatellite fragments by capillary electrophoresis also use fluorescent dyes to great effect. It is also used to follow up bone marrow transplant patients. In the United States, 13 core microsatellite loci have been decided upon to be the basis by which an individual genetic profile can be generated.

These profiles are stored on a local, state and national level in DNA databanks such as CODIS. The British data base for microsatellite loci identification is the UK National DNA Database (NDNAD). The British system uses 10 loci, rather than the American 13 loci.

Y-STRs (microsatellites on the Y chromosome) are often used in genealogical DNA testing.

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