What Is the Method for Double-Stranded DNA Testing?

Buccal swabs are used in a DNA test.

DNA testing is a biochemical technique that scientists use to accurately identify an individual based on her genetic makeup. DNA testing companies use this method to offer a range of services, including paternity testing, maternity testing and siblingship testing. This technique is highly sensitive and is able to discriminate between individuals with a high degree of accuracy. In the March 7, 1985 issue of ''Nature," Sir Alec Jeffreys and his team at the University of Leicester first described how DNA testing could provide an 'individual specific DNA fingerprint' for aid in human genetic analysis.

History

Deoxyribonucleic Acid (DNA) is a double-stranded molecule, similar in appearance to a circular staircase, that is found in all nucleated cells. James D. Watson and Francis Crick first proposed the double-helix model of DNA in the April 25, 1953 issue of ''Nature.' The two strands of DNA are composed of four building blocks, known as nucleotides, that base pair in a complementary fashion. It is the order of bases along a single strand of DNA that gives rise to the genetic code.

Obtaining a DNA Sample

Obtaining a quality DNA sample is an essential step in any DNA test. Sources of DNA from an individual include hair cell, skin cell or blood samples. The method that is most commonly used by DNA testing companies to obtain DNA samples is the buccal swab. For example, the DNA Diagnostic Centre administers their customers with a total of four buccal swabs that resemble standard cotton buds. Customers are asked to rub the swabs on the inside of their cheeks to collect cells.

Short Tandem Repeat Profiling

Short tandem repeats (STR) are repeating units of nucleotides that are typically two to five nucleotides in length. These repeating units are adjacent to each other along a DNA strand and can range from two to 16 repeats in individuals. Once an individual's DNA is received by a laboratory, their arrangement and number of STRs are analyzed using polymerase chain reaction (PCR), and this is known as STR profiling. This is the most widely used technique for the purpose of DNA testing.

Visualization of STR Profiles

Visualization is performed by gel electrophoresis or capillary electrophoresis. Gel electrophoresis exploits the negative charge of DNA molecules to help discriminate between different-sized DNA fragments. The DNA fragments move toward the positive end of an electrical field, with smaller fragments moving more quickly. Fragments are then visualized by staining a gel with ethidium bromide or a silver compound. During capillary electrophoresis, fluorescently labeled DNA is inserted into a tiny glass tube and then subjected again to an electric field.

Considerations

PCR is a technique used to amplify the amount of genetic material in vitro, as well being used to resolve individual STR profiles. The advantages of using STR profiling is that an individual's STR profile is unique to him, even in the case of siblings. The only exception to this is in the case of identical twins who share 100 percent genetic identity. A disadvantage of this technique lies in the fact that the techniques accuracy relies highly on the quality of the DNA sample provided.

References

• ''Nature'' magazine: Molecular Structure of Nucleic Acids
• ''Nature'' magazine: Hypervariable 'minisatelite' Regions in Human DNA
• Colorado State University: Gel Electrophoresis of DNA and RNA
• Davidson College: Capillary Electrophoresis
• Medical Genomics: Paternity Testing

Resources

• DNA Diagnostics Centre: DNA Test Services