Testing DNA with STR Analysis

DNA testing is a common topic for discussion and debate, but few people understand exactly how it works. Many different DNA tests are currently available, and STR analysis is one of the most widely used. STR stands for short tandem repeat and refers to a type of polymorphism, or difference in DNA between individuals.

DNA (deoxyribonucleic acid) is the genetic material containing instructions for development and function in all living things. It is made up of four nucleotide bases:

• adenine
• cytosine
• guanine
• thymine.

These bases combine in pairs to form ribbon-like strands of DNA. Most of our DNA is common to all human beings, but a small portion, less than one percent, is unique for each individual. It is this portion that is used for STR analysis and other DNA tests.

The human genome contains many repeated sequences ranging in length from two to six base pairs. DNA regions that contain these units are called short tandem repeats.

Understanding STR Analysis

STR DNA analysis has been in use since the 1980 's for genetic fingerprinting in criminal investigations. This test analyses the short tandem repeats on 13 different genetic regions, or loci. Samples from crime scenes can be compared with DNA stored in databases maintained by law enforcement agencies, such as the FBI's CODIS. There is only about a one in one billion chance that two people will match on all 13 regions, unless they are identical twins. This makes it possible to create a unique genetic profile for each individual. STR analysis accuracy is excellent, ensuring that this will remain a trusted method for solving crimes.

Short tandem repeats can be easily amplified by a polymerase chain reaction (PCR), which means that only a small DNA sample is needed for a successful test. Here is how this is accomplished:

• A polymerase chain reaction replicates and increases the original sample to obtain a better quantity for testing.
• Once this has been done, scientists perform STR analysis by capillary electrophoresis. This separates molecules so that minor differences can be detected.

• The analyst can then determine the number of sequence repeats in the sample. Gel electrophoresis and dyes may also be used to help genetic variations become more visible.

STR Analysis Applications

STR DNA analysis is commonly used to exonerate innocent people accused of crimes and to prove beyond doubt that an individual was present at a crime scene. There are also many important medical applications for this test. Here are just a few of the ways STR analysis helps ease health and family concerns:

• establishing relationships through paternity testing
• monitoring organ transplant recipients to assess the possibility of rejection or relapse
• testing for genetic conditions such as Huntingdon 's disease.

Ethical Concerns

While STR analysis clearly benefits both the health and criminal justice fields, DNA testing raises some important ethical issues. The storage of DNA in national databases raises questions about privacy and the ownership of genetic material. Concerns have also been voiced about the storage of DNA samples collected from people later found to be innocent.

Resources

DNA Diagnostics Center (2008).Short Tandem Repeats. Retrieved September 1, 2008, from the DNA Diagnostics Center Web site: http://www.forensicdnacenter.com/dna-str.html.

Explore DNA (2008).STR Analysis — Short Tandem Repeat. Retrieved September 1, 2008, from the Explore DNA Web site: http://www.exploredna.co.uk/str-analysis.html.