Short Tandem Repeat (STR) analysis is a powerful technique for human identification. This method leverages unique genetic variations to create a distinct DNA profile for an individual. Its precision and reliability make it an indispensable tool across various disciplines. STR analysis plays a significant role in criminal investigations, helping to identify suspects and link evidence to crimes. The technique is also widely applied in establishing biological relationships, such as in paternity disputes.
Understanding Short Tandem Repeats
Short Tandem Repeats, or STRs, are specific segments of DNA characterized by short sequences of base pairs that are repeated multiple times in a row. These repeat units typically range from 2 to 7 base pairs in length. The number of times this short sequence is repeated at a particular location on a chromosome varies considerably among individuals. This variation in repeat numbers makes STRs highly polymorphic, meaning they show significant differences from person to person.
These distinctive regions are found throughout the human genome, often in non-coding areas of DNA. Variations in STRs typically do not affect an individual’s physical traits or health. The high variability in the number of repeats at different STR locations, called loci, forms the foundation for individual identification. Scientists can analyze multiple STR loci simultaneously to create a unique genetic profile.
The STR Analysis Process
The process of STR analysis involves several steps to generate a DNA profile from a biological sample.
DNA Extraction
DNA is isolated from collected biological materials, such as blood, saliva, or skin cells. Proper collection and handling are important to prevent contamination and degradation of the DNA sample.
PCR Amplification
Polymerase Chain Reaction (PCR) creates millions of copies of specific STR regions. Primers are designed to bind to regions flanking the STR sequences; one of these primers is typically labeled with a fluorescent tag. This amplification makes the STR fragments detectable for analysis.
Capillary Electrophoresis
Following PCR, the amplified DNA fragments are separated based on their size using a technique called capillary electrophoresis. In this method, the DNA fragments migrate through a thin capillary tube containing a polymer gel when an electric current is applied. Smaller fragments move faster than larger ones.
As the fragments pass a detection window, a laser excites the fluorescent tags. A detector records the emitted light, and specialized software determines the exact size of each fragment based on its migration time. These size measurements correspond to the number of repeats at each STR locus. The data is then translated into a numerical profile, representing the number of repeats at each analyzed locus, forming the individual’s unique genetic fingerprint.
Key Applications of STR Analysis
STR analysis is an indispensable tool across a wide range of fields for human identification.
Forensic Science
It plays a significant role in criminal investigations. DNA profiles generated from crime scene evidence, such as blood or saliva, can be compared to profiles of suspects or to national DNA databases, like the Combined DNA Index System (CODIS) in the United States. CODIS stores STR profiles from convicted offenders and forensic samples, helping to link crimes and identify perpetrators.
Paternity Testing
STR analysis is widely used in paternity testing to establish biological relationships. By comparing the STR profiles of a child, mother, and alleged father, laboratories can determine with high accuracy whether a biological relationship exists. Analyzing multiple STR markers provides precision in these tests, as the probability of unrelated individuals sharing the same profile across numerous markers is extremely low.
Disaster Victim Identification
The technique also serves a vital function in disaster victim identification. In mass casualty events, STR analysis helps identify human remains by comparing DNA profiles from the remains with those of missing persons or their relatives.
Ancestry and Genealogy Research
STR analysis has uses in ancestry and genealogy research. By examining STR markers, scientists can trace family lineages and genetic ancestry, providing insights into population origins and historical migration patterns. This technology offers accurate and reliable genetic identification in diverse scenarios.