DNA profiling is a powerful technique for identifying individuals and establishing biological relationships. This process analyzes specific regions of DNA to create a unique genetic profile. Short tandem repeats (STRs) are central to this method, serving as variable markers within the human genome.
Understanding Short Tandem Repeats
Short tandem repeats, often called microsatellites, are specific segments of DNA characterized by a short sequence of base pairs, typically two to seven, repeated multiple times. These repeating units are located at distinct positions, or loci, across an individual’s chromosomes. For instance, a common repeat might be “TCAT” repeated several times. STRs vary in the number of times these sequences are repeated among individuals. While most human DNA is shared, these regions exhibit individual variation.
Key Characteristics for Profiling
STRs are useful for DNA profiling. The number of repeats at any given STR locus varies significantly, creating high polymorphism. Apart from identical twins, each individual possesses a unique combination of repeat numbers across STR locations. The human genome contains numerous STRs, providing many distinct locations for analysis.
STRs are relatively small, generally 80 to 400 base pairs. This compact size allows for effective amplification, even from degraded or very small DNA samples frequently encountered at crime scenes. Furthermore, STRs follow Mendelian inheritance patterns, meaning they are passed down predictably from parents to offspring. This predictable inheritance makes them valuable for determining biological relationships, such as in paternity testing.
Analyzing multiple STR loci simultaneously increases the discriminatory power of a DNA profile, making it improbable for two unrelated individuals to share the exact same profile. For example, the United States utilizes a standardized set of STR loci, including the 20 core loci in the Combined DNA Index System (CODIS), to ensure robust and comparable profiles across different laboratories.
The DNA Profiling Process
Generating a DNA profile using STRs involves a series of practical steps. The process begins with DNA extraction, where DNA is carefully isolated from a biological sample, such as blood, saliva, or tissue. After extraction, the quantity of the isolated DNA is determined, ensuring there is enough material for subsequent analysis.
The next step is Polymerase Chain Reaction (PCR) amplification, which is used to create millions of copies of the specific STR regions of interest. During PCR, primers, which are small DNA pieces designed to target the regions flanking the STRs, attach to the DNA strands. These primers are often tagged with fluorescent labels, allowing the amplified DNA fragments to be detected later. This amplification process is important, as it allows for analysis even from minute amounts of starting DNA.
Following amplification, the copied STR fragments are separated by size using a technique called capillary electrophoresis. In this method, an electric current pulls the DNA fragments through a thin tube containing a polymer. Smaller fragments, representing fewer repeats, move faster through the polymer than larger fragments with more repeats. As the fluorescently labeled fragments pass a detector, a distinct pattern of peaks, known as an electropherogram, is generated, indicating the size and quantity of each STR allele.
The final stages involve data analysis and interpretation, where the electropherogram data is converted into a numerical profile, representing the number of repeats at each analyzed locus. This unique numerical profile can then be compared to other known DNA profiles in databases or from reference samples. This comparison allows for the identification of individuals or the establishment of relationships with high statistical confidence.
Practical Applications
STR-based DNA profiling has broad real-world applications across various fields. In forensic science, it is a standard tool for criminal investigations. DNA profiles obtained from crime scene evidence, such as blood or hair, can be compared to profiles of suspects to link individuals to a crime or to exonerate the innocent. It is also used to identify victims, particularly in cases where traditional identification methods are not feasible.
Beyond forensics, STR analysis is widely employed in paternity and kinship testing. By comparing the STR profiles of a child, mother, and alleged father, biological relationships can be established due to the predictable inheritance of these markers. The technique is also valuable for human remains identification, especially in mass disasters or when dealing with historical remains, by comparing DNA from the remains to family reference samples. It also plays a role in identifying missing persons by matching their DNA profiles with those of relatives or unidentified remains.