Gel electrophoresis is a powerful analytical tool used to separate biological molecules like DNA, RNA, and proteins. This technique plays a role in modern forensic science, assisting in identifying individuals and solving crimes.
The Scientific Principle
Gel electrophoresis separates molecules based on their size and electrical charge. The process involves a gel matrix, often made of agarose or polyacrylamide, which acts like a sieve. Molecules are loaded into wells at one end of this gel, and an electric current is applied across it.
DNA molecules are negatively charged due to their phosphate backbone, causing them to migrate toward the positively charged electrode. Smaller molecules navigate through the gel’s pores more easily and therefore move faster and travel farther than larger molecules. This differential migration results in the separation of molecules by size, creating distinct bands along the gel. Proteins, while also separable by gel electrophoresis, often require treatment to impart a uniform negative charge for size-based separation.
Key Forensic Applications
One of the primary uses of gel electrophoresis in forensics is DNA profiling, also known as DNA fingerprinting. This technique separates DNA fragments to create a unique genetic pattern for an individual.
Forensic scientists focus on specific regions of DNA called Short Tandem Repeats (STRs), which are short, repeating DNA sequences that vary in length among individuals. This enables the identification of suspects, links crime scenes to individuals, helps identify human remains, and establishes paternity.
The patterns from crime scene evidence are compared to known samples from suspects or victims; a match provides strong evidence of a connection. Beyond human identification, gel electrophoresis can assist in species identification by analyzing DNA from biological samples found at a crime scene, providing context for the investigation.
From Sample to Conclusion
The process begins with the careful collection of forensic samples, such as blood, hair, or saliva, from a crime scene. Once collected, DNA must be extracted and purified from these biological materials. Often, the amount of DNA obtained is very small, necessitating an amplification step using a technique called Polymerase Chain Reaction (PCR). PCR creates millions of copies of specific DNA segments, providing enough material for analysis.
After amplification, the prepared samples are loaded into small wells at one end of the gel. An electric current is then applied, causing the negatively charged DNA fragments to migrate through the gel. Smaller fragments travel faster and further through the gel’s matrix, resulting in a separation of DNA based on size.
Once the separation is complete, the DNA fragments, which are otherwise invisible, are made visible. This is commonly achieved by staining the gel with a DNA-binding dye, revealing distinct bands under ultraviolet (UV) light. Forensic scientists then analyze these unique band patterns. The patterns from the crime scene sample are compared to those from reference samples, such as suspect DNA or victim DNA, to establish connections or exclusions. Specialized software often aids in interpreting complex patterns and generating a comprehensive report, which can then be presented as evidence in legal proceedings.