Gel electrophoresis is a laboratory technique that separates biological molecules based on size and electrical charge. This method is a cornerstone in forensic investigations, providing a powerful tool for analyzing crime scene evidence. Its significance stems from its ability to differentiate individuals or identify biological substances, contributing crucial information to legal proceedings. This technique helps forensic scientists analyze biological traces.
How Gel Electrophoresis Works
Gel electrophoresis operates on the principle that charged molecules migrate through a gel matrix when an electric current is applied. The gel, often made of agarose or polyacrylamide, acts like a sieve, allowing smaller molecules to move more easily than larger ones. DNA and RNA molecules are naturally negatively charged, moving towards the positive electrode. Proteins may require treatment with a denaturing agent like SDS to give them a uniform negative charge, ensuring separation primarily by size.
Samples are loaded into wells at one end of the gel, which is submerged in a buffer solution that conducts electricity and maintains a stable pH. As the electric current runs, molecules separate into distinct bands based on their size. Smaller fragments travel further down the gel, while larger fragments remain closer to the starting wells. These separated molecules, visible as banding patterns after staining, serve as the visual output of the electrophoresis process.
Identifying Individuals Through DNA
A primary application of gel electrophoresis in forensics is DNA profiling, also known as DNA fingerprinting. This technique analyzes specific regions of DNA, called Short Tandem Repeats (STRs), which vary in length among individuals. Forensic scientists extract DNA from crime scene evidence (e.g., blood, hair, or saliva) and amplify these STR regions using Polymerase Chain Reaction (PCR). The amplified DNA fragments are then separated by size using gel electrophoresis, producing a unique banding pattern for each individual.
Comparing these DNA profiles allows investigators to link suspects to crime scenes by matching patterns from evidence with those from individuals. A complete match provides strong evidence of a suspect’s presence. DNA profiling also identifies victims in mass disasters or cold cases and establishes familial relationships, such as in paternity disputes, by comparing shared genetic markers. The distinctiveness of STR patterns makes DNA profiling a powerful tool for individual identification.
Beyond DNA: Other Forensic Uses
While DNA analysis is prominent, gel electrophoresis also extends to other forensic applications involving different biological molecules. RNA analysis, for instance, can provide insights beyond individual identification. Messenger RNA (mRNA) degradation patterns can help estimate the age of a biological stain, such as a bloodstain, offering clues about the time a crime occurred. Specific RNA markers can also differentiate between various body fluids like blood, saliva, or semen, even from minute or mixed samples.
Proteins can also be analyzed using gel electrophoresis, particularly in forensic toxicology and species identification. Protein profiling can aid in detecting specific protein toxins in biological samples, relevant in poisoning cases. Proteins can also be used for species identification, determining if a biological sample originated from a human or an animal. This versatility demonstrates how gel electrophoresis provides valuable information from diverse biological evidence types.
Making Sense of the Results
Interpreting the banding patterns from gel electrophoresis is a crucial step in forensic investigations. Forensic scientists analyze the separated bands, comparing the positions and intensities of fragments across different samples. A “match” between samples means their banding patterns are identical, suggesting they originated from the same source. Conversely, an “exclusion” occurs when patterns do not match, ruling out a common source.
Interpretation often involves comparing the unknown sample’s profile to known reference samples (e.g., from suspects or victims) and to a DNA ladder with fragments of known sizes. For DNA profiling, the statistical significance of a match is calculated, indicating the probability that a random individual would have the same DNA profile. These findings contribute to building a comprehensive case, corroborating other evidence, and providing objective data for court.