DNA evidence holds immense power in criminal justice because the specific sequence of a person’s genetic material is a unique identifier. The ability to use deoxyribonucleic acid, or DNA, to distinguish one individual from another was first recognized in the mid-1980s, offering a powerful new tool to investigators. This early form of DNA profiling was immediately seen as a major scientific advancement, but its practical application in forensic settings was severely limited. The method itself was technically demanding and could only be successfully applied under a narrow set of ideal conditions. The result was that consistently using it to secure convictions or exonerations in court proved difficult for years.
Analyzing DNA Before Amplification
The primary technique used for forensic DNA analysis before the widespread adoption of the Polymerase Chain Reaction (PCR) was Restriction Fragment Length Polymorphism (RFLP). This method relied on naturally occurring variations in the length of specific DNA fragments between different people. The process began by treating the extracted DNA with special proteins called restriction enzymes, which act like molecular scissors.
These enzymes recognize and cut the DNA strand only at very specific, short sequences of genetic code. Because the DNA sequence varies from person to person, the distance between these cut sites also changes, leading to fragments of differing lengths. The resulting fragments were then separated by size using a technique called gel electrophoresis, where an electric current pulls the negatively charged DNA through a porous gel. Smaller fragments move faster and farther through the gel than larger ones, creating a pattern of separated bands.
After separation, the DNA fragments were transferred from the gel onto a nylon membrane using a process called Southern blotting. To visualize the unique pattern, scientists then applied a labeled probe that only attached to specific variable regions on the fragments. This multi-step process was slow, taking weeks to complete, and required physically long and intact DNA fragments to produce a reliable profile.
The Requirement for Large, Pristine Samples
The most significant technical hurdle in using RFLP was the sheer quantity of DNA required to perform a successful analysis. RFLP methods generally necessitated a sample containing several micrograms of non-degraded DNA. A microgram is a massive amount when compared to the trace amounts often left at a crime scene. A large blood stain or a significant tissue sample might yield enough material, but such ideal scenarios were uncommon in forensic investigation.
Most crime scene evidence, such as trace saliva on a cigarette butt, a single hair shaft without the root, or a small drop of dried blood, contained far less than the minimum microgram quantity. When the sample amount was too low, the RFLP process simply failed to produce a discernible pattern, leading to an inconclusive result. Furthermore, the entire, limited sample was consumed during the testing process, leaving no material left for re-testing or for a second opinion if the initial analysis failed.
This high sample quantity requirement meant that a large percentage of potential biological evidence collected was simply unusable for DNA profiling. The low success rate with typical crime scene samples severely limited its utility in the criminal justice system.
Vulnerability to Degradation and Contamination
Beyond the issue of sample quantity, the RFLP technique was extremely vulnerable to the poor quality and condition of biological evidence found outside of a laboratory setting. A successful RFLP analysis depended on having long, intact strands of high molecular weight DNA. Forensic samples, however, are frequently exposed to environmental factors that actively break down DNA molecules.
Heat, moisture, sunlight (UV light), and the presence of bacteria or fungi at a crime scene all cause the long DNA strands to fracture into numerous small fragments. When these degraded fragments were used in RFLP, they failed to produce the necessary long restriction fragments. Instead of distinct, clear bands, the analysis resulted in a “smearing” pattern on the gel, making it impossible to accurately determine fragment lengths or compare the sample to a suspect’s profile.
Moreover, pre-PCR methods were ill-equipped to handle samples that contained DNA from more than one person. A mixed sample, such as blood from both a victim and an assailant, would yield a confusing array of restriction fragments from multiple individuals. Without a method to target and separate the various DNA sources, the resulting pattern was often too complex to interpret reliably, further complicating the analysis of evidence from real-world crime scenes.
How PCR Transformed Forensic Analysis
The introduction of the Polymerase Chain Reaction (PCR) revolutionized forensic DNA analysis by directly addressing RFLP’s limitations. PCR is a technique that allows scientists to create millions or billions of copies of specific DNA segments from a tiny starting sample. This amplification capability drastically reduced the amount of DNA needed for analysis, dropping from the microgram level required by RFLP to nanograms or even picograms.
This change made previously unusable evidence, such as a few skin cells, viable for generating a profile. PCR also made it possible to analyze degraded DNA because it targets and amplifies smaller, remaining fragments, eliminating the need for the long, intact strands RFLP required. The ability to amplify specific, short regions transformed the speed and reliability of forensic testing, making DNA evidence a consistently obtainable tool for the justice system.