Identifying individuals by comparing the lengths of specific DNA regions separated on a gel is possible, a technique known as DNA fingerprinting or DNA profiling. This process does not typically involve comparing “genes,” which are DNA segments that carry instructions for building proteins. Instead, identification relies on highly variable, non-coding regions of DNA.
Understanding DNA Differences
The human genome contains both coding and non-coding regions. While coding regions, or genes, are largely similar across individuals, non-coding regions exhibit significant variations. Approximately 95% of human DNA consists of these non-coding segments. Despite this, their variability makes them highly valuable for identification.
The variations used for identification are primarily found in segments known as Short Tandem Repeats (STRs) and Variable Number Tandem Repeats (VNTRs). These are specific locations in the DNA where a short sequence of nucleotides is repeated multiple times in a row. For instance, an STR might involve a sequence like ‘AGAT’ repeated several times. The crucial aspect is that the number of these repeats, and consequently the overall length of that DNA segment, varies considerably among individuals. This variation in repeat numbers creates unique markers that can distinguish one person from another.
Separating DNA Fragments with Gel Electrophoresis
After obtaining a DNA sample from sources such as blood, hair, or saliva, the specific variable regions (STRs or VNTRs) are isolated and then amplified, often using a process called Polymerase Chain Reaction (PCR). These amplified DNA fragments are then loaded into a gel, which acts as a porous matrix. The gel is submerged in a buffer solution within an electrophoresis chamber.
An electric current is then applied across the gel. DNA molecules possess a negative charge due to their phosphate backbone. As a result, when the current is applied, the negatively charged DNA fragments are repelled by the negative electrode and migrate towards the positive electrode. The gel’s mesh-like structure acts as a molecular sieve, impeding the movement of the DNA fragments. Smaller fragments navigate through the pores more easily and therefore travel faster and further through the gel than larger fragments. This differential migration separates the DNA fragments by size, creating distinct bands along the gel.
Matching DNA Patterns for Identification
Once the DNA fragments have separated on the gel, they are visualized. This staining reveals a unique pattern of bands, often referred to as a “DNA fingerprint” or “DNA profile.” Each band corresponds to a DNA fragment of a particular length. The specific arrangement and position of these bands on the gel create a pattern that is highly individual.
Identification involves comparing the DNA banding pattern from an unknown sample to patterns from known individuals. If both samples originate from the same person, their DNA profiles will exhibit matching patterns across all analyzed regions. To ensure a high level of certainty, forensic analysis typically examines multiple STR locations, or loci, across the genome. For instance, the Combined DNA Index System (CODIS) in the United States currently utilizes 20 core STR loci for identification. A match across these numerous loci provides an extremely high statistical probability that the two DNA samples came from the same individual, with the exception of identical twins who share identical DNA.
Practical Uses of DNA Fingerprinting
Its primary use is in forensic science, where it plays a significant role in criminal investigations. DNA samples collected from crime scenes, such as blood, hair, or skin cells, can be compared to DNA profiles of suspects to establish connections or rule them out. This technology also assists in identifying victims in mass disasters or missing persons cases by comparing their DNA with that of family members.
Beyond criminal justice, DNA fingerprinting is widely applied in paternity testing. Since a child inherits half of its DNA from each parent, comparing the child’s DNA profile with that of an alleged father can conclusively determine biological relationships. The ability to identify individuals based on these unique genetic patterns is crucial for personal identification and legal proceedings.