DNA analysis is a critical tool in forensic investigations because it can uniquely identify individuals based on their genetic material. The process involves collecting biological samples, extracting DNA, and comparing genetic profiles to link suspects, victims, or evidence. Here’s a step-by-step explanation:
1. Sample Collection
- Biological material is collected from the crime scene or individuals.
- Common samples include blood, saliva, hair, skin cells, semen, or other body fluids.
- Proper collection and storage are essential to avoid contamination or degradation.
2. DNA Extraction
- DNA is separated from cells and other components in the sample.
- Techniques include:
- Organic extraction: Using chemicals to break down cell membranes and isolate DNA.
- Silica-based extraction: DNA binds to a silica surface and is purified.
- Chelex or magnetic bead methods: Rapid extraction for small samples.
- The goal is to obtain clean, high-quality DNA for analysis.
3. DNA Quantification
- The amount of DNA is measured to ensure there is enough material for testing.
- Too little DNA may require amplification, while degraded DNA may need special techniques.
4. DNA Amplification (PCR)
- The Polymerase Chain Reaction (PCR) is used to make millions of copies of specific DNA regions.
- PCR targets regions called Short Tandem Repeats (STRs), which are highly variable among individuals.
- Amplification allows even tiny amounts of DNA from crime scenes to be analyzed.
5. DNA Profiling
- The amplified DNA is separated and visualized to create a genetic profile.
- Techniques include:
- Gel electrophoresis: Separates DNA fragments by size.
- Capillary electrophoresis: Modern, automated method for high-resolution analysis.
- The resulting profile shows the pattern of STRs at multiple genetic loci.
6. Comparison and Matching
- The DNA profile from the evidence is compared with:
- Suspect’s DNA
- Victim’s DNA
- DNA databases (e.g., CODIS)
- A match can identify a suspect, while exclusion can clear someone from suspicion.
7. Interpretation and Reporting
- Forensic experts calculate the probability of a random match to support conclusions.
- Reports are used in court as scientific evidence.
- Expert testimony explains how the analysis was performed and the significance of the match.
8. Quality Control and Validation
- Every step follows strict protocols to prevent errors.
- Controls and replicates are used to ensure accuracy and reliability.
Summary
DNA analysis in forensic investigations involves:
- Collecting biological samples from crime scenes or suspects
- Extracting DNA from cells
- Measuring DNA quantity
- Amplifying DNA using PCR
- Creating a DNA profile through STR analysis
- Comparing profiles to suspects or databases
- Interpreting results for legal use
- Ensuring quality control at every step
In simple terms, DNA analysis turns even a tiny biological sample into a unique genetic fingerprint that can identify or exclude individuals in criminal cases.