DNA fingerprinting works by isolating and analyzing specific DNA sequences, called variable number tandem repeats (VNTRs) or short tandem repeats (STRs), that are highly variable between individuals. This creates a unique genetic "fingerprint" for identification.
Here's a breakdown of the process:
-
Sample Collection: DNA is extracted from a biological sample, such as blood, saliva, hair, or tissue.
-
DNA Isolation: The DNA is separated from other cellular components. This often involves chemical or physical processes.
-
DNA Amplification (PCR): Polymerase Chain Reaction (PCR) is used to make multiple copies of the specific VNTR or STR regions. PCR amplifies the selected DNA regions, ensuring enough material for analysis. This step is crucial, especially when dealing with limited sample quantities.
-
DNA Fragmentation (Restriction Enzymes - Optional): In older methods of DNA fingerprinting, restriction enzymes (proteins that cut DNA at specific sequences) were used to cut the DNA into fragments of different sizes. This step is less common with modern STR-based methods.
-
Separation of DNA Fragments (Gel Electrophoresis): The DNA fragments are separated based on size using gel electrophoresis. DNA fragments are placed in a gel and an electric current is applied. Smaller fragments move faster and further through the gel than larger fragments.
-
Visualization: The separated DNA fragments are visualized. In older methods, radioactive probes were used to bind to specific DNA sequences, and the resulting pattern was detected using X-ray film. Modern methods often use fluorescent dyes that bind to the DNA fragments and are detected by specialized imaging equipment.
-
Analysis and Comparison: The resulting pattern of DNA fragments (the DNA fingerprint) is then compared to other samples. If the patterns match, it indicates that the samples likely came from the same individual. Statistical analysis is performed to determine the probability of a match occurring by chance.
Summary of Steps in Table Format:
| Step | Description | Purpose |
|---|---|---|
| 1. Sample Collection | Obtaining a biological sample (e.g., blood, saliva). | To provide the source of DNA for analysis. |
| 2. DNA Isolation | Separating DNA from other cellular components. | To purify the DNA for subsequent steps. |
| 3. PCR Amplification | Making multiple copies of specific DNA regions (VNTRs/STRs). | To increase the amount of DNA available for analysis, especially with limited samples. |
| 4. Fragmentation (Optional) | Cutting DNA into fragments of different sizes using restriction enzymes. | (Older Methods) Creates differently sized fragments for separation and analysis. |
| 5. Gel Electrophoresis | Separating DNA fragments based on size using an electric field through a gel matrix. | To create a distinct banding pattern based on fragment sizes. |
| 6. Visualization | Making the separated DNA fragments visible (e.g., using radioactive probes or fluorescent dyes). | To allow the banding pattern to be observed and analyzed. |
| 7. Analysis and Comparison | Comparing the banding patterns to determine if samples match. | To identify individuals, establish relationships, or link evidence to suspects in criminal cases. |
Key Considerations:
- STRs are commonly used today: Modern DNA fingerprinting primarily uses STRs because they are easily amplified by PCR and provide high discriminatory power.
- Statistical significance: The more VNTR/STR loci analyzed, the lower the probability of a coincidental match, making the DNA fingerprint more reliable.
- Applications: DNA fingerprinting is used in forensic science, paternity testing, and identifying genetic diseases.
In conclusion, DNA fingerprinting is a powerful technique that utilizes the unique genetic variations between individuals to create a profile that can be used for identification purposes.
