Principle of DNA fingerprinting

The Principle of DNA Fingerprinting

The core principle behind DNA fingerprinting lies in the fact that while the vast majority of the human genome is identical across individuals, certain regions exhibit significant variability. These variable regions, particularly Short Tandem Repeats (STRs), contain repeating DNA sequences whose number of repeats differs between individuals. This difference in repeat number creates unique DNA profiles.

Steps Involved in DNA Fingerprinting

1. DNA Extraction

The process begins with the extraction of DNA from biological samples such as blood, saliva, hair follicles, semen, or tissue. Various methods are employed, including physical disruption of cells followed by chemical purification to isolate the DNA.

2. Restriction Digestion (Historically, less common with STR analysis)

In the earlier VNTR-based methods, DNA was digested with restriction enzymes. These enzymes recognize specific DNA sequences and cut the DNA at those sites, generating DNA fragments of varying lengths. However, with the advent of STR analysis, this step is often bypassed as PCR amplification directly targets STR regions.

3. PCR Amplification (STR Analysis)

Short Tandem Repeats (STRs) are amplified using Polymerase Chain Reaction (PCR). PCR uses specific primers designed to flank the STR regions, allowing for exponential amplification of these regions. This generates a sufficient amount of DNA for analysis.

4. Separation of DNA Fragments/Amplicons

The amplified STR fragments (or restriction fragments in older methods) are separated based on their size using gel electrophoresis. Capillary electrophoresis is now the standard method, offering higher resolution and automation. Smaller fragments migrate faster through the gel, resulting in a pattern of bands or peaks.

5. Hybridization (Historically, less common with STR analysis)

In VNTR-based methods, a labeled DNA probe complementary to the VNTR sequence was used to hybridize with the DNA fragments on the gel. The probe binds to the fragments containing the VNTR, making them visible.

6. Detection and Analysis

The separated DNA fragments are detected using various methods. In STR analysis, fluorescently labeled primers are used during PCR, and the amplified fragments are detected by a laser scanner as they pass through a capillary. The resulting data is analyzed to determine the number of repeats at each STR locus. A DNA profile is generated, representing the unique combination of STR alleles for an individual.

Applications of DNA Fingerprinting

• Forensic Science: Identifying suspects in criminal investigations, linking suspects to crime scenes, and exonerating wrongly accused individuals.
• Paternity Testing: Determining biological parentage with high accuracy.
• Immigration Disputes: Establishing familial relationships for immigration purposes.
• Missing Person Identification: Identifying remains in mass disasters or long-term missing person cases.
• Wildlife Conservation: Tracking animal populations, identifying poaching activities, and preserving biodiversity.
• Medical Diagnostics: Identifying genetic predispositions to diseases and tracking the spread of infectious agents.

Ethical Considerations

While DNA fingerprinting is a powerful tool, it raises ethical concerns regarding privacy, data security, and potential for misuse. The storage and use of DNA databases require careful regulation to prevent discrimination and ensure individual rights are protected. Concerns also exist about the potential for genetic profiling and its implications for social justice.