DNA Profiling Techniques in Forensic Biochemistry: RFLP, STR, and VNTR Analysis

Introduction to DNA Profiling in Forensic Biochemistry

DNA profiling utilizes molecular genetics to distinguish between individuals based on variations in their DNA sequences. These techniques are fundamental in forensic science, paternity testing, evolutionary biology, and the identification of remains. The primary focus is on polymorphic regions of DNA, which vary among individuals and can be analyzed using several biochemical techniques.

DNA Polymorphisms and Their Forensic Applications

Polymorphic and Anonymous DNA Sequences

• Polymorphic sequences are regions of DNA that vary between individuals. These are the basis for DNA profiling.

• Anonymous sequences are non-coding regions of DNA with no known function, chosen for forensic analysis to preserve privacy and confidentiality.

• Most human DNA (>99%) is conserved and does not vary between individuals; only a small percentage is polymorphic and useful for forensic analysis.

Types of Repeating DNA Sequences

• Variable Number Tandem Repeats (VNTRs): Repeats of 10–100 base pairs (bp) in length. The number of repeats varies between individuals and is inherited from parents.

• Short Tandem Repeats (STRs): Repeats of 2–9 bp. STRs are shorter than VNTRs and can be found in both anonymous and non-anonymous regions.

• Both VNTRs and STRs are inherited in a codominant fashion, meaning both maternal and paternal alleles can be detected.

Example: TH01 STR Locus

• The TH01 locus contains a variable number of [TCAT] repeats.

• Genotypes are described by the number of repeats on each chromosome (e.g., 5-3 genotype).

• Analysis of multiple loci increases the discriminatory power of DNA profiling.

Restriction Fragment Length Polymorphism (RFLP) Analysis

Principle of RFLP

• RFLPs arise from differences in DNA sequences that create or abolish restriction enzyme (RE) recognition sites.

• Digestion of DNA with REs produces fragments of varying lengths, which can be separated by agarose gel electrophoresis.

• Comparison of fragment patterns allows identification of individuals.

RFLP Protocol Overview

1. Digest DNA samples with restriction enzymes (e.g., EcoRI and PstI).

2. Separate fragments by agarose gel electrophoresis.

3. Visualize DNA bands using intercalating dyes (e.g., SYBR Safe) under UV light.

4. Compare fragment patterns between crime scene DNA and suspects.

Key Steps in Agarose Gel Electrophoresis

• Preparation of a 1% agarose gel in TAE buffer.

• Loading of DNA samples and a DNA ladder for size estimation.

• Running the gel at 100–120 V until the dye front migrates sufficiently.

• DNA migrates towards the positive electrode due to its negative charge.

Polymerase Chain Reaction (PCR)

Principle and Components

• PCR is a technique to amplify specific DNA regions, even from trace amounts of DNA.

• Key components: template DNA, sequence-specific primers, Taq DNA polymerase (thermostable), dNTPs, MgCl2, and buffer.

• Primers are designed to flank the region of interest (e.g., STR or VNTR loci).

PCR Cycle Steps

1. Denaturation: DNA strands are separated by heating (94–96°C).

2. Annealing: Primers bind to complementary sequences (45–65°C).

3. Extension: Taq polymerase synthesizes new DNA strands (72°C).

These steps are repeated for 25–35 cycles, resulting in exponential amplification of the target DNA region.

Applications and Modifications

• PCR can be modified to add restriction sites to PCR products for cloning purposes.

• Reverse Transcription PCR (RT-PCR) is used to detect RNA viruses (e.g., SARS-CoV-2) by converting RNA to cDNA before amplification.

Short Tandem Repeat (STR) Analysis

STR Analysis in Forensics

• STR analysis is preferred when DNA is limited or degraded (e.g., from skin cells on a weapon).

• PCR is used to amplify STR loci (e.g., TH01), and the number of repeats is determined by the length of the PCR product.

• Agarose gel electrophoresis separates PCR products, allowing comparison between crime scene and suspect DNA.

Variable Number Tandem Repeat (VNTR) Analysis

VNTR Analysis for Exclusion

• VNTR loci (e.g., D1S80) are analyzed to exclude the possibility of investigator DNA contaminating crime scene samples.

• VNTRs are highly variable, making it extremely unlikely for two unrelated individuals to share the same VNTR profile.

• PCR is used to amplify VNTR loci from cheek cell DNA, followed by agarose gel electrophoresis to determine genotype.

Example: D1S80 VNTR Locus

• Located on chromosome 1p, with a 16 bp repeat unit.

• Individuals may have between 16 and 40 repeats; heterozygosity is common.

Summary Table: Comparison of RFLP, STR, and VNTR Analysis

Technique	Target Sequence	Sample Requirement	Detection Method	Forensic Use
RFLP	Polymorphic RE sites (often VNTRs)	Large, high-quality DNA	Restriction digestion + gel electrophoresis	Obsolete in forensics; used in research
STR	Short tandem repeats (2–9 bp)	Small, degraded DNA	PCR + gel electrophoresis	Current forensic standard
VNTR	Variable number tandem repeats (10–100 bp)	Moderate DNA quality	PCR + gel electrophoresis	For exclusion and research

Key Equations and Concepts

• PCR Amplification: The theoretical yield of PCR after n cycles is given by: $N = N_0 \times 2^n$ where $N_0$ is the initial number of DNA molecules, and $n$ is the number of cycles.

• Restriction Enzyme Digestion: The presence or absence of a restriction site alters the length of DNA fragments produced.

Practical Considerations in Forensic DNA Analysis

• Strict separation of samples and protocols is essential to avoid cross-contamination.

• Control samples (e.g., from investigators) are analyzed to exclude contamination.

• Multiple loci are analyzed to increase the specificity of DNA profiling.

Applications and Relevance

• Forensic identification of suspects and exclusion of innocent individuals.

• Paternity testing and identification of remains.

• Detection of viral infections (e.g., COVID-19) using RT-PCR.

Additional info:

• Southern blotting is often used after RFLP analysis for detection of specific DNA fragments, but is not required for STR or VNTR analysis due to the specificity of PCR.

• STR and VNTR loci are chosen for their high variability and lack of association with known traits, ensuring privacy in forensic investigations.