Step-by-Step DNA Profiling Workflow in Forensic Science Laboratory

 Introduction

DNA profiling is one of the most powerful tools used in forensic science for individual identification. In Forensic Science Laboratories (FSLs), DNA analysis follows a strict, standardized workflow to ensure accuracy, reliability, and legal admissibility of results.

This article explains the step-by-step workflow of DNA profiling in FSL, starting from sample receipt to STR analysis, with a practical laboratory perspective useful for students, interns, and competitive exams.

1. Sample Receipt & Documentation

 Purpose

To ensure proper identification, traceability, and legal integrity of forensic evidence.

 Types of Biological Samples Received

• Blood (liquid or stains)
• Semen (swabs, clothing)
• Saliva (bite marks, saliva remains)
• Tissue, bone, teeth
• Hair with root
• Touch DNA samples

 Practical Procedure

• Samples are received with forwarding letter (FIR, case details)
• Unique Laboratory ID number assigned
• Seals are checked and documented
• Entries made in case register & LIMS
• Stored under controlled conditions (refrigerated or frozen)

Key Concept

Chain of Custody must be maintained at every step.

2. DNA Extraction

Aim

To isolate pure DNA from biological material.

Practical Steps (Generalized)

1. Sample cutting/swabbing
2. Cell lysis using buffer & Proteinase K
3. Removal of proteins and inhibitors
4. DNA precipitation or binding
5. DNA elution

Output

Extracted DNA solution ready for quantification.

3. DNA Quantification

 Why Quantification Is Important

• Determines how much DNA is present
• Prevents PCR failure due to too much or too little DNA
• Detects degradation or inhibitors

 Methods Used

• Real-Time PCR (qPCR) – preferred in forensic labs
• Human-specific quantification kits

 Practical Outcome

• DNA concentration expressed in ng/µL
• Decision taken whether: i) To proceed ii) Dilute iii) Re-extract

4. PCR Amplification

 Aim

To amplify specific DNA regions (STR loci).

What is Amplified?

• Autosomal STRs
• Sex-determining marker (Amelogenin)

 Practical PCR Components

• Template DNA
• Primers
• dNTPs
• Taq polymerase
• Buffer

PCR Conditions

• Initial denaturation
• Cycles of : 1 Denaturation 2. Annealing 3. Extension 

5. STR Analysis  

Purpose

To separate, detect, and generate DNA profiles from PCR-amplified STR fragments using automated instruments.

Automation in Modern FSLs

In present-day forensic laboratories, STR analysis is fully automated using genetic analyzers.
The analyst does not manually run gels or separate fragments—the system performs separation, detection, and data generation automatically.

The analyst’s role is limited to: Sample preparation , Loading , Software-based analysis & interpretation

Practical Steps Performed by the Analyst

1. PCR product is mixed with: Hi-Di formamide and Internal size standard
2. Samples are loaded into: Automated genetic analyzer (e.g., capillary-based systems)
3. Run is initiated through dedicated software

What the Machine Does Automatically

• Injects samples into capillaries
• Separates DNA fragments based on size 
• Detects fluorescently labeled STRs
• Converts signals into digital data

No manual intervention during the run 

Output Generated

• Electropherogram
• Allele calls at each STR locus
• Peak height and size information

All results are displayed through analysis software, not manual observation. 

The forensic analyst examines:

• Peak balance and height
• Stutter peaks and artifacts
• Allelic drop-in / drop-out
• Mixed DNA profiles

6. Report Generation

🔹 Final Report Includes

• Sample description
• Methods used
• Results & interpretation
• Opinion (match / exclusion / inconclusive)
• Authorized signature

Reports must be clear, unbiased, and court-defensible.

Common Practical Challenges in DNA Profiling

• Degraded samples
• Low template DNA
• PCR inhibition
• Mixed DNA profiles
• Contamination risks

Conclusion

The DNA profiling workflow in FSL is a systematic, legally controlled, and scientifically robust process. Understanding each step—from sample receipt to STR analysis—helps students bridge the gap between textbook theory and real forensic practice.