Forensic Molecular Evidence Protocol

Genomic DNA Extraction from Hair Samples (Spin Column)

Isolating trace nuclear genomic DNA from human hair follicles through extensive Proteinase K keratin digestion and silica-membrane spin column purification for downstream forensic STR profiling.

Chain-of-Custody & Evidence Notice: Sample processing adheres to standard forensic evidence handling procedures to eliminate exogenous environmental DNA contamination.
Method: Silica Spin Column Sample: Hair Follicle / Root Field: Forensic Molecular Biology Author: Usama Bin Ali (Bioinformatics Researcher) ~14 min read
Hair root under microscope
Forensic evidence collection bag
Forensic lab microcentrifuge
Forensic STR PCR gel electrophoresis

Forensic Overview & Biological Basis

Hair shafts consist primarily of cornified keratinFibrous protein forming structural units of hair, requiring strong enzymatic digestion. protein containing degraded nuclear DNA (primarily suitable only for mitochondrial DNA sequencing). In contrast, the basal hair follicleThe root region containing nucleated epithelial sheath cells rich in genomic DNA. contains intact nucleated cells necessary for nuclear genomic DNA extraction and Short Tandem Repeat (STR) profiling.

Why Hair Samples Are Used in Forensics

Advantages

  • Non-invasive sample collection.
  • Highly stable over long periods at room temperature.
  • Crucial evidence at crime scenes when blood is absent.

Limitations

  • Hair shafts without roots yield limited nuclear DNA.
  • Keratin matrix requires prolonged Proteinase K incubation.
  • Requires ultra-clean handling to avoid PCR inhibitors.

01 Hair Root Sample Collection

Surgically verify the presence of an intact follicular tag under a stereomicroscope.

02 Surface Decontamination Washing

Wash hair 3x with sterile 70% ethanol and nuclease-free water to strip external environmental DNA and dirt.

03 Hair Cutting & Mechanical Disruption

Minced root tissue (1–2 mm fragments) increases surface area for enzymatic digestion.

04 Lysis Buffer Addition

Add lysis buffer with high detergent content to break cellular membranes.

05 Keratinase & Proteinase K Digestion

Incubate with Proteinase K (20 mg/mL) and DTT at 56°C for 2–4 hours until hair fragments dissolve entirely.

06 Silica Spin Column Binding

Chaotropic salts promote selective binding of genomic DNA to the silica matrix.

07 Centrifugation & Flow-Through Removal

Centrifuge at 11,000 x g for 1 minute; discard flow-through containing digested keratin.

08 Washing (Ethanol Wash Buffers)

Wash column twice to remove residual digestion salts and cellular debris.

09 Low-Volume DNA Elution

Elute in 25 μL TE buffer to maximize DNA concentration for low-yield forensic samples.

Step 10: Nanodrop & Low-Yield Assessment

Low concentration (~2–15 ng/μL) is typical for single-hair extractions; purity ratios $A_{260}/A_{280} \approx 1.7-1.9$.

Step 11: Gel Verification & Comparison Slider

Intact Hair Root DNA Band
Hair Shaft Degraded DNA
↔
Degraded Hair Shaft DNA Intact Hair Follicle Band

Step 12: Forensic PCR Amplification

Amplification of polymorphic STR loci (e.g., TH01, vWA, D16S539) for human identity matching.

Step 13: STR Amplification Verification

Verify distinct amplicon band sizing on high-resolution polyacrylamide or 2% agarose gel.

Forensic Applications

Crime Scene Investigation

Matching suspect profiles to hair fibers found at evidence scenes.

Missing Persons Identification

Kinship matching using personal items (hairbrushes).

Complete 12-Stage Hair DNA Workflow

Stage 1–5: Follicle Washing & Keratin Digestion

Decontamination wash, mincing, and overnight Proteinase K breakdown.

Stage 6–12: Spin Column Elution & STR PCR

Silica matrix purification, low-volume elution, and STR locus amplification.

References

  1. Yoshii, T., et al. (1993). DNA typing of bone and hair sheath samples. International Journal of Legal Medicine, 105(6), 335-339.
  2. FBI Laboratory (2019). Quality Assurance Standards for Forensic DNA Testing Laboratories.