DNA Extraction and Quantification from Biological Evidence

All extraction methods share the same goal: separate DNA from everything else in the sample. They differ in how they achieve that separation and in the trade-offs between yield, purity, DNA integrity, hazard, cost, and automation compatibility.

Organic extraction was the first widely adopted method. Cells are lysed with detergent and proteinase K, which digests proteins. The lysate is mixed with phenol-chloroform, which denatures and partitions proteins into the organic lower phase while DNA remains in the upper aqueous phase. The aqueous phase is recovered and DNA is precipitated with ethanol. The method yields high-molecular-weight double-stranded DNA in good purity, but phenol is corrosive and toxic, chloroform is a carcinogen, and the multi-step protocol is difficult to automate at scale. Most modern high-throughput laboratories have replaced it with solid-phase methods.

Chelex extraction is rapid and requires minimal equipment. The sample (a cutting from a stain or a swab) is incubated with proteinase K and then with Chelex 100 resin suspended in water. Boiling denatures proteins and lyses cells while the resin chelates magnesium and other divalent ions that activate DNases. The supernatant after centrifugation contains single-stranded DNA. The method is inexpensive and appropriate for low-volume work or field-adjacent laboratories, but the single-stranded product is less stable than double-stranded DNA from organic or solid-phase extraction, and the method is not compatible with RFLP analysis or some quantification kits.

Solid-phase extraction exploits the affinity of DNA for silica under specific ionic and pH conditions. In high-salt, low-pH buffer, DNA adsorbs onto a silica membrane or magnetic silica bead. Wash steps remove haemoglobin, humic acids, and other inhibitors. A final elution with low-salt buffer or water releases purified, double-stranded DNA. Commercial kits, including the QIAamp DNA Investigator (QIAGEN) and the PrepFiler (Thermo Fisher Scientific), are validated for forensic use, compatible with liquid-handling robots, and produce extracts with consistent inhibitor removal. Solid-phase methods are the current standard in accredited forensic DNA laboratories.

Sexual assault evidence frequently contains a mixture of epithelial cells from the victim and sperm cells from a perpetrator. If both cell types are extracted together, the resulting DNA profile is a mixture, which is more difficult to interpret than a single-contributor profile. Differential extraction exploits a structural difference between the two cell types to separate them before lysis.

Sperm nuclei are stabilised by disulfide cross-links between protamine proteins in the nuclear envelope. These cross-links resist the detergents and proteinase K concentrations that lyse epithelial cells. The differential extraction protocol applies a mild lysis step, low detergent concentration and proteinase K, and then centrifuges the suspension. The supernatant containing lysed epithelial cell DNA is removed and saved as the epithelial fraction. The remaining pellet contains intact sperm cells, still unlysed. A second lysis step adds a reducing agent, typically dithiothreitol (DTT), which breaks the disulfide bonds and allows sperm cells to lyse. This produces the sperm fraction.

The two fractions are extracted to completion separately and profiled independently. In an ideal result, the epithelial fraction gives the victim's profile and the sperm fraction gives the perpetrator's profile. Incomplete separation is common when sperm concentration is low or when the sample has been mixed for an extended time. The analyst documents the separation efficiency and notes any contribution from the opposite cell type in each fraction.

Standard extraction protocols are optimised for fresh blood and saliva stains on cotton swabs. Forensic evidence arrives in many other forms, each with substrate-specific complications.

Hair shafts contain mitochondrial DNA in the shaft itself but no nuclear DNA unless the root sheath is attached. A hair with an intact follicular tag can yield nuclear DNA for STR profiling; a shed hair without a root yields only mitochondrial DNA, which cannot distinguish between maternal relatives. Extraction from hair requires extended proteinase K digestion and solid-phase cleanup to remove melanin, a potent inhibitor.

Bone and teeth are the most DNA-stable biological matrices in forensic and identification work. The mineral phase of bone and dentine physically protects DNA from environmental nucleases and moisture. Pulverisation in liquid nitrogen followed by decalcification with EDTA or acid is required before standard lysis can proceed. Teeth, particularly the pulp chamber and the cementum layer, preserve DNA for centuries under favorable burial conditions. The Forensic Anthropology subject covers skeletal recovery and sample selection in detail.

Touch DNA samples contain trace amounts of cellular material, typically shed skin cells, from contact with surfaces. The DNA mass may be below 100 picograms. Standard extraction protocols are used, but every step must be optimised to minimise loss: small-volume elution, minimised tube transfers, and carrier RNA or BSA to prevent DNA adsorption to tube walls. Touch DNA extraction is closely tied to low-template PCR strategies discussed in the forensic biotechnology subject. For the biological background on touch samples, see Touch DNA and Trace Biological Material.

Quantitative real-time PCR (qPCR) measures the concentration of target DNA in an extract by monitoring fluorescent signal in each amplification cycle. When signal crosses a threshold value, the cycle number at that crossing, called the Ct or Cq value, is recorded. A lower Ct means more target DNA was present at the start. A standard curve generated from DNA of known concentration converts Ct values into DNA mass per microlitre.

Forensic quantification kits amplify a human-specific target, typically in a repetitive region of the genome present at high copy number to increase sensitivity. Current commercial kits, including the Investigator Quantiplex Pro (QIAGEN), the PowerQuant System (Promega), and the Plexor HY (Promega), simultaneously measure total human DNA, male-specific DNA from a Y-chromosome target, and a degradation index. The Y-chromosome target identifies samples likely to contain a male contributor even when the sample appears female-biased at STR loci, which is relevant in sexual assault casework.

The degradation index compares the amplification efficiency of a short target (around 80 base pairs) to a long target (around 300 base pairs) in the same reaction. Intact, high-molecular-weight DNA amplifies both targets equally well. Degraded DNA, in which most molecules are fragmented below 200 base pairs, amplifies the short target efficiently but produces little signal for the long target. A high degradation index (greater than approximately 10 in most kit protocols) warns that STR profiling using standard multiplex kits will likely yield a partial or no profile, because STR amplicons range from about 100 to 450 base pairs.

Inhibition is detected when the internal positive control (IPC), a non-human DNA target included in every quantification reaction at a fixed concentration, amplifies at a higher Ct than expected. A delayed IPC Ct indicates that something in the extract is slowing polymerase. Remediation options include diluting the extract, adding BSA to sequester inhibitors, switching to a polymerase formulation marketed for inhibitor tolerance, or re-extracting with an inhibitor-removal cleanup step.

The quantification result feeds directly into amplification setup. Most validated STR multiplex kits specify an optimal input mass, typically 0.5 to 1.0 nanograms of DNA, with defined acceptable ranges. Above the maximum, artefacts including off-ladder peaks and pull-up increase. Below the minimum, allele dropout occurs, where one or both alleles at a locus fail to amplify, making the profile incomplete. Quantification allows the analyst to calculate the volume of extract to add to each PCR reaction.

When quantification indicates a sample contains human DNA below a useful profiling threshold, typically below 0.01 to 0.1 nanograms per microlitre depending on the laboratory's validated range, the analyst must decide whether to proceed with low-template amplification, concentrate the extract, or designate the sample as unsuitable. Low-template protocols amplify more PCR cycles and may use larger input volumes. They increase sensitivity but also increase the prevalence of stochastic artefacts: allelic drop-in (amplification of sporadic contaminant alleles), increased stutter, and peak height imbalance between heterozygous alleles.

Severely degraded samples with a high degradation index may contain adequate total DNA mass but still fail standard STR profiling because the DNA is fragmented. Options include mini-STR kits, which amplify shorter amplicons of 70 to 180 base pairs to recover allele information from fragmented templates, or massively parallel sequencing approaches that reconstruct genotype information from very short reads. These advanced strategies are covered in the forensic biotechnology curriculum.

Extraction and quantification steps are high-contamination-risk procedures. The analyst is handling open biological material, and any exogenous human DNA introduced at this stage will appear in the profile. Standard precautions include pre-PCR and post-PCR area separation, UV decontamination of surfaces and equipment, single-use consumables, and full personal protective equipment including gloves, mask, and lab coat. Many laboratories collect elimination databases of all personnel who enter the laboratory to enable rapid identification of any inadvertent contamination event.

Chain of custody documentation must accompany the evidence item from collection through extraction to profile. Any break in continuity, an unrecorded transfer, a missing signature, or a discrepancy in item description, can make the extracted DNA inadmissible. Under the Bharatiya Sakshya Adhiniyam 2023 (India) and the Evidence Act provisions applicable in many common-law jurisdictions, continuity of evidence is a condition for admissibility. In England and Wales, the Criminal Procedure Rules 2020 require forensic science providers to disclose the basis for expert opinion, including the extraction method, contamination controls, and quantification data.

In the United States, laboratory accreditation under the FBI Quality Assurance Standards for Forensic DNA Testing mandates proficiency testing, technical and administrative review of all cases, and annual audits. The Supreme Court decision in Melendez-Diaz v. Massachusetts (2009) established that forensic laboratory reports are testimonial and the analyst who performed the testing may be required to testify. Courts across jurisdictions have increasingly scrutinised the extraction and quantification documentation as part of challenges to DNA evidence reliability.

For biological evidence collected at scenes outside the laboratory, proper preservation before extraction is the single largest variable affecting DNA yield. Blood stains should be air-dried before packaging; wet stains sealed in plastic promote bacterial growth and accelerate degradation. Samples should be refrigerated or frozen until extraction. Bones and teeth can be stored at room temperature if dry. Swabs from sexual assault evidence should be frozen as soon as possible. The Blood as Biological Evidence and Semen, Saliva and Other Body Fluids topics cover substrate-specific collection guidance.