Differential Extraction
Definition
A two-step DNA extraction procedure used when a stain contains both sperm cells and non-sperm epithelial cells. The first step lyses epithelial cells under mild conditions; the second step adds a reducing agent to lyse the more resistant sperm cells. The two fractions are profiled separately, producing a victim profile and a contributor profile from the same stain.
- Primary use
- Sexual assault evidence containing mixed victim and suspect cells
- Key protection
- Protamine coating around sperm nuclei makes them resistant to initial cell lysis
- Output
- Two separate DNA fractions for distinct profiling of victim and suspect
Common questions
Why is differential extraction used in sexual assault cases?+
Sexual assault swabs usually contain mostly victim cells mixed with a smaller number of suspect (sperm) cells. Differential extraction separates them into two fractions, so you can get a clean male DNA profile even when victim DNA would normally dominate the sample.
How does the two-step process work?+
The first step uses a lysis buffer that breaks down epithelial cells (from the victim). The second step adds DTT, which breaks through the protective protamine coating around sperm nuclei, releasing the suspect's DNA. This gives you two separate DNA fractions to analyze.
What's the main advantage over regular extraction?+
Without differential extraction, victim DNA overwhelms the sample and you may miss or fail to profile the suspect. Separating them first lets you profile both contributors independently and build a reliable case profile.
Related terms
- Azoospermia
- The absence of spermatozoa from ejaculated semen, resulting from obstructive causes (vasectomy, ductal blockage) or non-obstructive causes (testicular failure). Azoospermic semen contains...
- PCR Inhibitor
- A co-extracted substance that interferes with Taq polymerase activity or primer annealing. Common inhibitors in forensic samples include haemoglobin (from blood on...
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- Chelex Extraction
- A rapid extraction method that uses the chelating resin Chelex 100 to bind divalent metal ions and prevent nuclease activity. Cells are...
- Destructive Testing
- Any test that consumes, extracts, or chemically alters the stain material: cutting a fabric swatch, swabbing a surface, adding a chemical reagent....
- Double-Swab Technique
- A collection method for dried biological stains in which a moistened swab rehydrates and lifts the stain from the surface, followed immediately...
- DTT (Dithiothreitol)
- A reducing agent that cleaves the disulphide bonds in the sperm protamine matrix, allowing proteinase K and SDS to lyse the sperm...
- Epithelial Cell Fraction
- The non-sperm cellular material in a sexual assault sample. Contains skin and mucosal cells from both victim and any perpetrator who made...
- Erase Sperm Isolation Kit
- An immunomagnetic sperm-cell isolation system using paramagnetic beads coated with antibodies against sperm-specific surface antigens. Achieves greater than 99% sperm recovery with...
- Female Fraction
- The lysate produced by the first (no-DTT) lysis step of differential extraction. Contains DNA from epithelial cells and other non-sperm cellular material...
- Laser Capture Microdissection (LCM)
- A UV-laser instrument technique for physically isolating individual cells or clusters of cells from a stained slide into a capture tube, without...
- Male Fraction (Sperm Fraction)
- The lysate produced by the second (DTT + proteinase K) lysis step after washing the sperm pellet. Ideally contains predominantly the suspect's...
Explained in these topics
- Differential Extraction in Sexual-Assault CaseworkA two-step selective lysis protocol that separates sperm-cell nuclei from epithelial-cell nuclei in a mixed sexual-assault swab. First step lyses epithelial ce...
- DNA Extraction and Quantification from Biological EvidenceA two-step lysis protocol applied to sexual assault samples that separates the sperm cell fraction from the epithelial cell fraction. Epithelial cells are lyse...
- Order of Testing and DNA PreservationA DNA extraction method used for sexual assault evidence that separates sperm cells from epithelial cells before cell lysis, yielding two DNA fractions that ca...
- Semen, Saliva and Other Body Fluids as EvidenceA two-step DNA extraction procedure used when a stain contains both sperm cells and non-sperm epithelial cells. The first step lyses epithelial cells under mil...
- Swabbing Technique and DNA YieldA laboratory technique that separates sperm cell fractions from epithelial cell fractions in a mixed sample, increasing the probability of obtaining a clean ma...