A forensic DNA laboratory reports a mixture ratio of 1:9 (minor:major contributor) in a sexual assault case. The analyst wishes to extract only the spermatozoa fraction for STR profiling. Which extraction method selectively lyses epithelial cells first while leaving sperm cells intact?

• A Organic (phenol-chloroform) extraction applied simultaneously to all cells
• B Chelex-100 extraction at 100°C to preferentially denature nuclear membranes
• C Differential extraction — low-detergent lysis first followed by proteinase K lysis with DTT for sperm ✓
• D FTA card direct PCR without any prior extraction step

Explanation

Differential extraction exploits the double-disulfide-crosslinked membrane of sperm cells that resists mild detergents. The first step uses a non-ionic detergent to lyse fragile vaginal epithelial cells; after centrifugation and washing, the intact sperm pellet is then lysed with a stronger lysis buffer containing dithiothreitol (DTT) to break disulfide bonds. This yields a sperm-enriched fraction, greatly improving the ability to develop a male contributor profile in mixed sexual assault evidence.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.