Differential Extraction

Differential Extraction

The Standard differential extraction (differential lysis) method was devised by Gill, Jeffreys, and Werrett in Nature,1985. It is a process used to separate male and female DNA from a sexual assault sample evidence. Differential extractions can be performed from both vaginal swabs and semen stains. The successful separation of cells in this extraction method depends on the skills and experience of the lab analyst.

The sample is removed and placed inside the tube. Salt, buffer, EDTA, detergent (sarkosyl) and proteinase k reagents are added. Incubate at 37C for 2 hours and centrifuge it. Remove supernatant leaving the pellet inside the tube. Wash the cell pellet and resuspend it in a sperm wash buffer. Vortex and spin again several times. It results in the separation of sperm/non sperm cell fraction. Histone proteins (4 %) are also present along with protamines, and cysteine.

Sperm loss, incomplete separation of the epithelial cells and sperm cells can occur using this method. There will be great variability in the quality of results from various labseven in the same sample. The procedure heavily depends upon the recovery of cells, washes, and digestions. Exogenous cells and epithelial cells might be also present in the DNA during the extraction. The age of the sample also affects the recovery of cells (since the DNA degrades). The procedure is time-consuming, depends upon the technique and analyst. DNA inhibitors can also be present along with the DNA and cause problems.

 

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