Differential Extraction

Differential extraction is a modified version of the organic extraction method that separates epithelial and sperm cells (Figure 3.2). Differential extraction was first described in 1985 (Gill et al. 1985) and is commonly used today by the FBI Laboratory and other forensic crime laboratories to isolate the female and male fractions in sexual assault cases that contain a mixture of male and female DNA. By separating the male fraction away from the victim's DNA profile, it is much easier to interpret the perpetrator's DNA profile in a rape case.

The differential extraction procedure involves preferentially breaking open the female epithelial cells with incubation in a SDS/proteinase K mixture. Sperm nuclei are subsequently lysed by treatment with a SDS/proteinase K/dithiothreitol (DTT) mixture. The DTT breaks down the protein disulfide bridges that make up sperm nuclear membranes (Gill et al. 1985). Differential extraction works because sperm nuclei are impervious to digestion without DTT. The major difference between the regular version of organic extraction described earlier and differential extraction is the initial incubation in SDS/proteinase K without DTT present.

Differential extraction works well in most sexual assault cases to separate the female and male fractions from one another. Unfortunately, some perpetrators of sexual assaults have had a vasectomy in which case there is an absence of spermatozoa. Azoospermic semen, i.e., without sperm cells, cannot be separated from the female fraction with differential extraction. In the case of

Figure 3.2

Schematic of differential extraction process used to separate male sperm cells from female epithelial cells.

Remove a portion of the mixed stain

Differential Extraction Sperm

Perpetrator's sperm mixed with victim's epithelial cells

SDS, EDTA and proteinase K (cell lysis buffer)

Incubate at 37 °C

Centrifuge

Perpetrator's sperm mixed with victim's epithelial cells

SDS, EDTA and proteinase K + DTT

'Male Fraction'

Incubate at 37 °C

Centrifuge

Sperm Pellet

REMOVE supernatant sperm pellet

Female Fraction'

REMOVE supernatant sperm pellet

Female Fraction'

vy azoospermic perpetrators, the use of Y chromosome specific markers permit male DNA profiles to be deduced in the presence of excess female DNA (see Chapter 9). Failure to separate the male and female portions of a sexual assault sample results in a mixture of both the perpetrator's and the victim's DNA profiles (see Chapter 7).

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Responses

  • lorraine
    What is differential extraction?
    1 year ago
  • fre-qalsi
    Why use differential extraction in forensic science?
    1 month ago
  • Ellie-louise
    How do differential extraction work?
    2 days ago

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