Investigating CSI – Background material Table of Contents I ...

B. Isolation of DNA
Differential Lysis
Once a suspected sample has been identified to contain sperm it is often
contaminated with other cell types. The most common contaminating cells are
the epithelial cells lining the vaginal wall, but can include epithelial cells from the
mouth (buccal cells) and skin, as well as those found in urine.
DNA from contaminating epithelial cells can be removed using a procedure
called a differential extraction (5), which takes advantage of unique properties
associated with each cell type. In this procedure, the cells are removed from the
suspected material by soaking them in a gentle solution. Epithelial cell DNA is
isolated under mild conditions that break open the epithelial cells but leave the
sperm cells intact DNA. The DNA in sperm can then be extracted using a more
harsh extraction procedure.
Y chromosome: Since the differential lysis procedure is unsuitable for saliva
evidence, the ratio of male to female cells becomes the deciding factor in cases
of sexual infidelity. For neat saliva stains or body surface swabs, it can be
possible to generate a clean or dominant male type using current STR
technology. For vaginal secretions it becomes more difficult to show the
presence of male DNA with in a sea of female DNA. Y-STR testing can help to
verify the presence of male DNA were infidelity involving oral sex is considered.
References:
5. Gill, P., Jeffreys, A.J., Werret, D.J., Nature 1985, 318, 577-579
Source: http://www.dnatesting.biz/Semen_Sperm_ID/semen_sperm_id.html
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