Differential extraction
Differential extraction refers to the process by which the DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 from two different types of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 can be extracted without mixing their contents. The most common application of this method is the extraction of DNA from vaginal epithelial cells and sperm
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 cells from sexual assault
Sexual assault
Sexual assault is an assault of a sexual nature on another person, or any sexual act committed without consent. Although sexual assaults most frequently are by a man on a woman, it may involve any combination of two or more men, women and children....

 cases in order to determine the DNA profiles of the victim and the perpetrator. Its success is based on the fact that sperm cells have protein disulfide bonds in their outer membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 which makes them more resilient to extraction than epithelial cells.

After determining that sperm cells are present (typically through staining and light microscopy) in a vaginal/rectal sample, the subject's epithelial cells are lysed
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....

 by a standard DNA extraction method, like a phenol/chloroform extraction and their DNA extracted
DNA extraction
DNA isolation is a routine procedure to collect DNA for subsequent molecular or forensic analysis. There are three basic and two optional steps in a DNA extraction:...

 through normal means. The epithelial DNA in solution is removed and saved, while the sperm cells are still intact. Differential extraction uses a chemical called dithiothreitol
Dithiothreitol is the common name for a small-molecule redox reagent known as Cleland's reagent. DTT's formula is C4H10O2S2 and the molecular structure of its reduced form is shown at the right; its oxidized form is a disulfide-bonded 6-membered ring . Its name derives from the four-carbon...

 (DTT) to disrupt the sulfur bonds in the coating of the sperm cell in order to extract its DNA. Once the sperm's outer membrane has been breached, it is prone to standard DNA extraction methods. This creates two different DNA fractions
Fraction (chemistry)
A fraction in chemistry is a quantity collected from a sample or batch of a substance in a fractionating separation process. In such a process, a mixture is separated into fractions, which have compositions that vary according to a gradient. A fraction can be defined as a group of chemicals that...

from one sample, hopefully that of the victim and that of the perpetrator.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.