DNA collection and analysis gives the criminal justice system a powerful tool for convicting the guilty and exonerating the innocent.
Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual using samples from blood, bone, hair, and other body tissues and products.
In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers).
If the sample profiles don't match, the person did not contribute the DNA at the crime scene. If the patterns match, the suspect may have contributed the evidence sample. DNA from crime scenes also can be compared to profiles stored in a database.
http://www.nij.gov/topics/forensics/evidence/dna/basics/Pages/welcome.aspx
SEMEN DNA ANALYSIS: The detection and investigation of a semen sample
For example, let us assume that a man's semen is alleged to be on a piece of fabric. To learn whether the cloth is stained by the suspect's semen, DNA testing can be done.Remember, no identifiable semen was found in Jaymie's body during the autopsy.
Here’s the procedure:
1. The fabric is examined for semen. Semen glows under ultraviolet light and also changes color when exposed to specific chemicals.
2. If semen is identified and the tests are positive, the lab technicians cut a swatch of the fabric from the stain and dissolve the organic matter in the stain by putting the swatch in a special solution.
3. The technicians then look for sperm cells in the solution with an ordinary low-power light microscope. The sperm look like little lifeless tadpoles.
4. If there are visible sperm under the microscope, the techs extract DNA from the sperm. This is done with a mild detergent that bursts non-sperm cells. A rinse with water removes broken cells. And then a stronger detergent is used to burst the sperm and recover their DNA.
5. The lab then compares the DNA from the semen with the suspect's DNA. A blood or saliva sample from the suspect will supply enough DNA for the comparison.
6. If the stain is small and the amount of DNA minute, a method based on the polymerase chain reaction, or PCR, is employed to make millions of copies of selected segments of the DNA. Note that PCR does not change the DNA but merely amplifies the amount.
7. The DNA from the semen is then compared with the suspect's. This can be done using what are called restriction fragment length polymorphisms, or RFLPs. To examine RFLPs, the DNA from the sample is cut (restricted) by special enzymes. These restriction enzymes cut different DNAs differently. If the restriction enzymes cuts the suspect's sample and the semen sample into the same number of fragments of the same length, then the semen may be the suspect's.
8. The odds that the DNA in the semen is the suspect's are then calculated. The odds may, for example, be one in 11 million that the DNA in the semen came from another man, not the suspect.
DNA tests of semen can take several days or several weeks before the results are available. DNA tests can show that the sperm is not from the suspect or these tests can show that the DNA in the sperm is not detectably different from the suspect's.
But DNA testing can never prove with complete certainty that the semen is from the suspect.
http://www.medicinenet.com/script/main/art.asp?articlekey=23321