Place the sample in a collecting tube. Add liquid detergent to the sample to break the cell membrane and nuclear membrane---this will release the DNA. Cell membranes are made of fat, so detergent breaks them apart like grease on a frying pan. This reaction will happen faster if you heat the sample.
Add salt to separate the DNA from everything else. Since DNA and salt both have a slight charge, the DNA will dissolve in salt water but nothing else will. Remember the old adage, "Like dissolves like."
Place the tube in a centrifuge, which spins the sample at a very high speed. Solid particles will move to the bottom of the tube and the DNA will stay in the liquid because it is dissolved.
Pour alcohol into the liquid part of the sample to make the DNA appear. Alcohol does not have a charge, so the DNA will not stay dissolved. This is the same principle we used in Step 2.
Spool the DNA onto a glass rod. You should be able to see the strands of DNA without a microscope.
Place the spooled DNA into a tube with a restriction enzyme in a buffering solution. Restriction enzymes, discovered in 1962, are made by bacteria for protection against foreign organisms, such as viruses. They make a cut at the exact same sequence of DNA bases every time. For example, the Smal restriction enzyme, extracted from the bacteria Serratia marcescens, recognizes the sequence CCCGGG. It cuts in between the CCC and GGG, creating smaller DNA fragments. (See Warnings for extra precautions.)
Repeat this process with the DNA from any possible suspects. Be sure to use the same restriction enzyme for each sample.
Set up the gel electrophoresis apparatus, identified by the Human Genome Project as an "essential tool of modern molecular genetics." This technique uses an electric field to separate DNA fragments of different lengths by density. Place the gel so that the end with the wells is labeled "negative."
Place the fragments of DNA into the wells of the gel using a micro-pipette. You will not notice any differences in the fragments at this point.
Connect the gel to an electrical power source. DNA has a slightly negative charge, so it will be attracted to the positive pole. This will cause the pieces of DNA to migrate. The smaller the fragment, the farther it will travel, thus creating a pattern across the gel.
Stain the gel to make the DNA banding pattern visible. Methylene blue is commonly used for this purpose. This step can also be done using a radioactive DNA probe.
Compare the banding patterns from the crime scene DNA and the suspect's DNA. If the suspect was at the crime scene, the patterns should be a match. According to the Genetic Science Learning Center of the University of Utah, "it is easier to exclude a suspect than to convict someone based on a DNA match. The FBI estimates that one-third of initial rape suspects are excluded because DNA samples failed to match."
Run the crime scene DNA sample through CODIS, the FBI's DNA database. The database maintains a record of DNA obtained from sex offenders, violent criminals and crime scenes throughout the country. The National DNA Index (NDIS) has profiles on more than 8,483,906 offenders, as well as 324,318 forensic profiles, as of June 2010. If there was a convicted criminal at the crime scene, CODIS will identify the perpetrator.