RFLP comparisons are much more specific tests, but require DNA samples of very high quality to work effectively. The procedures are also much more labor-intensive than PCR. First a restriction endonuclease is used to cut the DNA into millions of fragments. The restriction endonuclease is able to cut a strand when it comes across a specific sequence of nucleotides. These fragments are separated by gel electrophoresis based on their relative sizes. Smaller fragments have less drag in the gel and are carried further by the current. NaOH is then added to the gel to alter the pH and break the duplex DNA molecules into singular strands. The fragments are transferred to a nylon membrane in a process called Southern Blotting. In this procedure the nylon membrane is placed on top of the gel. Absorptive material is then placed on top of the membrane, pulling the gel through the nylon into the material. The DNA is carried in the gel medium to the nylon membrane where it is fixed to the sheet. The membrane is baked to more tightly attach the DNA fragments. To ensure no other DNA molecules can attach to the sheet, protein or detergent is added, which blocks all of the vacant spots where a DNA fragment could attach. Now the only way a DNA molecule can bond to the sheet is if it is complementary to the single stranded DNA already fixed to it. Probe fragments of specific genes are then made radioactive and washed over the membrane. If the complementary segment is present on the membrane, the probe will attach. After washing away the unattached probes, X-ray film or a phosphorimager can then be used to detect the presence of an attached probe and thus the presence of the gene in the sample. The variability in the types and sizes of probes allow RFLP to reveal a large amount of very specific information. The membrane can be cleaned of any probe and retested with another providing an even larger amount of data.
As the O.J. Simpson trial best exemplified, one must not be too quick to judge DNA fingerprinting as a fail proof method of determining truth. In this trial both PCR and RFLP methods of analysis were applied to 45 bloodstains. At the scene of the murder, 8 drops of blood along the walkway and back gate were found to be O.J. Simpson’s. Seven bloodstains in O.J.’s Bronco were found to be Nichole Brown and/or Ronald Goldman’s. Three drops of Nichole Brown’s blood were found on O.J. Simpson’s socks and 11 bloodstains found on the Rockingham glove contained the victims’ DNA. In spite of all of this biochemical data indicating O.J. Simpson as the murderer, the defense was able to convince the jury that genetic evidence is only as reliable as those who are in charge of the tests and a verdict of not guilty was pronounced.
Nevertheless, today’s courts are filling up with appeals on the grounds of new reliable genetic evidence. Many innocent men and women can now be justly returned to their freedom. The number of overturned rulings has even fueled the movement against the death penalty as people repeatedly witness just how many false convictions there are. Cases like those of Ronald Cotton are showing that even an eyewitness many not be as reliable as the witness provided in the genetic fabric of life itself.
The maximum potential for this technology is far from being attained. The days may be approaching when a hair follicle dropped into a computer produces the picture of the guilty party in seconds. Why bother even sending police out to look for them? If that hair follicle has the criminal’s antigen information on it, isn’t it possible to produce a virus that targets that individual's unique HLA profile? Then release the virus and wait at the hospital or morgue for the right symptoms. It sounds drastic, but it may be possible…
