DNA Fingerprinting

Background information:

During the investigation of violent crimes such as homicide, fingerprints are not the most common type of evidence recovered by police officers. They are more likely to recover hair or blood. Forensic scientists have been able to use a new technique developed by molecular geneticist Dr. Ray White in 1980. It is popularly called "DNA fingerprinting" and was first admitted as evidence in a court to convict someone in 1987.

In every cell of a person's body, DNA is found in the nucleus. No two people, except identical twins, have the same DNA. DNA is made of a series of 4 bases--adenine, thymine, cytosine and guanine. The order of the bases determines a person's genetic code. Some parts of the code are very similar in all people; for example, the code for building a human heart. However, some portions of the code varies greatly from person to person. These are called polymorphic (poly=many and morphic=shape) segments. Person A might have 6 repeats of a segment, person B may have 17 repeats and person C 12. When several different sequences are considered, the chances that any two individuals will have exactly the same variation are very remote.

To perform DNA fingerprinting, the DNA of cells from a crime scene is copied over and over by a process called polymerase chain reaction (PCR). A single drop of blood or the root of a single hair could be enough to identify the DNA. Using a search warrant, cells are taken from a suspect by drawing blood or taking hair. The DNA from the crime scene and suspect are cut with chemical scissors called restriction enzymes. Then the cut DNA is placed in wells at the top of a gel. The fragments are separated by electrophoresis. If the bands of the suspect's DNA matches the bands of the evidence DNA, the chances are very great that the suspect was present at the crime scene. Interestingly, the first time DNA was used to convict a person in a jury trial in North Carolina was in 1992, in a case of rape that occurred in Catawba County.

Procedure: Case 96-3337

  1. Sign and date the evidence

  2. Pour the agarose gel you receive from your teacher into the gel electrophoresis chamber. Place a comb into the agarose mixture. Allow it to harden. When the agarose is solid, cover it with TBE electrophoresis buffer. Remove the comb.

  3. In the first well pipette DNA from Exhibit M (the cells found under the nails of the deceased). Using a clean pipette, place DNA from Exhibit N (suspect 1) into the second well. Again using a clean pipette, place DNA from Exhibit O (suspect 2) into the third well.

  4. Put the lid on the gel chamber. Be sure the wells are at the black (negative) electrode. Plug the leads into the power supply and allow the fragments to separate. Your teacher will possibly have to turn off the power later in the day.

  5. The next day, visualize the bands by placing the gel carefully in methylene blue stain. Let the stain set for 30 minutes, then remove the gel while wearing gloves on your hands. Place the gel in distilled water for destaining. Finally, set the gel on the white light source and compare the bands.

  6. Measure and draw the bands on your report form exactly as they appear on the gel.

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