Fingerprinting Simulation

Advanced Preparation

  1. Purchase the dye kit from Wards catalog # 36 W 5130 "Introduction to Agarose Gel Electrophoresis".
  2. Prepare 1X TBE buffer.
  3. Prepare and pour 2% agarose gels. When pouring gels be sure the comb is placed in the middle of the gel tray. When the gel has solidified, place the gel tray and gel inside a zip-lock plastic bag. The gels may be refrigerated or may be kept at room temperature until ready to use.
  4. Use 5 different colored eppendorf tubes. Prepare enough tubes of each color for each pair of students.
  5. Take eppendorf tubes that are one color and label them A. Aliquot 12 µl of dye mixture for sample A.
  6. Take eppendorf tubes that are a second color and label them B. (This is the victim's ex-boyfriend's DNA.) Aliquot 12 µl of sample B into each of the tubes labeled B.
  7. Take eppendorf tubes that are a third color and label them C. (This is the cat burglar's DNA.) Aliquot 12 µl of sample C into each of the tubes labeled C.
  8. Take eppendorf tubes that are a fourth color and label these tubes D. Duplicate either the ex-boyfriend's DNA (sample B) or the cat burglar's DNA (sample C). This is the DNA of the rapist/killer. Aliquot 12 µl of either sample B or C into each of the tubes labeled D.
  9. Take the remaining eppendorf tubes that are a fifth color and label these tubes E. (This is a combination of the victim's blood and the rapist/killer.) Aliquot 12 µl of sample E into each of the tubes label E.
  10. Each lab group will receive 5 different tubes (Samples A, B, C, D, E) to run on their gel. Since each sample was aliquoted into a different colored tube, students will notice immediately if they are missing a sample.

Introduction

Many new scientific advances have resulted from the development of techniques such as restricting (cutting) DNA and analyzing human genes and human DNA. One of those advances is DNA fingerprinting which is based on the concept that every individual's DNA is unique. DNA fingerprinting analyses have been used in court as evidence to prove an individual's guilt or innocence. Additionally, DNA fingerprinting has been used to solve or reopen old rape and murder cases to find the killer/rapist. As long as one is able to obtain a sample of DNA (either through sperm samples, blood samples, or tissue samples), DNA fingerprinting can be accomplished.

The actual process of DNA analysis is based on the fact that different sizes of DNA will move at varying rates of speed in an electrical field. DNA samples taken from suspects and crime scenes are restricted by enzymes that cut the DNA only at specific sites. Since each person's DNA is unique, the pieces of DNA that result from the cuts are different sizes for each person. The larger sizes of DNA move at a slower rate because it takes them longer to wiggle their way through the maze of agarose gel; whereas the smaller sizes of DNA will move quickly through agarose gel. In addition, DNA is negatively charged so that in an electrical field DNA will move towards the positive electrode. Therefore, in an agarose gel, the pieces of DNA can be separated and viewed to determine which of the suspects' DNA has been found at a crime scene. If DNA is collected from two different sources, it can be tested to determine if the DNA came from the same individual. This determination is made after the DNA is cut into pieces and run on an agarose gel. When the DNA pieces move exactly the same distance on the gel, they are considered the same pieces. Therefore, if the cut up pieces of DNA of a suspect exactly matches the cut up pieces of DNA collected at a crime scene, there is a 99.6% chance that the suspect is guilty. The 0.4% difference is caused by the fact that not all the DNA is being used in the DNA comparison. If the cut up pieces of DNA of the suspect do not match the cut up pieces of DNA from the crime scene, the suspect is definitely innocent!

Student Objectives

  1. Students will be able to analyze and interpret experimental results.
  2. Students will be able to explain how gel electrophoresis works.
  3. Students will be able to explain how the judicial system can use gel electrophoresis in court cases.

Class Time Needed

One class period is needed for the students to complete this activity.

Materials

  1. Ward's dye kit (catalog #36 W 5130)
  2. 1XTBE buffer
  3. 2% agarose gels (one/2 students)
  4. Micropipet
  5. Eppendorf tubes (5 colors/2 students)
  6. Eppendorf tube rack
  7. Box of micropipet tips (one/2 students)
  8. Power supply (one/4 students)
  9. Electrophoresis gel box (one/2 students)

Recipes for Consummables

  1. 1X TBE buffer is made by mixing 100 ml of 10X TBE in 900 ml distilled water.
  2. 2% agarose gels are made by mixing 4 g of agarose in 200 ml 1X TBE buffer. Heat until dissolved and the solution is clear.
  3. Sample A is only the dye mixture provided by the manufacturer.
  4. Sample B is made by mixing 250 µl of Janus green and 250 µl of Bromophenol blue to make a total of 500 µl.
  5. Sample C is made by mixing 150 µl of Safranin O, 150 µl of Orange G, and 150 µl of Janus green for a total of 450 µl.
  6. Sample D is either Sample B or C.
  7. Sample E is made by mixing 150 µl of dye mixture and 150 µl of Sample D.

Procedure

  1. At each student station place a gel box, a micropipet, a box of pipet tips, the tube rack with 5 different colored tubes in it (samples A, B, C, D,E), a gel tray and gel, and a 500 ml container of 1X TBE.
  2. For every 2 student stations set up a power supply.