Introduction

In the following lab analysis you are a district attorney attempting to solve a murder/rape. You have been able to obtain samples of DNA through sperm left by the killer. The police have a man in custody who was seen by neighbors in the area at the time of the murder. This man claims that he is merely a cat burglar who was scouting out the area at the time of the murder. He claims that he watched the victim open her door to a man at the approximate time of the murder. He describes the man as a tall, blond man with a muscular build. The police discover that this description fits the victim's ex-boyfriend. Upon questioning, the ex-boyfriend admits that he went to the victim's home but left her alive.

You have obtained DNA samples from both suspects, the victim, the sperm left by the rapist/killer, and blood from the crime scene. You will now analyze the samples to determine who is the killer/rapist.

Procedure

1. Place your tray with the agarose gel in the gel box.

2. Pour in enough 1XTBE buffer to fill the box so it is slightly higher than the gel itself.

3. Place a clean, plastic micropipet tip on the micropipet. Dial the pipet to 12 ul.

4. Draw up the entire amount of sample A into the tip of your micropipet. Transfer this substance to your gel and load the substance into the first well in the gel. (The first well is Lane #1 in the diagram on the next page.) (Be careful not to puncture the well of the gel.)

5. After placing the sample into the gel well, discard the used tip into the waste container.

6. Repeat steps 3, 4, and 5 with the other 4 samples. Sample B is loaded into well 2; sample C into well 3; sample D into well 4; and sample E into well 5.

Sample A is the victim; sample B is the victim's ex-boyfriend; sample C is the cat burglar; sample D is the sperm collected at the crime scene; and sample E is a blood mixture of the victim and the rapist/killer.

7. Place the lid on the electrophoresis tray and attach the cords to the power supply. (The black anode to the black cord and the red anode to the red cord.)

8. Plug in the power supply and set at 100 volts. Allow the gel to run for 20 minutes. View the gel to determine when it is finished (when the fastest moving dye is near the end of the gel); then turn off the power supply. Unplug the cords from the power supply and remove the lid. Place the gel on a white sheet of paper for viewing. (This may make it easier to see the gel bands.)

Analysis

1. As you are watching your samples, explain how gel electrophoresis works and why the dyes move in the directions that they do.

2. After the 20 minutes are concluded, draw the results of separating the dye samples on the diagram of the gel below. Be as accurate as possible. Make sure you record the correct direction (+ or -), measure the distance that each dye moved and the color of each dye.

3. Is it possible for the sperm of the suspected rapist/killer to have a different pattern from his blood sample? Why?

4. Why is it important to have the blood sample from the crime scene?

5. Would it be possible to use this evidence in a court of law? Why?

Conclusion

Who is the most likely suspect in this case? Why?