Incriminating Evidence
“A Gel Electrophoresis Lab”
Johnny Wells, a famous and wealthy movie star, has been murdered. His body was found in his home by his
regular housekeeper. The police interviewed the housekeeper as well as many of Johnny’s friends and relatives. So far
they have narrowed their list of suspects to three---Suzanna Wells and Mr. and Ms. Starlight.
Suzanna Wells and Johnny divorced about five years ago. They had been angry with each other ever since the
divorce and Johnny became jealous of anyone Suzanna met. On more than one occasion, Johnny had been scene at
Suzanna’s home confronting her about her dates with other men. Johnny was known to occasionally become physically
aggressive with Suzanna especially if he had been drinking.
Gloria Starlight was Johnny’s mistress. They had been dating for over a year and Johnny had promised to marry
her on multiple occasions. However, something always happened to delay their wedding. Last week Gloria was
overheard in a heated debate with Johnny when she said she was tired of playing games with him and he confessed that
he still had feelings for his ex-wife.
Ben Starlight was Johnny’s manager and Gloria’s former husband. Gloria and Johnny’s affair was rumored to be
the cause of the couples’ recent divorce. However, Ben was still acting as Johnny’s manager despite the growing
tensions.
DNA samples were taken from all three suspects for comparison testing. DNA was found in a blood sample at
the crime scene. This DNA did not match Johnny Wells. To compare the crime scene DNA with that of the suspects, it
was extracted and replicated through PCR.
Materials:
DNA Samples
Sample A—DNA Marker (just used for comparison like a control)
Sample B—Crime Scene evidence
Sample C—Suzanna Wells
Sample D—Gloria Starlight
Sample E—Ben Starlight
Electrophoresis Buffer
Micropipettes and disposable tips
Electrophoresis chamber
Agarose gel
Ethidium bromide cards
Light box
Destaining tray
Power supply
Plastic wrap
Gloves
Goggles
LAB Safety
1. Gloves and goggles must be worn.
2. Exercise caution when using the electrophoresis chamber. It has an electrical current when in use.
3. Always wash hands.
Micropipette tips:
1. Remember to depress to the soft stop before entering into the DNA sample. You can puncture right through
the foil.
2. You need every drop of DNA so make sure that it has all settled to the bottom before trying to draw it up. If is
appears to be stuck to the sides, gently tape the quickstrip on the table until everything settles.
3. Do not put any pressure on the gel. It will puncture or tear. If it does, tell your teacher right away.
4. Remember when you are dispensing the DNA into the gel. DO NOT let go of the button until you are all the
way out.
DAY 1 Procedure (WEAR GLOVES AT ALL TIMES)
1. Place the agarose gel in the electrophoresis chamber. (This step may already be done).
2. Add enough electrophoresis buffer to cover the gel. (This step may already be done).
3. Using a micropipette load 20ul of DNA sample A into the first well. Make sure to use the technique demonstrated by
your instructor. BE CAREFUL NOT TO PUNCTURE THE GEL OR OVERFILL THE WELL.
4. After changing to a clean micropipette tip, load 20 ul of DNA sample B into well 2.
5. Continue loading 20ul of each sample (C-E) in order, making sure to change tips each time. Each sample should go
into a separate well. Make sure that the samples are loaded into the wells in the correct order.
6. Place the top on the electrophoresis chamber (red on red; black on black) and let your instructor know. She will show
you how to connect to the power supply. Run the gel for 30 minutes at 150 volts.
7. Label the diagram on your answer sheet.
8. Answer questions 1-8 on your answer sheet.
9. Turn off the power supply. Slide the gel from the gel bed to a destaining tray.
10. Using a pipette, moisten the gel with a few drop of buffer solution from your electrophoresis chamber.
11. Wearing gloves, remove the plastic protector sheet and place the ETBR card on your gel.
12. Place the gel bed and a small beaker on top to weigh it down.
13. Wait 15 minutes.
14. Place your gel on the UV transilluminator and photograph and sketch your results. Make sure to wear gloves and
use UV protection goggles.
15. Cleanup your area. Buffer and distilled water can go do the sink. Give gels to your instructor.
16. Based on your sketch answer the following questions.
GROUP MEMBERS: ________________________________________________________________________
DNA Gel Electrophoresis Lab
Questions
1. What are the four stages of DNA testing?
2. Which stages have already been done for you today?
3. Who are the only people with exactly the same DNA?
4. What cells can be used for DNA testing?
5. What kind of DNA testing is this, STR or RFLP? How can you tell?
6. Why does the DNA migrate down the gel?
7. Fragments of DNA are separated by their _______________________.
8. What is the purpose of the ethidium bromide?
9. How many genes are used in forensic DNA testing?
10. How many did we look at today?
11. Why do we usually see two bands for each sample?
12. One of your suspects only has one DNA band, what does that indicate about his/her genotype?
13. What can you say about each of your suspects in regards to the crime scene sample? Who matches? Who doesn’t?
14. Take a photo of your gel with your IPAD. Write directly on your photo
with any app that allows you to write directly on pictures. Fully
annotate (explain) your results. Insert your photo into this worksheet.