DNA analysis has become a cornerstone of modern forensic science. Because every individual (except identical twins) has a unique DNA profile, scientists can use DNA evidence from crime scenes to help identify suspects or eliminate individuals from suspicion. One common method for comparing DNA samples is gel electrophoresis, a technique that separates DNA fragments based on size. When DNA is cut using restriction enzymes, the resulting fragments form a pattern – or “DNA fingerprint” – that can be visualized and compared.
In this engaging, hands-on lab activity, students take on the role of forensic scientists to solve a fictional crime. A DNA sample has been recovered from a crime scene, and two suspects have been identified. Students will compare the DNA from the crime scene with samples from each suspect to determine whose DNA was left behind.
I created my own worksheet to go with this activity, but if you order the kit from Edvotek, it comes with a student handout and instructions.
Key steps in the activity include:
- Reviewing the background of the case and the suspects.
- Learning how restriction enzymes cut DNA at specific sequences.
- Loading and running DNA samples through an agarose gel using electrophoresis.
- Staining and visualizing the DNA bands to create a DNA fingerprint for each sample.
- Comparing the banding patterns to identify the individual whose DNA matches the crime scene sample.
Lab Sequence
*I do this lab over two days because my class periods are only 50 minutes long. You can shorten it by pouring the gels ahead of time.
Day 1 – Students read the background information explaining how DNA fingerprints are made using restriction enzymes, and PCR. I have students pour the agarose gels, though I make the gel mixture ahead of time. They will then melt it with hot plate and set up their molds with the comb wells. The molds are then left to solidify and covered with saran wrap. (Note, I have left these in the refrigerator for days and they still work!)
Day 2 – Students use micropipettes to load the samples into the gels. They may have used these tools previously, but I remind them about the first and second stops. I also give them the tip on bracing elbows on the table to load the gels. This part can be tricky! Students then hook up the battery. The gels will take about 15 minutes to run. I keep a photo of previously run gels for students that may have had difficulty loading the gels.
