7. DNA Fingerprinting - Lesson Plan
Day 1+ - Investigating the crime scene
If you set up a crime scene, make lots of observations of the crime scene. Start with all the kids outside the crime scene area, drawing pictures and writing down the things they notice. Finally, allow one student at a time enter the crime scene area wearing gloves to collect evidence. Evidence should be kept in plastic bags. Analyze any non-DNA evidence first. Dust for fingerprints. Collect hair and fiber samples. Perform paper chromatography on the ransom note and compare it against the pens in the possession of the various suspects. (See Sources section below for resources and lesson plans describing how to conduct these tests.)
Day 2 – Creating the DNA sequences and replicating DNA
- Each person should lay a meter stick down the middle of their strip of adding tape with the centimeter markings facing upward.
- Create a strand of DNA on the adding tape by writing a letter for a nucleic acid (A, T, C, or G) every centimeter along the top of the adding tape and writing the matching base below the meter stick. Remind students that they are creating their own individual DNA sequence and every person’s DNA is different, therefore they should independently make up their DNA sequence and not copy another student’s sequence.
- The first step of DNA replication uses scissors to represent the enzyme helicase. Helicase unzips the DNA down the middle of the DNA ladder, thus each student should cut their adding tape DNA down the middle between the bases, making sure there is white space on the paper both above and below the cut.
- Next, DNA polymerase finds the matching base and creates a new strand alongside the old strands. Your pencil is DNA polymerase and should follow along the half strands, filling in the matching bases. If you want, use a different colored pencil to create the new strand, that way you can tell the difference between the newly assembled strand and the original strand. Similarly, if you want, instruct students to fill in the matching bases continuously from left to right on the top “leading” strand but to fill in the matching bases on the bottom “lagging” strand 20 bases at a time from right to left. (Start at base #20, then match #19, #18, and so on until you hit #1. Then start at base #40, then match #39, #38, and so on until you hit #21.)
- Each person should end up with two exact copies of their DNA.
Day 3 – Running the “gel” and analyzing DNA fingerprint results
- Give students a quick overview of how DNA fingerprinting works:
- collect the DNA sample
- cut the sample with restriction enzymes
- sort the pieces by length using gel electrophoresis
- Collect one copy of each students’ adding tape DNA. These may be pinned to a bulletin board and will form the class “DNA library”. Students should keep the other copy at their desks. Students can be told that you are “extracting a DNA sample” from each student to compare against the DNA of the culprit and other suspects. The “extraction process” used in this case results in paper strips of DNA rather than real DNA like the strawberry DNA extraction.Cutting DNA at sequence -AT-
- Briefly describe what restriction enzymes are and how they work. The DNA sample the students kept will be cut using a restriction enzyme known at “AT”. Instruct students to read through the top row of their DNA. Any time they see the sequence AT (the complementary sequence below will read TA), they should cut the DNA in between the A and the T. On average, each student will make between 6 and 7 cuts although, depending on the sequence they created, some may make no cuts and some may make 20 or more.
- Next, students should measure the length of each DNA segment in centimeters and write this information on the back of each segment. Since each base pair was written 1 centimeter apart, the length of the DNA segment equals the number of base pairs on that segment.
- Line students up on the starting line of the gels. Explain how smaller segments will move farther. The rule is that each piece moves 100 inches minus the length of that piece. Thus a DNA segment only 1 base pair long can travel 99 inches, 2 base pairs travel 98 inches, and so on. An uncut DNA sample will not move at all and should stay on the staring line.
- Give students a few minutes to separate and sort their DNA pieces. When a given student is done, make sure they stand outside of the gel. It is easy to disturb DNA segments simply by walking past them.
- When all students have finished, stand back and look at the patterns made by the DNA segments. Some questions to ask include:
- How many “bands” does each person have?
- How far did the different segments move?
- Did anyone have exactly the same pattern of bands as someone else? Why or why not?
- Could this method be used to match a suspects DNA to a DNA sample taken from a crime scene? How?
- How reliable is this method? Is it possible to have matching patterns but a different DNA sequence? Is it likely?
- Should someone be convicted solely on DNA evidence? Should someone be released if DNA evidence shows they do not match the sample taken from the crime scene?
- Another direction to take this discussion is the ethics of maintaining a DNA library. DNA libraries generally only store information on STRs or restriction fragment length polymorphisms, not a person’s full genetic code. It is exceedingly useful in identifying offenders in situations where a DNA sample is collected from the scene of a crime. On the other hand, the existence of such a library may violate privacy rights. You could ask:
- Now that you know how DNA fingerprinting works, how do you feel about the way I collected and saved a copy of your DNA in a DNA library?
- Could a DNA library be useful in solving crimes?
- How could a DNA library be abused or misused by the police or government?
- Should governments maintain a DNA library? Why or why not?