Cristina R. David
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Who Stole Mrs. Pontius's Gel Pen Lesson

IV. Materials:

• DNA fingerprinting activity packet, which includes:

  • Description of crime

  • Instructions for activity

  • DNA sequences of suspects

  • Gel electrophoresis chart

  • Analysis questions

• Scissors

• Glue stick

• Pen or Pencil

V. Anticipatory Set:

Introduce the activity by asking students if anyone is interested in a career in forensic science.  Ask students if they know what is involved in forensic science.  If students are struggling with these questions, ask students if they have ever seen any crime investigation shows such as “CSI”.  Ask them what the forensic scientists do on these shows.  What type of technologies do they use?  Students may give answers such as the analysis of fingerprints, blood, hair samples, footprints, insects, bullets, etc.  Explain that even a small drop of blood, one strand of hair, or a broken fingernail at a crime scene can be used to help convict a criminal.  The reason is because that these things contain DNA that is unique to every person (except identical twins).  Forensic scientists can use a technology called DNA fingerprinting to analyze the DNA contained in the blood, hair, fingernail etc. and compare it to the DNA of a suspect. 

VI. Modeling:

1.  First, quickly review the process of gel electrophoresis. 

• Explain how restriction enzymes are used to cut up the very long DNA strand into smaller segments.  The restriction enzymes are very precise and cut the DNA at a very specific place in the DNA sequence. 

• The DNA fragments are placed in wells at one end of the gel and an electric voltage is applied

• Because DNA molecules are negatively charged, they move towards the positive end of the gel.  The smaller fragments move farther than the larger fragments

2.  Explain how that the DNA of each individual is unique (except identical twins).  Remind students that DNA contains genes which code for certain traits.  However, DNA also has segments that do not code for anything.  These segments are used by forensic scientists because they are different in every person.  These segments are cut up with restriction enzymes and separated using a gel electrophoresis

3.  Explain that the DNA contained in a very small sample of body fluid or body tissue found at a crime scene may be too small to analyze, so forensic scientists can use the technique that they learned previously called polymerase chain reaction (PCR).  PCR can multiply the amount of DNA in the small sample so that there is enough to analyze in the gel electrophoresis.

4.  Introduce the DNA fingerprinting activity.  Explain the crime that has been committed and the DNA evidence that was left at the scene.  Assign students to play the roles of each of the suspects.  Have those students read their “description of suspicion”.

5.  Have students look at the DNA sequences of the crime scene DNA and the DNA of each of the suspects.  Tell them that they will be simulating the work of restriction enzymes with scissors.  The restriction enzyme will cut up the DNA sequences at very specific spots.  Show students one location where they will be cutting – they should mark the cutting locations with a pen or pencil before cutting with scissors.

6. Have students look at the chart.  Explain that this represents a gel electrophoresis.  They will be pasting the DNA segments that they cut up according to the size of the fragment (just like in a gel electrophoresis).  Demonstrate how to locate the correct placement for a DNA fragment

7.  Finally, explain that when they have pasted all of their DNA fragments, they will be to identify the guilty suspect by comparing the DNA fragment patterns of all the suspects to that of the crime scene DNA

VII. Guided Practice:

Have students begin the activity by marking the locations in the DNA sequence where the restriction enzyme will be cutting.  Monitor students in this process and make corrections as needed.  Once all locations have been marked, students can then cut the sequences, mark the number of base pairs on the back, then paste them onto the chart. 

VIII. Independent Practice:

Once students have completed their chart, they will be able to answer the eight analysis questions for homework.  These questions will help them review and understand the process of DNA fingerprinting.  It will also help them realize that each of the steps in the activity represented a real step in DNA fingerprinting. 

IX. Assessment:

Attainment of the objectives for this activity will be assessed using three components:

1. The ability of student to complete the activity in class – determine where to cut the DNA sequences and how to place the sequences on the gel electrophoresis chart.

2. A completed gel electrophoresis chart with correctly placed DNA fragments, labeling of positive/negative ends, and the identification of the guilty suspect.

3. Eight analysis questions with completed answers

X. Closure:

Explain to students that we have been learning all about DNA, how it contains genes that code for our traits.  We have also been learning about certain types technologies that allow scientists to alter these traits – this is called genetic engineering.  We just learned about one technology today called DNA fingerprinting that allows scientists to identify an individual.  We also learned about the technologies of gel electrophoresis, restriction enzymes, and PCR and how they are used in DNA fingerprinting.  These technologies also have many other uses in DNA fingerprinting.  We will be learning about some of these other uses AND even more DNA technologies that are used in the field of genetic engineering.