In this lesson, students will investigate a fictional anthrax case using paper to model the Sanger method of DNA sequencing. After receiving an envelope containing strips of paper that represent the incomplete copies of anthrax DNA from a suspected lab, students will sort the strips to determine the DNA sequence from that lab. Then, as a class, students will compare their sequences to the one found in the fictional anthrax-laced letter to determine which lab is guilty.
Students will be able to:
- Define the term DNA sequencing
- Describe the steps of one well-known DNA sequencing method (the Sanger method)
- Demonstrate how scientists use DNA sequencing to make distinctions between different strains of anthrax
- Discuss the role mutations play in the identification of DNA samples from different labs
Grade Level: 9-12
- One class period
- NOVA scienceNOW: Anthrax Investigation Video (14:10)
- How DNA Replicates QuickTime Video
- The Nuts and Bolts of DNA Replication QuickTime Video
- HHMI: Sanger Method of DNA Sequencing Video
- Anthrax Investigation Student Handout PDF Document
- Anthrax Investigation Teacher Blackline Master PDF Document
Before the Lesson
- Provide computer access for each group.
- Make one copy of the anthrax DNA fragments from the guilty lab available in the Teacher Blackline Master. Cut it into strips and place them in an envelope.
- Make enough copies of anthrax DNA fragments of the innocent labs, also available in the Teacher Blackline Master, for the remaining pairs of students. Cut each copy into strips, and place a complete set of strips in an envelope for each pair of students.
- Print out one Student Handout per student.
- Read the Teacher Notes—Deadly Letters: The Anthrax Mystery PDF Document for background information about the methods used in the lesson and to see a list of glossary terms that you can share with your students.
1. As a class, begin by watching the NOVA scienceNOW: Anthrax Investigation Video. Then, discuss students' questions about the segment.
2. Explain to students that they will be conducting their own investigation of a fictional anthrax case. Tell them that the Bacillus anthracis from different labs may have slightly different DNA, and this feature could be useful in the investigation. Ask: What would have caused the DNA differences, from lab to lab, in the first place?
3. To refresh students' understanding of DNA replication, show them the following animations (in the order suggested). Students may benefit from seeing the animations more than once.
As a class, review what mutations are and how they can cause differences among populations of bacteria in different labs. Relate the information in the animations to the NOVA scienceNOW: Anthrax Investigation video segment. Emphasize that mutations arise over time in a population, and they are the source of variation among the lab populations. This variation ultimately enables investigators to compare the anthrax DNA from different labs and find a match between the DNA found in the letter and the DNA from one of the labs.
4. Tell students that, to find a match, investigators first had to establish the DNA sequence of the Bacillus anthracis in the letter and also the sequences from the suspected labs. This activity employs a well-known type of DNA sequencing called the Sanger method. Show students the HHMI: Sanger Method of DNA Sequencing Video. If necessary, you can show the video multiple times or let students go through it individually or in small groups.
As a class, summarize the activity, and then relate the information in the activity to the Anthrax Investigation video segment. Help students define DNA sequencing as a technology used to determine the order of nucleotides in DNA.
5. Divide the class into pairs. Give each student the Anthrax Investigation Student Handout PDF Document, and distribute the envelopes containing DNA samples (one envelope per pair). Explain that the envelopes contain "anthrax" DNA samples from suspected labs, and that the samples have already undergone the first part of the Sanger sequencing process. Envelope contents represent many incomplete copies of the DNA strand, each one of a varying length and each one ending on the left with a "fluorescent" A, T, G, or C. Students will need to sort the copies by length and determine the sequence. For example, the sequence would be TCGG for strips aligned like this:
6. When student pairs have determined their anthrax DNA sequence, check their answers on their handout.
7. Have groups write their anthrax DNA sequences on the board. Make a chart to organize their responses. For example:
8. Ask the class to review the sequences and see if they can spot any differences.
9. On the board, write the anthrax DNA sequence found with the fictional "guilty" letter: TGACAATCAG. Ask if any group's sequence matches the one found in the "guilty" envelope. What is the significance of the match?
10. Ask students how the activity compares to the real investigation shown in the Anthrax program. Point out how important it is to have samples from every lab. Otherwise, the guilty lab might be missed.
11. Optional Extension Activities:
- Have students research other types of DNA sequencing (such as Shotgun sequencing or High Throughput sequencing) and report their findings to the class.
- As a class, discuss differences between human genome sequencing and bacterial sequencing.
- Show students the DNA sequence from the Bacillus anthracis AMES strain, described in the video segment available at the National Center for Biotechnology Information, a free resource with genetic information about many different organisms. Mention that scientists share DNA information to promote research and understanding.
Remind students that genes are segments of DNA that are transcribed to mRNA and then translated into amino acids, the building blocks of proteins. Ask students to imagine that the DNA from this activity represents a small section of a gene. Ask them to transcribe and translate the DNA sequence. A table of the genetic code, usually found in biology textbooks, is required for this extension.
Check for Understanding
As a class, ask students to discuss how they were able to solve the anthrax mystery.