Students will look at the three proposed theories of DNA replication. They will use a hands-on simulation in which they collect data and consider one of the three theories. Then they will compare it with the actual data collected by Meselson and Stahl. This is the second day of a two day lesson. The first day can be found at this link. Here is an overview of what students will learn today.
Ask students summarize their lab results from yesterday's simulation for the generations zero through three. Students should have found that each round of the game produced very different data.
Possible student answer: In the conservative model round, generation zero had one all white template. In generation one, there were two DNA strands. One was completely white and one was completely blue. In generation two, there were four DNA strands. One was completely blue and three were completely white. In generation three, there were eight DNA strands. One was completely blue and seven were completely white.
In the semi-conservative model round, generation zero had one all white template. In generation one, there were two DNA strands. Each strand was half white and half blue. In generation two, there were four DNA strands. Two were completely half white and blue and two were completely white. In generation three, there were eight DNA strands. Two were completely half white and half blue. Six were completely white.
Once students have responded to the writing prompt, give them some time to copy the images of three test tubes on the whiteboard. Then, have students reconstruct the class test tube density gradient that they made in the previous class period.
Refer to the student handout for more detailed directions.
(Note: Because of a long weekend, I reviewed the rules of the game with my students so that they would understand the first two models that they tested before we move to the last model. We also reviewed the entire structure of DNA because my students could not remember. Depending upon when the second lesson is taught, it may be useful to have a quick spiral review with students. Once students understand the first two models, then students can tackle the more difficult dispersive model by modeling possible results.)
Generation zero: Student groups should build a 10 nucleotide DNA strand on a large surface like a lab table. The arrangement of nitrogen bases can be of the students choosing. Once the DNA template is made students should either draw a picture of their template indicating what the nitrogen bases are or they can take a picture of the template with their phone or computer. (Note: students should have some N14 DNA nucleotides left. )
Generation one: Have students cut the DNA template into two equal sections of 5 nucleotides each. Have students swap the tops and bottoms of the strand. They should attach a sugar phosphate backbone. (See image.) Student groups should receive a container of 20 DNA nucleotides made with nitrogen-15. They should add the N15 nucleotides to the pile of N14 nucleotides. Then they should generate daughter strands. Once the new DNA strands are made students should either draw a picture of their template indicating what the nitrogen bases are or they can take a picture of the template with their phone or computer.
(Note: Students may have some N15 DNA nucleotides left. For the purposes of their lab, the N15 nucleotides are blue as that students can distinguish between them. Remind students that DNA nucleotides with N15are not really colored. They do have a heavier mass though.)
Generation two: Have students cut the DNA template into two equal sections of 5 nucleotides each. Have students swap the tops and bottoms of the strand. They should attach a sugar phosphate backbone. (See image.) Students groups should receive a large container of 250 DNA nucleotides made with nitrogen-14. They should separate the two DNA strands and make new daughter strands from the parent template. Once the new DNA strands are made students should either draw a picture of their template indicating what the nitrogen bases are or they can take a picture of the template with their phone or computer.
(Note: To match the actual experiment conducted by Meseleson and Stahl, one needs to use ten times the amount of the N15 DNA nucleotides.)
To save time and since students are already familiar with the method of recording data, students should record data on the class test tube while they playing the game.
Show students the results of the actual experiment. (Note: the images in the presentation are also included in the student handout along with some guiding questions.) Assist students in interpreting the data. Poll students to decide which models best represent the data collected by Meselson and Stahl. Then allow students time to record their findings on the student handout or in their lab notebooks.
(Note: When students are gone for time-sensitive labs, it is important that their lab partners record data for them. This is a common courtesy and as long as it is not abused, it can be a great help to students that are sick. The absent student must still do the data analysis and learn the rules of the game.)
Give students some time to organize their data. They have had to complete the round of the game quickly and may have not had time during the data collection portion of the lab. (Note: it is important to take the time to do this. Otherwise, students will have difficulty with the writing prompt they will complete next.)
Once students have organized their data, direct their attention to the white board, where drawings of three large test tubes have been placed. Encourage students to use those graphs to help them explain their writing prompts that can be found on the student handout. (Note: I noticed that my students did not immediately understand the differences between the cartoon graph of the cesium chloride density gradient and the change in density as a function of time graph on the student handout. This is a skill that I will need to address with next year's biology class.)
Have students explain in their lab notebooks which model they believe best represent Meselson and Stahl's data. Students should give several pieces of evidence to support their conclusion. Students should also explain why they might have some randomness in their lab results. (Note: This is a nature occurrence and high schoolers are developmentally able to begin to understand what causes "noise" in the data.)
(Note: Here are two examples of student work. While both students understand that the semi-conservative model is the more accurate model, they still need to work on supporting their argument with evidence. This student does a much better job in her explanation than this student.)
To help students better understand how quickly DNA replication occurs, show students the Dolan DNA lab video clip that models DNA replication in real-time.
Homework: Students will create a comic strip showing the steps in DNA Replication using the following rubric. This comic strip will be due in three days. Also, students will watch an interview with Matthew Meselson that explains why semi-conservative replication of DNA is the best model. Students can use evidence given by Dr. Meselson to support their claims for today's writing prompts.