Modeling Replication (Part 1/2)

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Objective

Students will evaluate data to determine which model best explains how DNA replicates.

Big Idea

Recreate Meselson and Stahl's experiment through this simulation to determine how DNA copies itself.

What Students Will Learn in this Lesson

1 minutes

Students will look at the two of the three proposed theories of DNA replication. They will use a hands-on simulation in which they collect data and consider two of the three theories. This is a two day lesson. Day two is found at this link. Here is an overview of what students will learn today. 

 

Hook/Check for Understanding

3 minutes

Introduce students to the three competing theories for DNA replication by having them read brief summaries of the dispersive, conservative, and semiconservative models. Explain to students that almost immediately following their publication proposing a structure for DNA, Watson and Crick published another paper explaining one possible model for the replication of the DNA molecule. The following year, Max Delbruck suggested another model that would take care of the problem of supercoiling. Two years later, a third theory was proposed by David P. Bloch. It suggested that the entire DNA strand was copied to make an identical completely new copy. 

Using highlighters, students should highlight the main points of each theory and make a sketch of the resulting DNA strands.  

Teacher Mini-Lecture: Meselson-Stahl Experiment Set-up

5 minutes

Using the attached powerpoint, briefly explain the experimental procedure that Meselson and Stahl used to determine which of the three models proposed was correct. Then explain how the game will be played in order to collect data to evaluate which model is correct.

Materials needed for this game (for each lab group)

  • student handout
  • 200 white N-14 nucleotides 
  • 25 blue N-15 nucleotides
  • clear tape
  • glue dots, optional

(Note: Nucleotides can be made using this game template. I laminate the nucleotides so that they can be reused from year to year. I assemble bags of white N-14 nucleotides and bags of blue N-15 nucleotides before class in resealable bags. For this simulation to work like the Meselson-Stahl experiment, it is important to have ten times the amount of white nucleotides as compared to blue nucleotides. To help students start correctly, I typically guide students through the initial construction of their DNA template. It gives us a chance to review the parts of the DNA molecule.)

Student Activity: The Meselson-Stahl Experiment (Conservative Model?)

12 minutes

Materials needed for this game (for each lab group)

  • student handout
  • 200 white N-14 nucleotides 
  • 25 blue N-15 nucleotides
  • clear tape
  • glue dots, optional

 

Generation Zero

Using the directions on the student handout, students should make a 10 nucleotide DNA template. Students should label this template generation zero. They should take a picture of their complete DNA strand for later reference.

Generation One

For generation one, they should be given 20 N-15 nucleotides (each containing a nitrogen base, deoxyribose, and phosphate group).  Students are to make an exact copy of the DNA template with the new materials. They should take a picture of their completed DNA strands.

Generation Two

For generation two, students should be given 200 N-14 nucleotides (each containing a nitrogen base, deoxyribose, and phosphate group). Students are to make two exact copies of the DNA templates with the new materials.  They should take a picture of their completed DNA strands.

Generations Three through Seven

Students should repeat the process used in the last step for four more generations. They should take a picture at the end of each generation.

Students should sketch their DNA templates in their lab notebooks or take a picture of them with their computers. (Note: Students will need to make a copy of those pictures and add them to their lab notebooks. They should label all the parts of the images.)

 

Determining the Density Gradient

5 minutes

Bring the class back together. Using the class powerpoint, explain to student how to graph their data using a large test tube with the density gradient labeled on it. Students should draw a horizontal line to represent their DNA strand at the proper location in the test tube. The proper location is based on the percentage of heavy versus light nitrogen. Students determine this amount by counting the total number of nitrogen and calculating the percentage of heavy or light.  

Once students have graphed their group's data using the directions for their student handout, bring the class back together. Draw a large test tube on the board, have student groups report their findings from generation 1-3. Ask for student volunteers from each group to draw where their DNA strands will fall on the density gradient. Make a new test tube graph for each generation.

(Note: I have my students save their models and have them explain their data to their classmates.  In these videos, two student groups explain why their models were not 100% blue or 100% white in the conservative models. One student group explain their data in this video. Another student group explains their data in this video.)

 

Student Activity: The Meselson-Stahl Experiment (The Semi-Conservative Model?)

15 minutes

Refer to the student handout for more detailed directions.  

Generation 1: 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 2: Student groups should receive a container of DNA nucleotides made with nitrogen-15. They should add the N15 nucleotides to the pile of N14 nucleotides. Students should separate the DNA template into two strands. 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 N15 are not really colored. They do have heavier mass though.) 

Generation 3: Students groups should receive a large container of 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 Medeleson and Stahl, one needs to use ten times the amount of the N15 DNA nucleotides.) 

Determining the Density Gradient

5 minutes

Again, bring the class back together. Using the class powerpoint, explain to student how to graph their data using a large test tube with the density gradient labeled on it. Students should draw a horizontal line to represent their DNA strand at the proper location in the test tube. The proper location is based on the percentage of heavy versus light nitrogen. Students determine this amount by counting the total number of nitrogen and calculating the percentage of heavy or light.  

Once students have graphed their group's data using the directions for their student handout, bring the class back together. Draw a large test tube on the board, have student groups report their findings from generation 1-3. Ask for student volunteers from each group to draw where their DNA strands will fall on the density gradient as they did in the conservative round of the game. Make a new test tube graph for each generation.

Putting It All Together

7 minutes

Have students summarize the class results from generations 1-7 by drawing a pictures on their student handout or lab notebook for both scenarios that we did today. Then have students predict the results for generation 8 and 9 for both scenarios. Have students put their answers in their lab notebook or student handout.