She Doesn't See It
Lesson 2 of 7
Objective: Students will construct a model of DNA to understand its basic structure.
In today's lesson, students will use Franklin and Wilkins' data to build a more accurate model of data. First, they will view clips from the movie The Race for the Double Helix to better understand how the model of DNA was developed and why model building is important in science. Students will again evaluate their model and two of their peer models. Finally, they will write a business letter to the K'Nex company making recommendations for a more accurate model of DNA. This is a day two of a three day lesson. You can find day one at this link and day three at this link. Here is an overview of what students will learn today.
Check for Understanding
Yesterday, students built a model of DNA based on their previous knowledge. Students will calculate now the percent error of their model using the student data sheet. Have students share what their percent error was. Students should then make their recommendations for their next model. They should explain where their model was too big or too small. They should also brainstorm how they might make the model the correct size.
Using the following video clips from Life Story, students will learn about the contribution of Wilkins, Franklin, and Gosling in determining the structure of DNA. (Note: In America, it can also be found under the name, The Race for the Double Helix.) (Part 1 can be found here and part 2 can be found here.)
Getting Under Way: 14:09-15:06
Everyone Dreams of Finding Buried Treasure: 34:08-35:40
A Report on Progress: 37:56-39:00
Students will complete a graphic organizer on the student handout that summarizes their findings about the structure of DNA.
For a shorter lesson, you may choose to watch Watson's inspirational TED talk instead.
Using the attached powerpoint, briefly explain how x-ray crystallography work and how Franklin and others read an image generated using x-ray crystallography. Make sure to point out the following to students during the lecture:
- X-ray crystallography is a standard method that is used to determine the structure of many biomolecules and viruses.
- To fully understand how it works one needs to understand diffraction and the different types of interference.
- Scientists reverse engineer the image to determine the 3-D structure of the molecule. This is because certain shapes will always give the same diffraction pattern.
- Scientists use Bragg's model of diffraction (2d sinθ=nλ) to make these assumptions.
- To further confirm the structure of the molecule, scientists apply other mathematical computations (which is why Franklin performed a Patterson function).
(Note: I keep this lecture brief and simple as much of it is based in mathematical and physics theory. I just want my students to understand that physics can be applied to better understand biology and chemistry. I also want them to appreciate why they need to understand complex math and physics.)
Students will be given X-ray crystallographs of both the A and B form of DNA and the famous photo 51. Using the directions of the student handout, they will read them and determine the pitch of both the forms. They will also determine the 3-D structure. Students will then compare their original model with the images on the student handout. They will recalculate their experimental error and explain which form of DNA their model best represents.
Putting It All Together
Students will discuss the benefits of the two methods in determining the structure of a macromolecule. In their lab notebooks, students will write a modified minute paper explaining why both methods are useful and important to determining the accurate structure of many biomolecules like DNA.