Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
DNA
The Secret of Life Playing God Human Race Curing Cancer Pandora's Box Timeline 3D DNA Explorer
For Teachers

Overview
Me First!
Sequencing DNA
Shedding Light on X-Ray Crystallography
Double helix

Tools
Glossary
E-Mail this page

Shedding Light on X-ray Crystallography
(high school science)
by Viki Babcock


Introduction:

X-ray crystallography images created by Rosalind Franklin provided crucial information to the development of an accurate model of the DNA structure. This lesson includes a teacher demonstration which 'illuminates' and clarifies how crystallography works, using some ordinary objects and an overhead projector.


Estimated class time:

Two 50-minute class periods, or one 90-minute block


Lesson Objectives:

Students will:
  • Identify laboratory techniques used in the development of a model of DNA
  • Observe images and deduce the shapes of objects from the images produced
  • Relate light-produced images to the technique of X-ray crystallography



Correlation to National Science Standards from http://bob.nap.edu/html/nses/html/6e.html#csc912

CONTENT STANDARD C: As a result of their activities in grades 9-12, all students should develop understanding of the molecular basis of heredity:

  • In all organisms, the instructions for specifying the characteristics of the organism are carried in DNA, a large polymer formed from subunits of four kinds (A, G, C, and T).
CONTENT STANDARD E: As a result of activities in grades 9-12, all students should develop understandings about science and technology:
  • Science often advances with the introduction of new technologies.



Materials needed:

  • TV/VCR
  • Copy of DNA, "Episode 1: The Secret of Life"
  • Overhead projector
  • Three-sided cardboard screen large enough to hide objects on projector
  • Common laboratory objects such as a graduated cylinder, culture dish, water bottle, funnel, etc.
  • Ball and stick molecular model kit
  • Internet Access



Teaching Strategy:

  • Ask the class to brainstorm ideas about how scientists find out information about things that cannot be seen directly. Accept all reasonable suggestions, compiling a list on the chalkboard or overhead transparency.

  • Introduce the video as an insight to the techniques scientists used to discover the structure of DNA, which could not be observed directly. Show the video, instructing students to take notes on the laboratory procedures discussed in the film.

  • After the video, ask students:

    1. What technique did Rosalind Franklin and Maurice Wilkins use?

    2. What did the image that Franklin produce look like?

    3. How were Watson and Crick able to use the information from Franklin's image?

  • To illuminate the process of X-ray crystallography, perform this demonstration. Set up the cardboard screen (a three-sided screen which blocks students' view of any object on the overhead projector, while allowing the image to be projected on the wall screen) and place a well-known object, such as a graduated cylinder, on the projector. With the room darkened, turn on the projector and have students write down their guesses about the shape of the object on the projector. Before identifying what the item is, turn off the projector, reposition the item on its side and turn on the projector again. Allow the students to modify their guesses and discuss their ideas with a partner before revealing the item. Repeat this procedure with several other items such as the culture dish, funnel, and wash bottle. Discuss the clues about the shapes of the objects that show up in the images produced by the projector.

  • Now challenge students to determine the shape of a molecule. Without letting the students see it, construct a simple model of water (H2O) or methane (CH4) out of the ball and stick molecular model kit. Place it on the projector and have students draw their guesses about the shape of the molecule. Reposition the model with a 90-degree turn and allow students to see the new image, if desired. Repeat with two more complex models.

    (In general, the closer "atoms" are to the stage of the projector, the more distinct their shadows. Students quickly pick up the idea that "fuzzier" means greater depth (or distance from the projector stage). In the methane molecule, there are three planes in the molecule. Three hydrogen atoms are in one plane, the carbon atom in another plane, and the fourth hydrogen atom in the third plane. Careful observation will allow students to correlate planar distance from the projector stage and sharpness of shadow image. The degree of sharpness depends to some extent whether you place the model on the center of the stage or nearer the edge. Additional "depth" can be observed with the models placed closer to the edge than the center. You will need to experiment beforehand with the relative positions of the model, projector, and screen. Reference: http://www.chemheritage.org/EducationalServices/
    pharm/tg/antibiot/activity/xray.htm
    )

  • For more in-depth discussion of how X-ray crystallography aided in determining the structure of DNA, direct students to NOVA: THE SECRET OF PHOTO 51 at http://www.pbs.org/wgbh/nova/photo51/about.html. Instruct them to explore the link entitled "Anatomy of Photo 51." Have students write a summary of what they learned from the site and how it relates to the teacher demonstration and attach it to their drawings from the classroom crystallography demonstration.



Assessments:

  • Participation in class discussion
  • Accuracy and participation in demonstrations
  • Illustrations and written summaries



Extensions:



back to top

For Teachers DNA Resources Feedback Support PBSSupport PBS Credits