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NOVA scienceNOW: RNAi

Viewing Ideas

Before Watching

1. If your class is studying the structure and function of animal cells, have students make a cell model. (See Table 1) Divide the class into teams and give each one the Before Watching student sheet. Have teams do Question 1, identifying the function of each cell component and brainstorming common materials that could represent them in a cell model. As a class, review the sheet. Provide the class with an assortment of materials and have teams construct and display their cell models. (Make sure students understand that nuclear DNA remains in the nucleus and that RNA and proteins are found in both the nucleus and the cytoplasm.)

   Table 1: Answers to the Making an Animal Cell Model Student Sheet

   Cell Organelles and Components	Function	Common Materials For Model
   Cell membrane	Double layer of lipid surrounding the cell. Controls the transport of materials into and out of the cell.	Gallon-sized zip-lock bag
   Endoplasmic reticulum	Important in the synthesis of lipids and proteins.	Narrow foam strips with circles of foam (i.e., ribosomes) glued on
   Vacuoles	Membrane-bound sacs that play a role in digestion of waste products.	Bubble-wrap
   Nucleus	Contains the nuclear DNA.	Small zip-lock bag
   DNA	Double-stranded. The cell's genetic material.	Length of twisted string
   Messenger RNA	A copy of the information carried by a gene on DNA. A template for single-stranded protein.	Short length of string
   Mitochondria	Provide energy for the cell.	Packing peanuts
   Golgi apparatus	Important in transport of large molecules.	Pipe cleaners
   Ribosomes	Translates messenger RNA into protein.	Marbles
   Cytoplasm	Contains the organelles. Primarily composed of water.	Water or gelatin

2. Analogies help students understand abstract concepts. To review protein synthesis, ask the class to read the music analogy on the Before Watching student sheet and fill in the table. Use the following version of the paragraph to review each step with the class. The italicized words indicate the part or process being modeled.

   A sheet of music (the DNA) is in a closed office (the nucleus). A person in the room copies the music (transcribes the DNA to make messenger RNA), passes through a door (nuclear membrane), and delivers the copy to a musician (the ribosome). The musician reads the copy of the sheet music and plays the music (translates the messenger RNA to make the protein).

   Answer Key to Question 2 on the Before Watching student sheet

   Steps in the process of synthesizing a protein	Making Music analogy
   DNA	Sheet music
   The nucleus	The closed office
   Transcribing DNA to make messenger RNA	Person copying the sheet music
   Messenger RNA	Copied sheet music
   Messenger RNA exits nucleus to cytoplasm	The door
   Ribosome	Musician
   Messenger RNA reaches ribosome in cytoplasm	Person brings copied sheet music to musician
   Protein made	Musician produces the music

   Have students make their own analogies for protein synthesis. Have them choose a topic (e.g., making a sailboat, ice cream sundae, pizza, jewelry, or a birdfeeder or dog house) and create a chart similar to the one on their sheet. Ask teams to act out their models for the class. Since analogies are imperfect, ask students to comment on which parts of their analogy closely match steps in the protein synthesis process and which parts do not. (For example, the music analogy is imperfect because the sheet music exists as a single copy, while DNA is actually double-stranded. In addition, the single person copying and delivering the messenger RNA represents many different synthesizing and transport enzymes and proteins.)

After Watching

1. It was an accidental finding that set in motion the chain of research that led to the discovery of RNAi. Geneticists attempting to develop an especially deep purple-colored petunia got unexpected results—white flowers. A decade of experimentation in different laboratories led to discoveries that enabled scientists to identify RNAi as responsible for the loss of purple pigment. Had geneticists dismissed the experiment that produced the white petunias, the discovery of RNAi might have been delayed.

   Write the Joseph Henry quote (see below) on the board. Ask students to consider its meaning by listing personal qualities that contribute to a "mind well prepared." (Answers may include being persistent, patient, open-minded, imaginative, and knowledgeable.) Next, have them share personal stories of discovery, invention, success, or creativity and identify what enabled them to be "well prepared."

   "The seeds of great discoveries are constantly floating around us, but they only take root in minds well prepared to receive them." Joseph Henry, American Physicist.

   As an extension, consider having students read about the accidental discoveries of X-rays and penicillin (pbs.org/wgbh/nova/cancer/discoveries.html). As a class, discuss how these discoveries are similar to the discovery of RNAi. (Dr. Fleming's contaminated flu culture led to the discovery of penicillin. If Fleming had been less prepared—less observant, patient, knowledgeable, and open-minded—he might have discarded the dish contaminated with mold. Röntgen's experimental objective had nothing to do with X-rays—he wanted to see if cathode rays could escape from a tube covered with black cardboard. However, he noticed that a screen over a yard away glowed unexpectedly. Rather than dismiss this occurrence, he explored it and ultimately discovered X-rays.)

2. One of the lead scientists of the Human Genome Project expressed his excitement about the discovery of RNAi. He believes it may help scientists learn about the function of individual genes by being able to "turn them off" one at a time. Discuss the following questions:

   • How can turning individual genes off help scientists better understand normal growth and development? (Scientists can see what process or characteristic does or does not occur when a gene is turned off.)

   • Some diseases involve an overproduction of proteins or the production of defective proteins. Have students brainstorm how RNAi might be used to control the overproduction of proteins or eliminate the production of defective proteins, thereby helping cure or control such diseases. (Researchers can put a double-stranded RNA form of the undesirable gene in a cell, triggering an RNAi response and preventing the translation of the damaged or over-produced protein.)

   • Which genetic diseases might not be able to be helped by RNAi? (Diseases caused by a missing gene or an inactive gene.)

3. Have students role play how RNAi interferes with protein production in an activity that steps them through two analogies. In the first analogy, a chef makes bread from a recipe copied from a master recipe. In the second analogy, a second, double-message version of the bread recipe is introduced. This difference is enough to trigger an RNAi response, which keeps both recipes from getting to the chef. This analogy shows how RNAi silences a gene; thus, the protein that the DNA coded for will not get made once it is targeted by RNAi. The classroom activity, "Acting Out Your Cell Biology", includes complete teacher notes and a reproducible student sheet with an assessment.

Web Sites

Animal Cell Organelles
www.cellsalive.com/cells/animcell.htm
Provides image and description of an animal cell and its components.

Molecular Expressions - Animal Cell Structure
micro.magnet.fsu.edu/cells/animalcell.html
Describes organelles and has detailed description of an animal cell.

RNA interference - Nature Reviews
www.nature.com/focus/rnai/animations/animation/animation.htm
Animates the action of RNAi.

Books

The Human Genome by Jeremy Cherfas. Dorling Kindersley, 2002.
Explains DNA inheritance and genetics.

Essential Cell Biology—textbook by Bruce Alberts, Dennis Bray, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. Garland Publishing, Inc., 1998.
Reviews proteins, DNA, protein synthesis, genetics, and other cell biology topics at the high school and college levels.