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Modeling Prokaryotic Operons

Adapted from
AP® Biology Daily Lesson Plans
by Kristen Dotti

Gene regulation can be a difficult topic to understand because the control mechanisms are too small to visualize. It may be easier for your students to compare different modes of gene suppression and gene induction if they build models of these processes. Below is a lesson plan to create models of repressible and inducible operons used by bacteria for protein transcript regulation.

Approximate time for activity: 40 minutes

Objectives

  1. The learner will improve his or her understanding of gene regulation by making models of repressible and inducible operons.
  2. The learner will review the terms and processes of protein transcription.

Materials

Each lab group will need:

  • 2 Styrofoam® Water “Noodles” in Different Colors
  • 7 Different Colors of Electrical Tape (or 7 different colors of Sharpie® markers)
  • 1 Wire Coat Hanger
  • Wire Cutters
  • 2 Racquet/Tennis Balls
  • 6 Stick-On Velcro® Tabs

Procedure

  1. Lead the lab groups through the process of making models of a repressible operon and an inducible operon using the above supplies and the following sample diagrams of a prokaryotic tryptophan operon and a prokaryotic lactose operon.
  2. For the repressible operon, use the prokaryotic tryptophan operon as an example:


    1. Using a serrated knife, cut an 8-inch segment from the first noodle (the following steps all apply to this noodle). This segment will be used as the repressor protein.
    2. Each end of the noodle/operon should feature an unlabeled/untaped section to show the continuation of the DNA strand.
    3. Wrap spirals of colored electrical tape (or shade the noodle with colored Sharpies) where each of the 5 gene domain regions would be found (trp E, trp D, trp C, trp B , and trp A), using a different color for each gene domain.
    4. Tape or shade in the regulatory gene (trp R) region as far upstream of the promoter region as possible.
    5. Using a Sharpie®, draw the shape of the active form of the repressor protein onto the lower portion of the noodle/operon, in the operator region. (See e in the diagram on the previous page.) Make the shape simple, like the one in the diagram, since you will need to carve it out using a serrated knife. Also, carve a matching shape into the regulatory repressor protein piece that you cut off in step a above.
    6. On the bottom side of the repressor protein, carve a U and wedge the racquet/tennis ball into the U.
    7. Cut a piece of wire from a coat hanger and shove the wire into the repressor protein and bend the repressor protein into a shape so that it will not fit the operator region if the co-repressor (tryptophan) is not in place.
    8. Write the word “tryptophan” on one of the racquet/tennis balls. Write “repressor protein” on the carved foam piece. Now label the various parts of the noodle/operon using a Sharpie®: “trp R,” “promoter/operator,” “trp E,” “trp D,” “trp C,” “trp B,” and “trp A.”
    9. Place stick-on Velcro® tabs on the parts of the operator and the repressor protein that fit together, so that they can stick together without being held in place. You may do the same for the repressor and the co-repressor/tryptophan ball.

  3. For the inducible operon, use the prokaryotic lactose operon as an example:
    1. Using a serrated knife, cut an 8-inch segment from the second noodle (the following steps all apply to this noodle). This will be used as the repressor protein.
    2. Again, each end of the noodle/operon should feature an unlabeled/untaped section, to show the continuation of the DNA strand.
    3. Wrap spirals of colored electrical tape (or shade the noodle with colored Sharpies) where each of the 3 gene domain regions would be found (lac Z, lac Y, and lac A), using a different color for each gene domain.
    4. Tape or shade in the regulatory gene (lac I) region, which is immediately upstream of the promoter region.
    5. Using a Sharpie®, draw the shape of the active form of the repressor protein onto the lower portion of the noodle in the operator region. Make the shape simple, like the one in the diagram, since you will need to carve it out using a serrated knife. Also, carve a matching shape into the regulatory repressor protein piece that you cut off in step a above.
    6. On the bottom side of the repressor protein, carve a wide, semicircle shape that is a little too wide to accommodate the racquet/tennis ball. You want the repressor protein to have 2 shapes, one that fits the operator shape perfectly when the inducer is NOT present and one that distorts the repressor so that the carved top shape appears to pop out of the operator when the inducer fits into the bottom (you can shove a piece of coat hanger wire into the repressor to make it hold 2 different shapes).
    7. Write “allolactose” on one of the racquet/tennis balls. Write “repressor protein” on the carved foam piece. Write “lac I,” “promoter/operator,” “lac Z,” “lac Y,” and “lac A” at the appropriate places along the noodle.
    8. You may place stick-on Velcro® tabs on both the operator and repressor protein parts so that they can stick together without being held in place. You may do the same for the repressor and the co-repressor/allolactose ball.
  4. Use these models as props during class, when discussing the operon hypothesis. Have pairs of students use the props as they simulate and narrate the process of repressing or inducing an operon to regulate the genes. Make sure everyone has a chance to run through a simulation with each operon.

AP® Biology Daily Lesson Plans

Make the topics of AP® Biology accessible by adding more hands-on activities to your lessons. See our printed or online catalog for the AP® Biology Daily Lesson Plans (cat. 74-6691) curriculum that contains student-centered activities for every day of the school year.

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