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The trp Operon
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The trp Operon
Modeling Gene Regulation
Cells in the skin on your fingers have the same genetic information as the cells in your pancreas yet your
finger cells do not produce insulin. How do pancreatic cells “know” to produce insulin? The basic
answer to this question lies in the fact that genetic information is not simply inherited and automatically
used in every cell all the time. Each cell is using or expressing only a small portion of the total genes
that it contains. The finger cells are not expressing the gene for insulin production.
In 1961, Fracois Jacob and Jacques Monod described the control of gene expression as the operon
model. In the operon model, sets of genes operate together to result in the production of a specific
protein (enzymes) following the central dogma. As shown in Figure 1 below, the central dogma
indicates that the genetic information coded for in the DNA strand directs the production of proteins.
The DNA is transcribed into RNA which is then translated into specific protein products. Operons are
groups of genes operating together in prokaryotic cells to allow the formation of necessary proteins.
Fig. 1
The basic structure of an operon is shown in Figure 2. The operon contains an operator which is a
region of a chromosome or DNA to which the repressor binds when the operon is turned off and not
producing a protein product. The area of the operon called the promoter indicates to the enzyme, RNA
polymerase, where to bind to make mRNA during the process of transcription. The repressor is a
protein that suppresses the transcription of a gene.
promoter
operator
gene 1
gene 2
gene 3
Fig. 2
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The trp Operon
One of the first operons discovered in E.coli was the operon that results in the production of tryptophan.
The tryptophan operon or trp operon contains a promoter, an operator and five genes that result in the
production of the enzymes that cause tryptophan synthesis to occur. See Figure 3. Tryptophan is an
essential amino acid used in a variety of metabolic pathways within an E.coli cell. As such, tryptophan
is frequently needed by the E.coli cell so; it is not surprising to find that the trp operon is normally
activated in the E.coli cell. Operons that are typically activated, but can be turned off are referred to as
repressible operons. Alternatively, those that are normally off but can be turned on are called inducible
operons. The trp operon is a repressible operon.
Fig. 3
While operons are not present in eukaryotes, the expression of genes in eukaryotes works in a similar
fashion. Eukaryotic gene expression is controlled at multiple levels. Generally speaking, eukaryotic
gene expression is more complex because regulation can occur before and after transciption.
PURPOSE
In this activity you will prepare a model of an operon and use that model to explain how an operon
functions. Additionally, you will practice defining terms related to the regulation of gene expression.
MATERIALS
1 operon model set
2 sets of terms puzzle pieces
scissors
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PROCEDURE
1. Read through the introduction section of this activity and listen as your teacher demonstrates the
components of an operon.
2. Write a definition or description for each of the gene expression terms in the table on the student
answer page.
3. Obtain the components needed to prepare an operon model from your teacher.
4. Construct a model of an operon by doing the following:
a. Obtain a flexible neck straw and cut its length in half using scissors. Keep the portion with the
flexible neck.
b. Place your flexible neck straw on the surface of your desk or lab table with the flexible neck
portion on your left-hand side. Wrap a piece of masking tape around the straw in the area just to
the right of the flexible neck area. The flexible neck area of the straw will represent the
promoter area and the masking tape will represent the operator.
c. Use clear tape to secure the five segments of colored straw end to end onto the right-hand end of
the flexible neck straw. Smooth the tape down as much as possible. Each colored straw segment
represents a gene. Collectively, the structure formed when the pieces are taped to the longer
straw now represents the trp operon.
d. The RNA polymerase is represented by the segment of straw with the larger diameter that can fit
over the entire operon model. The work of RNA polymerase will be simulated by sliding this
larger diameter segment down the straw. Try sliding your RNA polymerase down the trp operon
model. Smooth down any tape ridges that block the sliding action. As RNA polymerase “reads”
the DNA code, a segment of messenger RNA will be formed. The messenger RNA is
represented in the model by the pieces of yarn.
e. The pony beads represent the ribosomes which are the cell organelles where the synthesis of the
enzymes would occur. Simulate the action of the ribosomes (represented by the pony beads) by
threading the yarn through the pony bead. Place the colored pieces of yarn below the matching
colored gene of the model. Each piece of yarn represents a different segment of mRNA which
codes for an enzyme needed in the production of the amino acid tryptophan.
f. Use the pipecleaner to represent the five enzymes produced by this operon. Coordinate the
colors as much as possible. Take the pipecleaner and fold it onto itself. This action represents
the folding of the enzyme into its native conformation. Place the folded enzyme below the
mRNA pieces in the model.
g. The repressor of the trp operon is represented by the paper clip. The repressor attaches to the
operator segment of the operon. When the repressor is in place the RNA polymerase is unable to
transcribe the DNA. To simulate this action in your model, attach the paperclip to the operator
segment of the straw. Notice that now the RNA polymerase is unable to move down the operon.
5. Cut apart the operon component labels and place them beside the appropriate part of the model.
When you have completed this portion, signal your teacher to come view your model and initial the
model completed box on your student answer page.
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The trp Operon
6. Label the diagram of the model on the student answer page using the following terms: gene 1, gene
2, gene 3, gene 4, gene 5, mRNA, operator, operon, repressor, RNA polymerase, enzyme products,
promoter and ribosome
7. Practice using the definitions of the components of the operon by completing the terms puzzle.
When you have your puzzle completed. Signal your teacher to initial the puzzle complete box on
your student answer page.
8. Answer the questions in the conclusion section of the student answer page
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Name _____________________________________
Period _____________________________________
The trp Operon
DATA AND OBSERVATIONS
Operon component
Definition/Description
Model component
operon
Straw with multi-colored
segments
Promoter region of operon
Flexible neck of straw
Operator region of operon
Masking tape strip
mRNA coded for by the
operon
Yarn
End product/enzyme coded for
by mRNA
Pipe cleaner
Repressor
Paper clip
Ribosome
Pony bead
Teacher Initials ________
Model completed
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Puzzle completed
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a._________
b.
c.
d.
e.
f.
g.
h.
i.______
k.______
j. ________.
l._________
CONCLUSION QUESTIONS
1. According to the central dogma, what is the relationship among DNA, RNA and proteins?
2. What is the difference in the role of an operator and the role of a promoter?
3. How are inducible and repressible operons different?
4. How are inducible and repressible operons similar?
5. Is the trp operon inducible or repressible?
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6. What would happen to the amount of tryptophan being produced in an E.coli cell when the repressor
is attached to the operator?
7. What would happen to the amount of tryptophan being produced in an E.coli cell if the RNA
polymerase was destroyed?
8. In your own words, explain how prokaryotic genes are expressed according to the Jacob and Monod
operon model.
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