Download The trp Operon - aandersonbiology

The trp Operon
The trp Operon
Modeling Gene Regulation
About this lesson
This lesson involves students creating a manipulative that serves as an introduction to the
concept of gene regulation. They students will construct a working model of the trp operon and
then use it to explain the process. This important lab will help reinforce and give real-world
applications to the concepts of DNA replication and protein synthesis.
This lesson is included in the LTF Module 10.
Objective
Students will:
 Prepare a model of an operon.
 Use the model to explain the basic components and functioning of an operon.
 Practice defining terms related to the regulation of genes.
Common Core State Standards for Science Content
LTF Science lessons will be aligned with the next generation of multi-state science standards that
are currently in development. These standards are said to be developed around the anchor
document, A Framework for K–12 Science Education, which was produced by the National
Research Council. Where applicable, the LTF Science lessons are also aligned to the Common
Core Standards for Mathematical Content as well as the Common Core Literacy Standards for
Science and Technical Subjects.
Code
Standard
(LITERACY)
RST.9-10.3
Follow precisely a multistep procedure when
carrying out experiments, taking measurements, or
performing technical tasks, attending to special cases
or exceptions defined in the text.
Level of
Thinking
Apply
Depth of
Knowledge
II
Connections to AP*
AP Biology: This lesson addresses concepts contained in Big Ideas 2 and 3 in the revised AP
Biology curriculum under the following sections: 2.C.1.a-b and 3.B.1.a-b.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
T E A C TH E E AR C HP EA RG E P S A G E S
Level
Biology
The trp Operon
Materials
Each lab group will need the following:
Additional teacher materials:
2 copies of operon terms and puzzle pieces
operon model set
paper clips, jumbo
pencils, colored, individual
scissors
tape, clear
bags, zipper-lock, quart
70 beads, pony
70 pipe cleaners, different colors
14 straws, flexible
14 straws, large diameter
5 rolls yarn
Assessments
The following types of formative assessments are embedded in this lesson:
 Visual assessment of students’ abilities to explain the process using the model
Teaching Suggestions










0.5-cm diameter flexible neck straw
1 3-cm long piece of 0.6-cm diameter straw
1 large paper clip
5 pieces of yarn approximately 6 cm long with colors to match the pipe cleaners
5 pieces of pipe cleaner with each pipe cleaner’s color matching the yarn (if possible)
5 pony beads (all the same color)
1 roll of clear tape
1 strip of gene labels
Colored pencils
Operon component labels
Alternatively, to save preparation time you may want to place all of the supplies in one central
location and have the students pick up their own components.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Students in the first year biology course need an introduction to the concept of an operon and
how an operon functions in very general terms. This modeling activity will add a concrete
dimension to the very abstract concept of gene regulation. By presenting the concept during the
first year course, the students will begin to formulate an understanding of the operon that will be
more fully developed in the AP Biology course. This lesson should be used after students have
been exposed to the processes of DNA replication and protein synthesis. You will need to
assemble an operon model for yourself to use during the presentation of this lesson (see student
procedure pages). Use a waterproof marker to label the operon components on your model.
Additionally, you will need to prepare the 14 student sets of the operon model by placing the
following in a re-sealable sandwich bag or other suitable container:
T E A C H E R
The following assessments are located on the LTF website:
 Short Lesson Assessment: The trp Operon
 DNA to Protein & Biotechnology Assessment
The trp Operon
Colored straws may be purchased at local discount stores. The exact diameter of the straw is not
important. What is important about the size is that one type of straw must be small enough to fit
inside the other type of straw. If pipe cleaners are unavailable, you can substitute colored
modeling dough, colored twist-ties, or colored craft pompoms.
Suggested Teaching Procedure
1. In an opening discussion, ask the students the following questions to introduce the topic of
operons:
 Does a cell in your finger contain the same genetic information as a cell in your pancreas?
(yes)
 Why do you suppose your finger cells are not producing insulin as do your pancreas cells?
(Genetic information is not simply inherited and automatically used in every cell all the
time. Each cell is only using or expressing a small portion of the genes that it contains.
The finger cells are not expressing the gene for insulin production.)
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
6. The ribbed area of the straw represents the promoter region, and the operator is represented
by the piece of masking tape on the straw. If a flexible straw is not available, use another
type or color of tape to illustrate the promoter region. Place a paper clip in the operator
region of your model to show the students how the repressor prevents RNA polymerase from
transcribing the genes.
7. Have the students complete the definition/description column of the table on the student
answer sheet using the information that you have presented along with the introduction
portion of the student handout.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
RNA polymerase
T E A C H E R
2. Explain that in this lesson they will explore Jacob-Monod’s model of gene expression, the
operon. Jacob and Monod devised this model in 1961 to explain how a particular set of genes
are expressed in prokaryotes. Although 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 at pre-transcriptional and post-transcriptional levels.
3. Explain/review the central dogma using the central dogma master to point out the
fundamental flow of genetic information into an expressed product.
4. Using your operon model and the gene terms master, describe the basic structural
components of an operon and the terms used to identify the components.
5. Use the sample student model to identify the parts of a typical operon and to demonstrate
how the trp operon works. Place the large-diameter piece of straw (RNA polymerase) over
the smaller straw (operon genes) to show how when the operon is on, the RNA polymerase
can move down the operon and transcribe the mRNA segments. Use the pieces of yarn to
represent the mRNA that is transcribed. You can place the model on the overhead to make it
easier for all students to view.
The trp Operon
8. Provide each pair of students with the operon model components and have the students
complete the modeling of the operon portion of the student answer page. The students are to
signal you when they have completely assembled their model and are ready to explain the
process so that you can initial their student answer page.
9. Give each student a set of the terms puzzle pieces. Explain to them that the purpose of the
puzzle is to give them a chance to practice using the definitions of the terms related to gene
expression. When the puzzle is completed, it will have a rectangular shape. The center piece
is the one with the DNA diagram on it. The blackline master to the term puzzle will serve as
your answer key. The students are to signal you when they have completed their puzzle so
that you can initial their student answer page.
10. Students should then answer the questions found on their student answer sheet.
T E A C H E R
Gene Regulation/Expression Terms
 Gene expression — Gene activity or transcription.
 Gene — A segment of DNA coding for a specific protein product.
 Operon — A group of genes operating together in prokaryotic cells.
 Operator — The region of a chromosome in an operon to which the repressor binds
when the operon is turned off.
 Promoter — The region of DNA that indicates to an enzyme (RNA polymerase) where
to bind to “promote” the synthesis of RNA.
 Repressor — A protein that suppresses the transcription of a gene.
 Inducible operon — An operon that is normally off but can turn on in the presence of a
specific metabolite.
 Repressible operon — An operon that is generally on but that can be turned off in the
presence of a specific metabolite.
P A G E S
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
Central Dogma
DNA
Transcription
Translation
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Protein
T E A C H E R
RNA
The trp Operon
Master for Gene Strip
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
TrpD
T E A C H E R
TrpE
The trp Operon
Master for Gene Strip
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
TrpE
TrpD
TrpC
TrpB
TrpA
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
TrpD
T E A C H E R
TrpE
The trp Operon
Repressible Operon
Regulatory site.
A protein that suppresses the
transcription of a gene
Repressor
Inducible Operon
Ex. Lac Operon
Operon
Genes that control the body plan
of animals by controlling the
development of a group of cells.
Gene Expression
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Operator
T E A C H E R
An operon that is
normally off, but can turn
on in the presence of a
specific metabolite
Sequence of DNA that
codes for a protein &
determines a trait
Segment on a DNA strand that
codes for a protein that controls
transcription of another gene group
Promoter
Operon
Region of DNA that
indicates to an enzyme
where to bind to make
RNA
Ex. Trp operon
Operator
An operon generally on, but
that can be turned off in the
presence of a specific
metabolite
A group of genes
operating together in
prokaryotic cells
Repressible Operon
Homeotic gene
Operon
Genes that control the body plan
of animals by controlling the
development of a group of cells.
Master for Terms Puzzle
The trp Operon
Repressible Operon
Regulatory site.
A protein that suppresses the
transcription of a gene
Repressor
Inducible Operon
Ex. Lac Operon
Operon
Genes that control the body plan
of animals by controlling the
development of a group of cells.
Gene Expression
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Operator
T E A C H E R
An operon that is
normally off, but can turn
on in the presence of a
specific metabolite
Sequence of DNA that
codes for a protein &
determines a trait
Segment on a DNA strand that
codes for a protein that controls
transcription of another gene group
Promoter
Operon
Region of DNA that
indicates to an enzyme
where to bind to make
RNA
Ex. Trp operon
Operator
An operon generally on, but
that can be turned off in the
presence of a specific
metabolite
A group of genes
operating together in
prokaryotic cells
Repressible Operon
Homeotic gene
Operon
Genes that control the body plan
of animals by controlling the
development of a group of cells.
Master for Terms Puzzle
The trp Operon
Master for Operon Model Component Label
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
T E A C H E R
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
The trp Operon
Master for Operon Model Component Label
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
P A G E S
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
T E A C H E R
Operon
Ribosome
Promoter
Operator
mRNA
end product/enzyme
Repressor
RNA polymerase
Gene
The trp Operon
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 uses or expresses only a small portion
of its total number of genes. 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, 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.
Figure 1
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
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
gene 1
operator
gene 3
gene 2
Figure 2
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 amino acid used in a variety of metabolic pathways within an E. coli
cell and can actually be synthesized by E. coli. 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. However, if E. coli has tryptophan present in the cell, it would be wasteful to have the trp
operon active making tryptophan.
Operons that are typically turned on but can be turned off when need be are referred to as
repressible operons. Alternatively, those that are normally off but can be turned on when need
be are called inducible operons. The trp operon is a repressible operon.
The Tryptophan Operon
RNA polymerase
trpP trpO trpE
trpD
trpC
tryptophan biosynthetic genes
regulatory regions
trpB trpA
mRNA
tryptophan repressor
free tryptophan
concentration too low
to bind to repressor
E
D
A
tryptophan biosynthetic
proteins
C
B
tryptophan
protein synthesis
Figure 3
Although operons are not present in eukaryotes, the expression of genes in eukaryotes works in a
similar fashion in that gene expression is controlled at multiple levels. Generally speaking,
eukaryotic gene expression is more complex because regulation can occur before and after
transcription.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
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
Each lab group will need the following:
2 copies of operon terms and puzzle pieces
operon model set
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:
a. Obtain a flexible neck straw and a larger straw.
b. Place your flexible neck straw on the surface of your desk or lab table with the flexible
neck portion on your left 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 colored gene strip onto the right end of the flexible neck
straw. Smooth the tape down as much as possible. Each colored 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. To
represent the linear nature of mRNA, tie the pieces of yarn together to form one strand.
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 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
that codes for an enzyme needed in the production of the amino acid tryptophan.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
f. Use the pipe cleaners to represent the five enzymes produced by this operon. Coordinate
the colors as much as possible. Take the pipe cleaner 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 paper
clip to the operator segment of the straw. Notice that now the RNA polymerase is unable
to move down the operon.
h. Now that you have assembled all of the pieces of the model, use it to practice explaining
how an operon works to your partner. Document your practice by having your partner
initial the box on your student answer sheet. Trade roles and listen as your partner
explains the process.
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 labeled
model, and listen as you and your partner explain how an operon functions. The teacher will
initial the model completed box on your student answer page.
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.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
The trp Operon
Modeling Gene Regulation
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
Partner Initials
________
________
Model completed and explained
Teacher Initials
________
Puzzle completed
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.
The trp Operon
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?
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.
Copyright © 2012 Laying the Foundation®, Inc., Dallas, Texas. All rights reserved. Visit us online at www.ltftraining.org.