Notes to Instructors
Chapter 18 The Genetics of Viruses and Bacteria
What is the focus of these activities?
Many students try to memorize these systems instead of understanding the logic
What are the particular activities designed to do?
Activity 18.1 How is gene expression controlled in bacteria?
Activity 18.2 Modeling the lac and trp operon systems: How can gene expression be
controlled in prokaryotes?
Activity 18.1 provides students with a mechanism to sort out similarities and differences
in the control of gene expression for inducible versus repressible operons. Activity 18.2
Activity 18.3 How is gene activity controlled in eukaryotes?
Activity 18.3 asks students to integrate information from both Chapter 18 and earlier
Activity 18.4 What controls the cell cycle?
Activity 18.4 asks students to review the major controls or checkpoints that regulate cell
What misconceptions or difficulties can these activities reveal?
Activity 18.1
Activity 18.2
Students tend to encounter a number of difficulties, misconceptions, and missing
124 Notes to Instructors
1. Many students try to memorize these systems. Modeling is used to help students
develop and understand the logic associated with the systems’ operation. As students
work on the chart comparing the two types of operons and as they model the system,
it is helpful to ask questions that focus on the logic. For example, if specific
enzymes function to break down compound X, is it energetically efficient (or logical
from an energy standpoint) for the cell to produce the enzymes when compound X is
not present? Is it energetically efficient for the cell to produce glucose by
hydrolyzing lactose if a supply of glucose is already present in the cell? Bacteria are
2. Students often have difficulty understanding why there are two levels of control for
the lac operon. Again, it helps to direct students to look at these from an energetics
viewpoint. As noted, if lactose is not present, there is no need for the cell to produce
the enzymes for its digestion. However, is it energetically efficient for the cell to
produce the enzymes for lactose digestion when glucose is readily available in the
3. Many students don’t understand why cAMP levels vary relative to the level of glucose
in the cell. To understand this, they need to realize that when glucose levels are high,
the rate of metabolism and cell respiration in the cell is also likely to be high. As a
Activity 18.3
Before you assign this activity, it is a good idea to remind students that they already know
the DNA →RNA →protein pathway. Given this understanding and their knowledge of
how DNA, RNA, and protein function in the cell, you can ask them to work in small
groups. Their assignment is to use simple logic (don’t look in the textbook) to answer
these questions:
• Under what conditions would a cell find it necessary to control expression of its
genes?
• Which of these types of control would occur over the long term and which would
have to occur quickly (in the short term)?
Notes to Instructors 125
Collect ideas from the students and write them on the board. Then ask the following question:
Doing this exercise before assigning Activity 19.1 serves two purposes:
• It teaches students the types of questions they should be asking themselves as
Activity 18.4
Many students are unaware of how complex the cell cycle controls are. It is often
counterintuitive to many that some of the controls actually function to disable or kill cells
with damaged control systems.
Answers
Activity 18.1 How is gene expression controlled in bacteria?
Fill in the chart to organize what we know about the lac and trp operons.
126 Activity 18.1
(Continued on next page)
Operon: lac trp
Is the metabolic
pathway
anabolic or
catabolic?
Catabolic
Hydrolyzes or breaks down
lactose(a disaccharide) into
glucose and galactose (two six-
carbon surgars)
Anabolic
Synthesizes tryptophan from
precursors
What
regulatory
genes are
Genes:
lacI
promotor,
Functions:
lacI produces an
active repressor
Genes:
trpR
promoter,
Functions:
trpR produces an
inactive repressor
Activity 18.1 127
What structural
genes are
included in
each operon
and what does
Genes:
lacZ
lacy
lacA
Products:
LacZ codes for the
-galactosidase
enzyme, which
digest lactose into
Genes:
tryA
tryB
tryC
tryD
Products:
These five genes
code for the five
enzymes required to
convert a precursor
Is the operon
inducible or
repressible?
The lac operon is inducible. The trp operon is repressible.
Is the repressor
protein
produced in
active or
inactive from?
The repressor protein is produced
in its active form. (The active
form binds to the operator and
stops transcription of the
structural genes)
The repressor protein is produced
in its inactive form.
Operon: lac trp
Activity 18.2 Modeling the lac and trp operon systems: How can
gene expression be controlled in prokaryotes?
Using the information in Activity 18.1 and in Chapter 18 of Campbell Biology,
9th edition, construct a model or diagram of the normal operation of both the lac and trp
operon systems.
In your models or diagrams, be sure to include these considerations:
regulatory and structural genes
inducible versus repressible control
anabolic versus catabolic enzyme activity
negative versus positive controls
Use your model to answer the questions.
1. Under what circumstances would the lac operon be “on” versus “off”? The trp operon?
The lac operon would be off when there is no lactose in the cell. The lac operon
It would be on at all other times.
2. How are the lac and trp operons similar (in structure, function, or both)?
Both have regulatory genes that produce repressor proteins that can interact with the
3. What are the key differences between the lac and trp operons?
The lac operon is inducible; the presence of lactose induces production of the
128 Activity 18.2
4. What advantages are gained by having genes organized into operons?
As they are needed, both systems are set up to simultaneously turn on (or off) all of
5. Strain X of E. coli contains a mutated lac regulatory gene on its bacterial genome.
As a result, the gene produces a nonfunctional lac repressor protein. You add a
plasmid (an extra circular piece of double-stranded DNA) to these cells. The plasmid
contains a normal regulatory gene and a normal lac operon.
Build a model or diagram of what one of these modified E. coli cells would look like.
Then answer the questions and use your model or diagram to explain your answers.
a. Before the addition of the plasmid, would the E. coli strain X cells be able to
produce the enzymes for lactose digestion? Explain.
Yes. The lacI gene ordinarily produces an active repressor protein that inhibits
b. After the addition of the plasmid, would the plasmid’s lac operon produce the
enzymes for lactose digestion constitutively (all the time) or only when lactose
was the available sugar source? Explain.
The plasmid contains a normal regulatory gene and a normal lac operon. As a
c. After the addition of the plasmid, would the bacterial genome’s lac operon
produce the enzymes for lactose digestion constitutively or only when lactose
was the available energy source? Explain.
The regulatory gene on the plasmid could produce enough repressor protein
d. If equal amounts of lactose and glucose were present in the cell, would the lac
operon in the bacterial DNA be off or on? Would the lac operon on the
introduced plasmid be off or on? Explain.
In combination with cAMP, CAP (catabolic activator protein) is an activator of
Activity 18.2 129
Activity 18.3 How is gene activity controlled in eukaryotes?
Human genes cannot all be active at the same time. If they were, all the cells in our
bodies would look the same and have the same function(s). For specialization to occur,
some genes or gene products must be active while others are turned off or inactive.
1. In eukaryotes, gene expression or gene product expression can be controlled at
several different levels. Indicate what types of control might occur at each level of
gene or gene product expression.
Note: The table presents a representative sample of the various types of control
mechanisms. It does not include all possible mechanisms of control.
Level Types of control
130 Activity 18.3
Single-celled organisms such as Amoeba and Paramecia often live in environments that
change quickly. Which of the following types of control allow organisms like Amoeba to
respond most quickly to frequent short-term environmental changes? Explain your
reasoning.
a. Control of mRNA transcription from DNA
b. Control of enzyme concentration by controlling the rate of mRNA translation
c. Control of activity of existing enzymes
d. Control of the amount of DNA present in the cell
The answers to this question may vary depending on how frequently the changes occur
Activity 18.4 What controls the cell cycle?
1. Checkpoints in the normal cell cycle prevent cells from going through division if
problems occur—for example, if the DNA is damaged.
a. What forms do the checkpoints take? That is, how do they control whether or not
cell division occurs?
A number of different cyclins are produced during interphase. Three of the major
b. In the space below, develop a handout or diagram to explain how these
checkpoints work under normal conditions. Your diagram should include a
description of each checkpoint, where it acts in the cell cycle, and what each does
to control cell division.
Refer to Figure 12.17, page 240, of Campbell Biology, 9th edition, which shows
Activity 18.4 131
18.3 Test Your Understanding
c. Cancer results from uncontrolled cell division. Explain how mutations in one or
more of the checkpoints might lead to cancer.
The basic function of each checkpoint is to determine whether or not the cell is
functioning normally and should enter into or continue through division. If these
132 Activity 18.3