Download 13.4 Gene Regulation and Expression

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13.4 Gene Regulation and Expression
Lesson Objectives
Describe gene regulation in prokaryotes.
Explain how most eukaryotic genes are regulated.
Relate gene regulation to development in multicellular organisms.
Lesson Summary
Prokaryotic Gene Regulation Prokaryotes do not need to transcribe all of their genes
at the same time. They can conserve energy and resources by regulating their activities,
producing only those genes necessary for the cell to function. In prokaryotes, DNA-binding
proteins regulate genes by controlling transcription. An operon is a group of genes that are
regulated together. An example is the lac operon in the bacterium E. coli:
▶ This group of three genes must be turned on together before the bacterium can use lactose
as food.
▶ When lactose is not present, the DNA-binding protein called lac repressor binds to a
region called the operator, which switches the lac operon off.
▶ When lactose binds to the repressor, it causes the repressor to fall off the operator, turning
the operon on.
Eukaryotic Gene Regulation Transcription factors are DNA-binding proteins. They
control the expression of genes in eukaryotes by binding DNA sequences in the regulatory
regions. Gene promoters have multiple binding sites for transcription factors, each of which
can influence transcription.
▶ Complex gene regulation in eukaryotes makes cell specialization possible.
▶ The process by which microRNA (miRNA) molecules stop mRNA molecules from
passing on their protein-making instructions is RNA interference (RNAi).
▶ RNAi technology holds the promise of allowing scientists to turn off the expression of
genes from viruses and cancer cells, and it may provide new ways to treat and perhaps
even cure diseases.
Genetic Control of Development Regulating gene expression is especially important
in shaping the way a multicellular organism develops. Gene regulation helps cells undergo
differentiation, becoming specialized in structure and function. Master control genes are
like switches that trigger particular patterns of development and differentiation in cells and
tissues.
▶ Homeotic genes are master control genes that regulate organs that develop in specific
parts of the body.
▶ Homeobox genes share a similar 130-base DNA sequence called homeobox. They code
for transcription factors that activate other genes that are important in cell development
and differentiation in certain regions of the body.
▶ Hox genes are a group of homeobox genes that tell the cells of the body how to
differentiate as the body grows.
Environmental factors can also affect gene expression.
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Prokaryotic Gene Regulation
1. How do prokaryotes conserve energy?
Prokaryotes regulate their activities, producing only those genes necessary for the cell
to function.
2. How do DNA-binding proteins in prokaryotes regulate genes?
They control transcription. Some of these regulatory proteins help switch genes on,
while others turn genes off.
3. What is an operon?
It is a group of genes that are regulated together.
4. What is in the lac operon in E. coli?
three genes
5. What is the function of the genes in the lac operon of E. coli?
They allow E. coli to use lactose for food when it is present.
6. What turns the lac operon off?
A repressor protein turns the operon off.
7. How does a repressor protein turn off the lac operon?
It binds to the operating region, blocking RNA polymerase from transcribing the lac
genes.
8. How does lactose turn on the lac operon?
It attaches to the repressor, which causes the repressor to fall off the operator.
Transcription can take place.
9. Complete the table to describe the role of each regulatory region or molecule in the
operation of the lac operon.
Regulatory Region
or Molecule
What It Does
Repressor protein
Binds to the operator, preventing transcription of the lac genes
Operator
Binding site for the repressor protein
RNA polymerase
When the repressor is not present, this enzyme carries out the
transcription of lac genes.
Lactose
Causes the repressor to drop off the operator so transcription of
the lac genes can begin
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Eukaryotic Gene Regulation
10. In what two ways is gene regulation in eukaryotes different from gene regulation in
prokaryotes?
a. Most eukaryotic genes are controlled individually.
b. Eukaryotic cells have more complex regulatory sequences than those of the lac
repressor system.
11. What is a TATA box? What does a TATA box do?
It is a short region of DNA that contains a sequence of T and A base pairs. The protein
that binds to this site helps position RNA polymerase.
12. What are transcription factors and what do they do?
They are DNA-binding proteins that bind to DNA sequences in the regulatory regions
of genes and help control gene expression.
13. Explain how gene regulation makes cell specialization possible.
It allows particular genes to be expressed in some kinds of cells but not others.
14. What is microRNA and how is it related to mRNA?
Small RNA molecules are called microRNA. They attach to certain mRNA molecules
and stop them from passing on their protein-making instructions.
15. Explain how the process of RNA interference works.
Certain small RNA molecules fold into loops. The Dicer enzyme cuts them into
microRNA (miRNA). The strands then separate. An miRNA piece attaches to a cluster
of proteins to form a silencing complex. The silencing complex binds to and destroys
an mRNA molecule that contains a base sequence complementary to the miRNA. In
this way, it blocks gene expression.
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Genetic Control of Development
For Questions 16–23, write the letter of the correct answer on the line at the left.
C
16. As an embryo develops, different sets of genes are regulated by
A. mRNA and lac repressors.
C. transcription factors and repressors.
B. operons and operators.
D. promoters and operators.
C
17. The process through which cells become specialized in structure and function is
A. transcription.
C. differentiation.
B. gene expression.
D. RNA interference.
B
18. Homeotic genes are
A. regulator genes that bind to operons in prokaryotes.
B. master control genes that regulate organs that develop in specific parts of
the body.
C. parts of the silencing complex that regulates gene action through RNA
interference.
D. base sequences complementary to sequences in microRNA.
A
19. What role do homeobox genes play in cell differentiation?
A. They code for transcription factors that activate other genes important in
cell development and differentiation.
B. They block certain gene expression.
C. They cut double-stranded loops into microRNA.
D. They attach to a cluster of proteins to form a silencing complex, which
binds to and destroys certain RNA.
D
20. In flies, the group of homeobox genes that determines the identities of each
segment of a fly’s body is the group known as
A. silencing complexes.
C. operators.
B. promoters.
D. Hox genes.
D
21. Clusters of Hox genes are found in
A. flies only.
B. flies and frogs only.
C. plants only.
D. nearly all animals.
B
22. The “switches” that trigger particular patterns of development and differentiation
in cells and tissues are
A. mRNA molecules.
C. silencing complexes.
B. master control genes.
D. Dicer enzymes.
A
23. Metamorphosis is
A. a series of transformations from one life stage to another.
B. the master switch that triggers development and differentiation.
C. the product of interactions among homeotic genes.
D. the process by which genetic information is passed from one generation to
the next.
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24. Environmental factors can influence gene expression. Fill in the table below to show how
organisms respond to conditions in their environment.
Environmental Factor
Influencing Gene
Expression
E. coli with limited food nutrient availability
supply
A tadpole in a drying
pond
lack of water
How the Organism Responds
The lac operon is switched on
when lactose is the only food
source.
The tadpole may speed up its
metamorphosis.
25. Many research studies have shown that different species may possess some of the exact
same genes but show vastly different traits. How can that happen?
The difference arises not from the genes themselves but from how they are regulated
and expressed. For example, a gene may be turned on at a different time in one
species than in another. Perhaps environmental factors have an effect, too.
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Chapter Vocabulary Review
For Questions 1–7, write True if the statement is true. If the statement is false, change the
underlined word or words to make the statement true.
RNA
1. DNA contains the sugar ribose.
True
2. Messenger RNA carries copies of the instructions for making proteins
from DNA to other parts of the cell.
Transfer RNA
3. RNA polymerase transfers amino acids to ribosomes.
True
4. The process of transcription produces a complementary strand of
RNA on a DNA template.
True
5. The enzyme that assembles a complementary strand of RNA on a
DNA template is RNA polymerase.
promoter
True
6. The region of DNA where the production of an RNA strand begins is
called the intron.
7. Exons are spliced together in forming messenger RNA.
For Questions 8–16, match the term with its definition.
Term
Definition
B
E
F
8. The sequence of bases that serves as the
“language” of life
A. polypeptide
B. genetic code
9. A sequence of three bases on a tRNA molecule C. codon
that is complementary to a sequence of bases
D. translation
on an mRNA molecule
10. How genetic information is put into action in a E. anticodon
F. gene expression
living cell
I
11. Having extra sets of chromosomes
G. mutation
D
12. The decoding of an mRNA message into a
protein
H. mutagen
G
13. A heritable change in genetic information
A
14. A chain of amino acids
C
15. The three consecutive bases that specify
a single amino acid to be added to the
polypeptide chain
H
16. A chemical or physical agent that causes a
change in a gene
I. polyploidy
For Questions 17–19, complete each statement by writing the correct word or words.
17. A group of genes that are regulated together is called a(n)
18. A region of DNA where a repressor can bind is a(n)
19. Master control genes, called
homeotic
operon
operator
.
.
genes, regulate organs that develop in
specific parts of the body.
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In the Chapter Mystery, you learned about
a fly that was genetically manipulated to
grow eyes in different places on its body.
Scientists continue to develop techniques
to modify a variety of other animals. But
should they?
MOUSE-EYED FLY
Learning
Should Genetic Experiments Be Performed
on Animals?
Some people think that the type of research that produced the mouse-eyed fly is perfectly
acceptable. Others think that it’s a terrible thing to do even to a fly. In the biological sciences,
animal testing has always been controversial, and it probably always will be.
Perspectives on Animal Genetic
Experiments
Animal rights groups have come a long way in recent years.
More people are responding to the issues brought up by animal
rights groups. For example, animal testing of cosmetics has been
banned in a number of countries, and in the United States many
companies have voluntarily stopped testing on animals. So what’s
the next frontier for animal rights activists? According to Edward
Avellone of Animal Rights Now!, it’s genetic experimentation.
“What purpose is there in creating a mouse with six legs or a
sheep with one eye in the middle of its forehead?” asks Avellone.
“Scientists are just playing around with a new technology. They’re
creating horribly deformed animals for no real reason.”
Some people disagree with this point of view. Says Ann
Wilber of Scientists for the Ethical Treatment of Animals, “We’re
responsible professionals, not monsters.” Wilber explains that
the one-eyed sheep was the unintended result of an attempt to
understand how the eye developed and how it works. “We’ve also
developed a sheep whose milk contains a protein that might cure
emphysema. There are reasons for what we do.”
But to Avellone, the point is not simply the motivation behind
the experimentation. It’s also the process of the experiment. “Only
10 percent of the animals they breed have the gene they want
to study. The remaining 90 percent [of the animals] are simply
killed.” Wilber admits that this situation is “sad, but true.” Still,
she says, “We’re working every day to improve our techniques
and therefore our success rate.” Even if the success rate never tops
10 percent, she asks, “Isn’t that a small price to pay for a cure for
cancer, or multiple sclerosis, or Parkinson’s disease?”
Continued on next page ▶
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Science and Civic Literacy
1. Edward Avellone makes several arguments against animal testing in genetics experiments.
Why does he think the process of animal genetic experimentation is flawed?
He says that only 10 percent of animals scientists breed have the gene they want to study.
He says the remaining 90 percent of the animals are killed because they are not useful.
2. Avellone argues that genetic experiments are unnecessary. What claim does he make
about scientists’ motivations for doing such experiments?
He says that scientists are performing experiments with no real scientific purpose,
and that they are “playing with new technology.”
3. What is Ann Wilber’s main argument in favor of genetic testing on animals?
It could lead to cures for serious human diseases.
4. Which of her arguments could be summed up as, “It’s a necessary evil”?
Killing 90 percent of the animals they breed is a small price to pay to cure diseases.
5. Do you agree with Avellone, with Wilber, or with neither of them? Why?
Accept any answers that are clearly argued, logical, and based on evidence, either
from the article or from other reliable sources.
Evaluating an Issue
The skills used in this activity include creativity and intellectual curiosity, communication
skills, interpersonal and collaborative skills, information and media literacy, and social
responsibility.
The issue of experimenting on animals, especially genetic experimentation, is complex;
you can’t construct an informed opinion after reading just one article. Working with a
group, use library and Internet resources to collect opinions on both sides of the issue. Work
through the material thoroughly and then make up your mind about how you feel about the
issue.
If everyone in the group agrees, create a multimedia presentation for your class in which you
present your point of view. If the members of your group disagree, stage a debate in front of
the class, with the same number of students arguing each side of the issue.
Evaluate students’ presentations based on the thoroughness of their research, the
inclusion of sources from both sides of the issue, and the quality of the sources they used;
also, consider the cogency of the arguments they make to support their point of view.
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