Transcript Ch 18 Lecture

AP Biology
Chapter 13:
Gene Regulation
Gene Regulation in Bacteria
and Eukaryotes
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Bacterial cells
• Genetic Organization?
• Grow rapidly and have short life span
• Controlling transcription is the most economical way for the
cell to regulate gene expression
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Eukaryote cells
• Long life span/respond to many different stimuli
• A single gene is regulated in different ways in different types
of cells
• Gene regulation complex
• Although transcriptional-level control predominates, control
at other levels of gene expression is also important
Gene Regulation in Bacteria
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Prokaryotic DNA is organized into units called operons,
which contain functionally related genes
Operons regulated as units, so functionally related proteins
are synthesized simultaneously only when needed
Each operon consists of:
 Regulatory gene, controls transcription of other genes
 Promoter, RNA polymerase recognizes as place to start transcribing
 Operator, governs access of RNA polymerase to promoter
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Structural genes, encode for related proteins
Inducible, Repressible, and
Constitutive Genes in Bacteria
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Inducible operon
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•
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Repressible operon
•
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•
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Normally turned off
Catabolic pathways
Normally turned on
Operate via feedback inhibition
Anabolic pathways
Constitutive genes
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•
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Constantly needed and therefore constantly transcribed
Examples: Ribosomal proteins, tRNAs, RNA polymerase, glycolysis
enzymes
Neither inducible nor repressible and active at all times
The activity of constitutive genes is controlled by how efficiently RNA
polymerase binds to their promoter regions.
Inducible Operons: lac operon
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Intestinal bacterium Escherichia coli (E.coli) lives on what its
host eats
Specific enzymes are needed to metabolize the type of food
that comes along
e.g. in newborn mammals, E.coli are bathed in milk,
containing the milk sugar lactose
The lactose operon contains three structural genes, each
coding for an enzyme that aids in lactose metabolism
 lactose not present: repressor active, operon off; no
transcription for lactose enzymes
 lactose present: repressor inactive, operon on; inducer
molecule inactivates protein repressor (allolactose)
 transcription is stimulated when inducer binds to a
regulatory protein
Lac Operon Video
Lac Operon Video
Allolactose bound to
repressor
Repressible Operons:
trp Operon
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Tryptophan synthesis (anabolism)
Promoter: RNA polymerase binding
site; begins transcription
Operator: controls access of RNA
polymerase to genes (tryptophan not
present)
Repressor: binds to operator
preventing attachment of RNA
polymerase ~ when tryptophan is
present ~ acts as a co-repressor)
Transcription is repressed when
tryptophan binds to a regulatory
protein
Trp Operon Video
Negative Control in the
Regulation of an Operon
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Negative regulators inhibit transcription
 Repressible and inducible operons are under
negative control
 When repressor protein binds to operator,
transcription is turned off
 Seen in lac and trp operons
Positive Control in the
Regulation of an Operon
Positive regulators stimulate transcription
 Some inducible operons (lac) are also under
positive control
 A separate protein binds to DNA and stimulates
transcription of the gene
 Positive control of lac operon requires that the cell
is able to sense the presence of glucose
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• More efficient for cell to utilize glucose before lactose
• Only when lactose is present and glucose is in short
supply does E.coli use lactose as energy source
Positive Regulation of lac
Operon
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Lac operon always has low affinity for RNA polymerase
Involves Two Proteins:
• CAP (catabolite activator protein)
• cAMP (cyclic AMP)
Together, CAP and cAMP cause RNA polymerase to bind tightly to promoter
region
Levels of cAMP increase as levels of glucose decrease
Lac operon is fully active only when lactose is available and glucose levels
are low
A Regulon
Group of functionally related operons controlled
by a common regulator
 Example: CAP regulates the catabolism of
lactose, galactose, arabinose and maltose
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Comprehension Check
Match these components of the lac operon with their functions.
C
______
b-galactosidase
D
______
cAMP-CAP complex
G
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lactose
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operator
E
F
______
promoter
______
regulator gene
B
A
______
repressor
H
______
structural gene
A. is inactivated when
attached to lactose
B. codes for synthesis of
repressor
C. hydrolyzes lactose
D. stimulates gene
expression
E. repressor attaches here
F. RNA polymerase attaches
here
G. acts as inducer that
inactivates repressor
H. codes for an enzyme
Comprehension Check
Listed below are characteristics of repressible and
inducible enzymes. Identify each of the following
as true of repressible or inducible enzymes.
Inducible genes are switched off until a specific
______
metabolite inactivates the repressor
Repressible genes are switched on until a specific
______
metabolite activates the repressor
Repressible Generally function in anabolic pathways
______
Inducible Usually function in catabolic pathways
______
______
Repressible Pathway end product switches off its own
production
______
Inducible Enzyme synthesis is switched on by the
nutrient in used in the pathway
Comprehension Check
The events listed below describe how the lac operon functions
when lactose is present and glucose is absent. Put the steps in
the correct order.
1
______
Allolactose binds to repressor
4
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cAMP accumulates
6
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cAMP activates CAP
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cAMP binds to CAP
5
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cAMP/CAP complex binds to CAP site in
7
promoter
3
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CAP concentration increases
9
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Genes transcribed
______
Repressor inactivated
2
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RNA polymerase binds to promoter
8
Eukaryotic Gene Expression
Not organized into operons
 Typical human cell only expresses about 20%
of its genes at any given time
 Remember: All body cells contain identical
genome
 Cells rely on differential gene expression
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Regulation allows cell differentiation and
organization into tissues/organs
 Each gene has regulatory sequences essential
to the control of transcription
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Gene Regulation in
Eukaryotic Cells
 Gene
regulation occurs at
the levels of
• Transcription
• mRNA processing
• Translation
• The protein product
Eukaryotic Promoters Vary in
Efficiency, Depending on UPE’s
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Like prokaryotes, transcription requires an
initiation and promoter sites
Eukaryotic Promoter consists of:
• RNA Polymerase-binding Site (TATA box)
• Upstream Promoter Elements (UPE’s)
• 8-12 bases upstream from TATA box
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Types/Number of UPE’s determine efficiency
of promoter
• UPE’s required for accurate and efficient initiation
of transcription
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In addition to UPE’s, eukaryotic genes also
controlled by Enhancers
• Enhancers facilitate RNA polymerase binding to
promoter
• Increase rate of transcription
Regulation
of Transcription
in Eukaryotes
Eukaryotic Regulatory Proteins
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Called “Transcription
Factors”
Similar to repressors
and CAP’s in
prokaryotes
Usually act as
activators
Enhancers only
become functional
when bound to
specific transcription
factors
Chromosome Organization may
Affect Gene Expression
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Genes are inactivated by changes in chromosome structure
• Heterochromatin is tightly wound and not transcribed (ex. Barr body)
• Euchromatin is loosely packed and easily transcribed
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DNA methylation
• Methyl groups added to cytosines
• Make transcription impossible
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Multiple copies of some genes present in one chromosome
• Tandemly Repeated Gene Sequences (VTNR’s)
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Gene amplification
• Cells produce multiple copies of a gene by selective replication
Differential mRNA Processing
(Posttranscriptional Control)
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Cells in each tissue produce own version of mRNA
Same gene can be used to produce a certain protein in one tissue and a
related, but slightly different protein in another tissue
Example: troponin
• a protein that regulates muscle contraction
• produced in different muscle tissues
Other Methods of
Posttranscriptional Control
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Proteolytic Protein Processing
Proteins produced in inactive form
 Become active via removal of a portion of their
polyepeptide chain
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Chemical Modification
Addition or removal of functional groups
 Affects enzyme activity
 Kinases (add phosphate groups)
 Phosphatases (remove phosphate groups)
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