Chapter 18
Regulation of Gene
Expression
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Conducting the Genetic Orchestra
• Prokaryotes and eukaryotes alter gene
expression in response to their changing
environment
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Bacteria often respond to environmental change
by regulating transcription
• Natural selection has favored bacteria that
produce only the products needed by that cell
• A cell can regulate the production of enzymes by
feedback inhibition or by gene regulation
• Gene expression in bacteria is controlled by the
operon model
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Precursor
Feedback
inhibition
trpE gene
Regulation of
a metabolic
pathway
Enzyme 1
trpD gene
Regulation
of gene
expression
Enzyme 2
trpC gene
trpB gene
Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
(b) Regulation of enzyme
production
Operons: The Basic Concept
• A cluster of functionally related genes can be
under coordinated control by a single on-off
“switch”
• The regulatory “switch” is a segment of DNA
called an operator usually positioned within the
promoter
• An operon is the entire stretch of DNA that
includes the operator, the promoter, and the
genes that they control
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Figure 18.UN01
Operon
Promoter
Genes
B
C
A
Operator
RNA
polymerase
A
B
C
Polypeptides
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• The operon can be switched off by a protein
repressor
• The repressor prevents gene transcription
• The repressor is the product of a separate
regulatory gene
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Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
• An inducible operon is one that is usually off;
a molecule called an inducer inactivates the
repressor and turns on transcription
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Pearson Education
Inc., publishing as Pearson Benjamin Cummings
©Copyright
2011 Pearson
Education,
Inc.
EXAMPLE:
The lac operon is an inducible operon and
contains genes that code for enzymes used in
the hydrolysis and metabolism of lactose
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Eukaryotic gene expression can be regulated at
any stage
• All organisms must regulate which genes are
expressed at any given time
• In multicellular organisms gene expression is
essential for cell specialization
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Differential Gene Expression
• Almost all the cells in an organism are
genetically identical
• Differences between cell types result from
differential gene expression, the expression of
different genes by cells with the same genome
• Errors in gene expression can lead to diseases
including cancer
• Gene expression is regulated at many stages
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Signal
NUCLEUS
Chromatin
Chromatin modification
DNA
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing
Active protein
Degradation
of protein
Transport to cellular
destination
Cellular function
Regulation of Chromatin Structure
• Chemical modifications to histones and DNA of
chromatin influence both chromatin structure
and gene expression
Regulation of Transcription Initiation
• Chromatin-modifying enzymes provide initial
control of gene expression by making a region of
DNA either more or less able to bind the
transcription machinery
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Animation: DNA Packing
15
Right-click slide / select “Play”
© 2008
Pearson Education
Inc., publishing as Pearson Benjamin Cummings
©Copyright
2011 Pearson
Education,
Inc.
Histone
tails
Amino acids
available
for chemical
modification
DNA
double
helix
A simple model
of histone tails
and the effect
of histone
acetylation
Nucleosome
(end view)
(a) Histone tails protrude outward from a nucleosome
Acetylated histones
Unacetylated histones
(b) Acetylation of histone tails promotes loose chromatin
structure that permits transcription
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16
A eukaryotic gene and its transcript
Enhancer
(distal control elements)
Poly-A signal
sequence
Termination
region
Proximal
control elements
Exon
Intron
Exon
Intron Exon
DNA
Upstream
Downstream
Promoter
Primary RNA
5
transcript
Transcription
Exon
Intron
Exon
Intron Exon
RNA processing
Cleaved 3 end
of primary
transcript
Poly-A
signal
Intron RNA
Coding segment
mRNA
3
5 Cap
5 UTR
Start
codon
Stop
codon
3 UTR Poly-A
tail
Mechanisms of Post-Transcriptional Regulation
• Transcription alone does not account for gene
expression
• Regulatory mechanisms can operate at various
stages after transcription
• Such mechanisms allow a cell to fine-tune gene
expression rapidly in response to
environmental changes
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Initiation of Translation
• The initiation of translation of selected
mRNAs can be blocked by regulatory proteins
• Alternatively, translation of all mRNAs
in a cell may be regulated simultaneously
• For example, translation can be simultaneously
activated in an egg following fertilization
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Protein Processing and Degradation
• After translation, various types of protein
processing, including cleavage and the addition of
chemical groups, are subject to control
• Proteasomes are giant protein complexes that
bind protein molecules and degrade them
Proteasome
and ubiquitin
to be recycled
Ubiquitin
Proteasome
Protein to
be degraded
Ubiquitinated
protein
Protein entering a
proteasome
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Protein
fragments
(peptides)
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A program of differential gene expression leads to
the different cell types in a multicellular organism
• During embryonic development, a fertilized egg
gives rise to many different cell types
• Cell types are organized successively into
tissues, organs, organ systems, and the whole
organism
• Gene expression orchestrates the developmental
programs of animals
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A Genetic Program for Embryonic Development
• The transformation from zygote to adult results
from cell division, cell differentiation, and
morphogenesis
From fertilized egg to animal: What a difference four days makes
(a) Fertilized eggs of a frog
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(b) Newly hatched tadpole
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Figure 18.19a
Key developmental events in the life cycle of Drosophila
Head Thorax
Abdomen
0.5 mm
Dorsal
BODY
AXES
Anterior
Left
Right
Posterior
Ventral
(a) Adult
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• Cell differentiation is the process by which cells
become specialized in structure and function
• The physical processes that give an organism its
shape constitute morphogenesis
• Differential gene expression results from genes
being regulated differently in each cell type
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Sequential Regulation of Gene Expression During
Cellular Differentiation
• In the process called induction, signal molecules
from embryonic cells cause transcriptional
changes in nearby target cells
• Thus, interactions between cells induce
differentiation of specialized cell types
• Determination commits a cell to its final fate
• Determination precedes differentiation
25
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Abnormal pattern formation in Drosophila
Eye
Leg
Antenna
Wild type
Mutant
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