CAMPBELL
BIOLOGY
Overview: Conducting the Genetic Orchestra
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
18
Regulation of Gene Expression
 Prokaryotes and eukaryotes alter gene expression
in response to their changing environment
 In multicellular eukaryotes, gene expression
regulates development and is responsible for
differences in cell types
 RNA molecules play many roles in regulating gene
expression in eukaryotes
Lecture Presentation by
Dr Burns
NVC Biol 120
© 2014 Pearson
Inc.
Copyright
© 2009Education,
Pearson Education,
Inc.
Regulating Gene Expression
© 2011 Pearson Education, Inc.
Bacteria often respond to environmental
change by regulating transcription
1. Control amount of mRNA that is
transcribed
 Natural selection has favored bacteria that produce
only the products needed by that cell
2. Control the rate of translation
 A cell can regulate the production of enzymes by
feedback inhibition or by gene regulation
3. Control the activity of the protein
 Gene expression in bacteria is controlled by the
operon model
© 2011 Pearson Education, Inc.
Constitutively expressed genes
 Constitutively expressed genes code for
proteins that are always needed, therefore
they are always being transcribed
 Other genes transcribed only when the
proteins are needed
Gene Expression in Prokaryotes
 Gene expression in bacteria was first
studied in e. coli
 The work was done by Jacob and Monod
in 1961
1
Operons: The Basic Concept
Repressor Protein
 A cluster of functionally related genes can be
under coordinated control by a single “on-off
switch”
 The operon can be switched off by a protein
repressor
 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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 The repressor prevents gene transcription by
binding to the operator and blocking RNA
polymerase
 The repressor is the product of a separate
regulatory gene
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Corepressor
Trp Operon
 By default the trp operon is on and the genes for
tryptophan synthesis are transcribed
 The repressor can be in an active or inactive
form, depending on the presence of other
molecules
 A corepressor is a molecule that cooperates
with a repressor protein to switch an operon off
 When tryptophan is present, it binds to the trp
repressor protein, which turns the operon off
 The repressor is active only in the presence of
its corepressor tryptophan; thus the trp operon
is turned off (repressed) if tryptophan levels are
high
 For example, E. coli can synthesize the amino
acid tryptophan
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Figure 18.3a
Figure 18.3b
trp operon
DNA
DNA
Promoter
Promoter Regulatory gene
Genes of operon
trpE
trpR
mRNA
3′
RNA
polymerase
Operator
Start codon
trpD
trpC
trpR
trpB
trpE
No
RNA
made
trpA
mRNA
5′
Stop codon
3′
mRNA 5′
5′
Active
repressor
Protein
Protein
Inactive
repressor
E
D
C
B
Polypeptide subunits that make up
enzymes for tryptophan synthesis
A
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
(a) Tryptophan absent, repressor inactive, operon on
2
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
 The trp operon is a repressible operon
 An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
© 2011 Pearson Education, Inc.
Lactose is an Inducer of the lac Operon
 When lactose is present, it is converted to
allolactose
 Allolactose binds to the lactose repressor protein
 Allolactose is an allosteric regulator because it
binds to the repressor protein at a site other than
the active site
 It causes the lactose repressor protein to leave
its place on the operator region of the lac operon
 Now transcription will proceed
Inducible Operon – Lac Operon
 The lac operon is an inducible operon and
contains genes that code for enzymes used in
the hydrolysis and metabolism of lactose
 By itself, the lac repressor is active and
switches the lac operon off
 A molecule called an inducer inactivates the
repressor to turn the lac operon on
© 2011 Pearson Education, Inc.
Lactose Digestion
 Lactose is a disaccharide.
 The enzyme β–galactosidase splits the
disaccharide into glucose and galactose
 The enzyme lactose permease functions to
transport lactose into the bacteria
 The enzyme lactose transacetylase also has a
role
 E. coli normally have low levels of these
enzymes, but if they are grown in a media of
lactose they will produce large quantities of these
enzymes
Figure 18.4a
Regulatory
gene
Promoter
Operator
DNA
lac I
IacZ
No
RNA
made
3′
mRNA
5′
Protein
RNA
polymerase
Active
repressor
(a) Lactose absent, repressor active, operon off
© 2014 Pearson Education, Inc.
3
Figure 18.4b
Inducible vs Repressible
lac operon
DNA
lac I
lacZ
RNA polymerase
mRNA
3′
Start codon
lacA
lacY
Stop codon
mRNA 5′
5′
 Inducible enzymes usually function in catabolic
pathways; their synthesis is induced by a
chemical signal
Protein
β-Galactosidase
Permease
Transacetylase
 Repressible enzymes usually function in
anabolic pathways; their synthesis is repressed
by high levels of the end product
Inactive
repressor
 Regulation of the trp and lac operons involves
negative control of genes because operons are
switched off by the active form of the repressor
Allolactose
(inducer)
(b) Lactose present, repressor inactive, operon on
© 2014 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Positive Gene Regulation
Positive Gene Regulation
 Some operons are also subject to positive control
through a stimulatory protein, such as catabolite
activator protein (CAP), an activator of
transcription
 When glucose (a preferred food source of E. coli) is
scarce, CAP is activated by binding with cyclic
AMP (cAMP)
 When glucose levels increase, CAP detaches from
the lac operon, and transcription returns to a
normal rate
 CAP helps regulate other operons that encode
enzymes used in catabolic pathways
 Activated CAP attaches to the promoter of the lac
operon and increases the affinity of RNA
polymerase, thus accelerating transcription
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Figure 18.5a
Figure 18.5b
Promoter
Operator
DNA
lac I
lacZ
CAP-binding site
cAMP
Active
CAP
Inactive
CAP
Allolactose
RNA
polymerase
binds and
transcribes
Inactive lac
repressor
(a) Lactose present, glucose scarce (cAMP level high):
abundant lac mRNA synthesized
© 2014 Pearson Education, Inc.
Promoter
DNA
lac I
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase less
likely to bind
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low):
little lac mRNA synthesized
© 2014 Pearson Education, Inc.
4
Figure 13-5
Page 262
DNA
cAMP
CAP dimer
Examples of Hormone induced responses
mediated by cAMP
Target Tissue
Adrenal Cortex
Ovary
Muscle
Bone
Heart
Liver
Kidney
Fat
Hormone
Major Response
ACTH
Cortisol Secretion
LH
Progesterone secretion
Adrenaline
Glycogen breakdown
Parathyroid
Bone reabsorption
hormone (PTH)
Adrenaline
Increase heart rate
Glucagon
Glycogen breakdown
Vasopressin
Water reabsorption
Adrenaline,
Triglyceride
ACTH, glucagon
breakdown
5
Eukaryotic gene expression is regulated at
many stages
 All organisms must regulate which genes are
expressed at any given time
 In multicellular organisms regulation of gene
expression is essential for cell specialization
Coordinately Controlled Genes in Eukaryotes
 Unlike the genes of a prokaryotic operon, each of
the co-expressed eukaryotic genes has a promoter
and control elements
 These genes can be scattered over different
chromosomes, but each has the same combination
of control elements
 Copies of the activators recognize specific control
elements and promote simultaneous transcription of
the genes
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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
© 2011 Pearson Education, Inc.
Organization of a Typical Eukaryotic Gene
 Associated with most eukaryotic genes are multiple
control elements, segments of noncoding DNA that
serve as binding sites for transcription factors that
help regulate transcription
 Control elements and the transcription factors they
bind are critical to the precise regulation of gene
expression in different cell types
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Eukaryotic Promoter region
© 2011 Pearson Education, Inc.
Fig. 16.9
 RNA polymerase binds to the promoter region
 In eukaryotes the DNA has a “TATA box” just
upstream from where the RNA polymerase
binds
 It is located upstream from the genes (25 – 35
bases upstream)
6
Figure 18.8-1
Enhancer
(distal control
elements)
Figure 18.8-2
Proximal
control
elements
DNA
Upstream
Transcription
start site
Exon
Intron
Exon
Poly-A
signal
sequence
Intron Exon
Transcription
termination
region
Downstream
Promoter
Enhancer
(distal control
elements)
Proximal
control
elements
DNA
Upstream
Transcription
start site
Exon
Enhancer
(distal control
elements)
DNA
Upstream
Exon
Intron
Exon
Promoter
Primary RNA
transcript
5
(pre-mRNA)
Figure 18.8-3
Intron
Intron
Poly-A
signal
sequence
Exon
Transcription
Exon
Intron
Transcription
termination
region
Downstream
Poly-A
signal
Exon
Cleaved
3 end of
primary
transcript
The Roles of Transcription Factors
Proximal
control
elements
Transcription
start site
Exon
Intron
Exon
Intron
Exon
Promoter
Poly-A
signal
sequence
Intron Exon
Transcription
termination
region
Downstream
Poly-A
signal
Exon
Cleaved
3 end of
primary
RNA processing
transcript
 To initiate transcription, eukaryotic RNA polymerase
requires the assistance of proteins called
transcription factors
Transcription
Exon
Primary RNA
transcript
5
(pre-mRNA)
Intron
 General transcription factors are essential for the
transcription of all protein-coding genes
Intron RNA
Coding segment
mRNA
G
P
P
5 Cap
AAA  AAA
P
5 UTR
Start
codon
Stop
codon
3 UTR Poly-A
tail
3
 In eukaryotes, high levels of transcription of
particular genes depend on control elements
interacting with specific transcription factors
© 2011 Pearson Education, Inc.
Fig. 16.11
Figure 18.10-3
Promoter
Activators
DNA
Enhancer
Distal control
element
Gene
TATA box
General
transcription
factors
DNAbending
protein
Group of mediator proteins
RNA
polymerase II
RNA
polymerase II
Transcription
initiation complex
RNA synthesis
7
Transcription Factors
 Proteins that regulate transcription in
eukaryotes are called transcription factors.
 There are thousands of transcription factors,
both repressors and enhancers
Turn
COO-
Finger 2
Finger 3
Zinc
ion
a-helix
Finger 1
 Examples of common motifs found in these
proteins:
Leucine
zipper
region
NH3+
 Helix-turn-helix
 Zinc fingers
 Leucine zipper proteins
DNA
DNA
DNA
(a)
(b)
(c)
Figure 18.9
Activation
domain
DNA-binding
domain
DNA
Animation: Initiation of Transcription
Right-click slide / select “Play”
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Transcription Factors
Figure 18.10-1
Activators
Promoter
DNA
 Some transcription factors function as
repressors, inhibiting expression of a particular
gene by a variety of methods
Enhancer
Distal control
element
Gene
TATA box
 Some activators and repressors act indirectly
by influencing chromatin structure to promote
or silence transcription
© 2011 Pearson Education, Inc.
8
Figure 18.10-2
Promoter
Activators
DNA
Enhancer
Distal control
element
Gene
TATA box
General
transcription
factors
DNAbending
protein
Group of mediator proteins
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
 Abnormalities in gene expression can lead to
diseases including cancer
 Gene expression is regulated at many stages
© 2011 Pearson Education, Inc.
Figure 18.6
Figure 18.6a
Signal
Signal
NUCLEUS
Chromatin
NUCLEUS
Chromatin
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethylation
DNA
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethylation
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
DNA
Intron
Gene available
for transcription
RNA processing
Cap
Tail
mRNA in nucleus
Gene
Transport to cytoplasm
Transcription
CYTOPLASM
RNA
mRNA in cytoplasm
Degradation
of mRNA
Primary transcript
Intron
RNA processing
Polypeptide
Protein processing, such
as cleavage and
chemical modification
Degradation
of protein
Exon
Translation
Cap
Active protein
Transport to cytoplasm
Transport to cellular
destination
Cellular function (such
as enzymatic activity,
structural support)
Figure 18.6b
Tail
mRNA in nucleus
CYTOPLASM
CYTOPLASM
Regulation of Chromatin Structure
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing, such
as cleavage and
chemical modification
Degradation
of protein
 Genes within highly packed heterochromatin are
usually not expressed
 Chemical modifications to histones and DNA of
chromatin influence both chromatin structure and
gene expression
Active protein
Transport to cellular
destination
Cellular function (such
as enzymatic activity,
structural support)
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9
Histone Modifications
 In histone acetylation, acetyl groups are attached
to positively charged lysines in histone tails
 This loosens chromatin structure, thereby promoting
the initiation of transcription
 The addition of methyl groups (methylation) can
condense chromatin; the addition of phosphate
groups (phosphorylation) next to a methylated
amino acid can loosen chromatin
Animation: DNA Packing
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Figure 18.7
Histone Modifications
Histone
tails
Amino acids
available
for chemical
modification
DNA
double
helix
 The histone code hypothesis proposes that
specific combinations of modifications, as well as
the order in which they occur, help determine
chromatin configuration and influence transcription
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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Epigenetic Inheritance
 Although the chromatin modifications just
discussed do not alter DNA sequence, they
may be passed to future generations of cells
 The inheritance of traits transmitted by
mechanisms not directly involving the
nucleotide sequence is called epigenetic
inheritance
DNA Methylation
 DNA methylation, the addition of methyl groups to
certain bases in DNA, is associated with reduced
transcription in some species
 DNA methylation can cause long-term inactivation
of genes in cellular differentiation
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10
Mechanisms of Post-Transcriptional Regulation
 Transcription alone does not account for gene
expression
 Regulatory mechanisms can operate at various
stages after transcription
RNA Processing
 In alternative RNA splicing, different mRNA
molecules are produced from the same primary
transcript, depending on which RNA segments are
treated as exons and which as introns
 Such mechanisms allow a cell to fine-tune gene
expression rapidly in response to environmental
changes
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© 2011 Pearson Education, Inc.
Figure 18.13
Exons
DNA
1
3
2
4
5
4
5
Troponin T gene
Primary
RNA
transcript
3
2
1
RNA splicing
Animation: RNA Processing
mRNA
1
2
3
5
or
1
2
4
5
Right-click slide / select “Play”
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mRNA Degradation
 The life span of mRNA molecules in the cytoplasm
is a key to determining protein synthesis
 Eukaryotic mRNA is more long lived than
prokaryotic mRNA
 Nucleotide sequences that influence the lifespan of
mRNA in eukaryotes reside in the untranslated
region (UTR) at the 3 end of the molecule
Animation: mRNA Degradation
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11
Initiation of Translation
 The initiation of translation of selected
mRNAs can be blocked by regulatory proteins that
bind to sequences or structures of the mRNA
 Alternatively, translation of all mRNAs
in a cell may be regulated simultaneously
 For example, translation initiation factors are
simultaneously activated in an egg following
fertilization
Animation: Blocking Translation
Right-click slide / select “Play”
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© 2011 Pearson Education, Inc.
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
Animation: Protein Processing
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© 2011 Pearson Education, Inc.
Figure 18.14
Proteasome
and ubiquitin
to be recycled
Ubiquitin
Proteasome
Protein to
be degraded
Ubiquitinated
protein
Protein entering
a proteasome
Protein
fragments
(peptides)
Animation: Protein Degradation
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12
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
A Genetic Program for Embryonic Development
 The transformation from zygote to adult results
from cell division, cell differentiation, and
morphogenesis
 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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Figure 18.16
Cell Differentiation and morphogenesis
 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
1 mm
(a) Fertilized eggs of a frog
2 mm
(b) Newly hatched tadpole
 Materials in the egg can set up gene regulation
that is carried out as cells divide
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Cytoplasmic Determinants and Inductive Signals
 An egg’s cytoplasm contains RNA, proteins, and
other substances that are distributed unevenly in
the unfertilized egg
 Cytoplasmic determinants are maternal
substances in the egg that influence early
development
 As the zygote divides by mitosis, cells contain
different cytoplasmic determinants, which lead to
different gene expression
Maternal Effect Genes
 The developing oocyte gets “polarized” by
mRNA from the mother’s nurse cells.
 The DNA in the mother is transcribed in the
mother and the mRNA is given to the oocyte
prior to fertilization.
 These genes are maternal effect genes
 This is what determines anterior vs posterior
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13
Figure 18.17a
(a) Cytoplasmic determinants in the egg
Cell Signaling - Induction
Unfertilized egg
Sperm
Fertilization
Nucleus
Molecules of two
different cytoplasmic
determinants
Zygote
(fertilized egg)
Mitotic
cell division
 The other important source of developmental
information is the environment around the cell,
especially signals from nearby embryonic cells
 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
Two-celled
embryo
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Figure 18.17b
(b) Induction by nearby cells
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Animation: Cell Signaling
Signaling
molecule
(inducer)
Right-click slide / select “Play”
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Sequential Regulation of Gene Expression During
Cellular Differentiation
 Determination commits a cell to its final fate
 Determination precedes differentiation
 Cell differentiation is marked by the production of
tissue-specific proteins
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Pattern Formation: Setting Up the Body Plan
 Pattern formation is the development of a spatial
organization of tissues and organs
 In animals, pattern formation begins with the
establishment of the major axes
 Positional information, the molecular cues that
control pattern formation, tells a cell its location
relative to the body axes and to neighboring cells
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14
The Drosophila Fly
 Development in drosophila flies have stages:
 Egg and sperm fuse to produce a zygote
 The zygote undergoes embryonic
development to become a larva
 The larva undergoes several molts and
grow until it becomes a pupa
 A pupa has a hardened external cuticle
 The pupa undergoes metamorphosis and
becomes a mature adult fly
86
The Life Cycle of Drosophila
Figure 18.19 Head Thorax
Abdomen
Follicle cell
1 Egg
developing within
ovarian follicle
Nucleus
Egg
0.5 mm
Nurse cell
 In Drosophila, cytoplasmic determinants in the
unfertilized egg determine the axes before
fertilization
Dorsal
BODY
AXES
Anterior
Left
Right
Posterior
2 Unfertilized egg
Depleted
nurse cells
Ventral
(a) Adult
Egg
shell
Fertilization
Laying of egg
3 Fertilized egg
 After fertilization, the embryo develops into a
segmented larva with three larval stages
Embryonic
development
4 Segmented
embryo
0.1 mm
Body
segments
Hatching
5 Larval stage
(b) Development from egg to larva
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Axis Establishment
 Maternal effect genes encode for cytoplasmic
determinants that initially establish the axes of the
body of Drosophila
 These maternal effect genes are also called eggpolarity genes because they control orientation of
the egg and consequently the fly
Animation: Development of Head-Tail Axis in Fruit Flies
Right-click slide / select “Play”
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© 2011 Pearson Education, Inc.
15
Maternal Effect Genes
 The developing oocyte gets “polarized” by
mRNA from the mother’s nurse cells. The
mRNA is given to the oocyte prior to fertilization
 This is what determines anterior vs posterior
Establishment of the A/P axis
 Nurse cells secrete maternally produced bicoid
and nanos mRNAs into the oocyte
 The two types of mRNA are transported by
microtubules to opposite poles of the oocyte
 bicoid mRNA to the future anterior pole
 nanos mRNA to the future posterior pole
Bicoid: A Morphogen Determining Head
Structures
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Movement of bicoid mRNA moves
maternal mRNA toward anterior end
Follicle
cells
Nurse
cells
 One maternal effect gene, the bicoid gene,
affects the front half of the body, the other,
nanos affects the back half.
Anterior
Posterior
Microtubules nanos mRNA moves
toward posterior end
 An embryo whose mother has no functional
bicoid gene lacks the front half of its body and
has duplicate posterior structures at both
ends
a.
Nucleus
Anterior
Posterior
bicoid
mRNA
Establishment of the A/P axis
nanos
mRNA
b.
© 2011 Pearson Education, Inc.
Fig. 19.13.a1
 After fertilization, translation of the bicoid and
nano mRNA will create opposing gradients of
Bicoid and Nanos proteins
16
Fig. 19.15.a
Hunchback and Caudal
 The bicoid and nano proteins control
(inhibit) the translation of two other
maternal mRNAs:
 Hunchback and Caudal mRNAs code for
transcription factors which control genes
necessary for anterior and posterior
(abdominal) structures
Fig. 19.15.b
Fig. 19.15.c
Figure 18.21
Figure 18.22
Head
Tail
100 m
RESULTS
Anterior end
A8
T1 T2 T3
A1
A2
A3
A4
A5
A6
Wild-type larva
A7
250 m
Fertilization,
translation of
bicoid mRNA
Bicoid mRNA in mature
unfertilized egg
Tail
Bicoid protein in
early embryo
Tail
A8
A8
A7
A6
A7
Bicoid mRNA in mature
unfertilized egg
Bicoid protein in
early embryo
Mutant larva (bicoid)
17
Hunchback gene
 The bicoid research is important for three reasons
– It identified a specific protein required for some
early steps in pattern formation
– It increased understanding of the mother’s role in
embryo development
– It demonstrated a key developmental principle that
a gradient of molecules can determine polarity
and position in the embryo
 Bicoid protein is also a transcription factor
for the embryo’s hunchback gene. The
hunchback mRNA is the first to be
transcribed in the embryo after fertilization.
 There is both maternal and embryonic
hunchback mRNA being transcribed in the
embryo
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Hunchback gene
Segmentation
 Nüsslein-Volhard and Wieschaus studied
segment formation
 Hunchback gene is a type of gap gene,
which maps out the largest subdivisions of
the A/P axis in the embryo
 The gap genes (there are nine gap genes)
code for transcription factors for pair-rule
genes
 They created mutants, conducted breeding
experiments, and looked for corresponding
genes
 Many of the identified mutations were
embryonic lethals, causing death during
embryogenesis
 They found 120 genes essential for normal
segmentation
© 2011 Pearson Education, Inc.
Fig. 19.13.a2
Segmentation Genes
 The embryo DNA produces mRNA
 Some of the embryonic genes code for the
body segments and pattern of the organism
1. Gap genes – refine the broad regions set by
maternal genes into more defined regions along
A/P axis
2. Pair-rule genes - Divide the embryo into seven
zones
3. Segment polarity genes - finish defining the
embryonic segments

Most segmentation genes code for transcription
factors
18
Fig. 19.13.b2
Fig. 19.13.d2
Fig. 19.13.c2
Homeotic Genes
 After the segmentation genes have
established the pattern of segments, then
homeotic genes code for the development
plan for the segments
 Like segmentation genes, these genes
code mainly for transcription factors
HOX genes
Production of Body Plan
 All the homeotic genes have a highly
conserved region that codes for a DNA
binding domain in the transcription factors
Drosophila HOM Chromosomes
Mouse Hox Chromosomes
Drosophila HOM genes
Antennapedia complex
Bithorax complex
Hox 2
lab pb Dfd Scr Antp
Ubxabd-Aabd-B
Hox 3
Head Thorax
 Therefore they are referred to as Hox
genes since they all contain the
“homeodomain”
a.
Hox 1
Hox 4
Abdomen
Fruit fly
embryo
Mouse
embryo
Fruit fly
Mouse
b.
19
Genetic Analysis of Early Development:
Scientific Inquiry
Figure 18.20
 Edward B. Lewis, Christiane Nüsslein-Volhard,
and Eric Wieschaus won a Nobel Prize in 1995
for decoding pattern formation in Drosophila
Eye
 Lewis discovered the homeotic genes, which
control pattern formation in late embryo, larva,
and adult stages
Leg
Antenna
Wild type
Mutant
© 2011 Pearson Education, Inc.
Cancer results from genetic changes that
affect cell cycle control
 The gene regulation systems that go wrong during
cancer are the very same systems involved in
embryonic development
Post-transcriptional control - summary
 How long a mRNA remains active is important
 Bacterial mRNA has a short half life – minutes
 Eukaryotic mRNA undergoes posttranscriptional
modification
 Eukaryotic mRNA can remain active for hours
© 2011 Pearson Education, Inc.
Post-transcriptional modifications
Post-translational Modification

1. A 5’ cap is added to the 5’ end which may
protect the mRNA from being degraded.
2. A poly-A tail is added near the 3’ end which
may help the mRNA to be exported from the
nucleus
3. Introns must be removed
4. Small RNAs can inhibit translation of mRNA
Modification of the protein or chain of amino acids:
1. Protein Kinases – phosphorylate proteins,
activating them
2. Enzyme inhibitors and activators
3. Proteolysis – some polypeptide chains are cut into
smaller chains
20
Post-translational Modification
4. Glycosylation – sugars are added to some proteins,
some of these sugar chains are important to
“address” the protein
5. Methionine is often removed
6. Proteases degrade the proteins
7. Proteasome degrades proteins using the marker ubitiquin
Important Concepts
 Understand how eukaryotes regulate gene
transcription, including the role of TATA box, and
transcription factors, the types of transcription
factors
 Know how gene expression is controlled post
transcriptionally and posttranslationally
Important concepts
 Know the vocabulary of the lecture
 Know in detail how e. coli regulates the
transcription of genes responsible for lactose
digestion.
 Understand the inducible genes and the
repressible genes
 Understand the differences between positive
control and negative control of gene transcription
 Know how the lac operon is controlled by positive
control, including role of cAMP and CAP, and
Important Concepts
 Understand what leads to differences observed
in cell types
 Know the stages of development in drosophila
flies
 Understand what maternal effect genes are,
include in your discussion, what the origin of the
genes are, at what point in development do these
genes appear, and what effect do they have on
the organism.
 Understand what segmentation genes (including
the three types of segmentation genes), and
homeotic genes are and what they do.
21