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Chapter 18- Regulation of Gene Expression
Overview: Conducting the Genetic Orchestra
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
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Regulating Gene Expression
1. Control amount of mRNA that is
transcribed
2. Control the rate of translation
3. Control the activity of the protein
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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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
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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
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
(a) (b) Regulation of enzyme activity
Regulation of enzyme production
Regulation of gene expression
trpE gene
trpD gene
trpC gene
trpB gene
trpA gene
Figure 18.2
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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Repressor Protein
The operon can be switched off by a protein
repressor
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
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
For example, E. coli can synthesize the amino
acid tryptophan
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Trp Operon
By default the trp operon is on and the genes for
tryptophan synthesis are transcribed
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
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Repressable Operon - Trp Operon
Promoter
DNA
Regulatory gene
mRNA
trpR
5
3
Protein Inactive repressor
RNA polymerase
Promoter
trp operon
Genes of operon
Operator
mRNA 5
Start codon Stop codon
trpE trpD trpC trpB trpA
E D C B A
Polypeptide subunits that make up enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
Figure 18.3b-1
(b) Tryptophan present, repressor active, operon off
DNA
mRNA
Protein
Tryptophan (corepressor)
Active repressor
Figure 18.3b-2
(b) Tryptophan present, repressor active, operon off
DNA
mRNA
Protein
Tryptophan (corepressor)
Active repressor
No RNA made
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
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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
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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
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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
(a) Lactose absent, repressor active, operon off
Regulatory gene
Promoter
Operator
DNA lacZ lacI DNA
mRNA
5
3
No RNA made
RNA polymerase
Active repressor Protein
Figure 18.4b
(b) Lactose present, repressor inactive, operon on
lacI
lac operon
lacZ lacY lacA DNA
mRNA
5
3
Protein
mRNA 5
Inactive repressor
RNA polymerase
Allolactose (inducer)
-Galactosidase Permease Transacetylase
Inducible vs Repressible
Inducible enzymes usually function in catabolic
pathways; their synthesis is induced by a
chemical signal
Repressible enzymes usually function in
anabolic pathways; their synthesis is repressed
by high levels of the end product
Regulation of the trp and lac operons involves
negative control of genes because operons are
switched off by the active form of the repressor
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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)
Activated CAP attaches to the promoter of the lac
operon and increases the affinity of RNA
polymerase, thus accelerating transcription
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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
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Positive Gene Regulation Figure 18.5a
Promoter
DNA
CAP-binding site
lacZ lacI
RNA polymerase binds and transcribes
Operator
cAMP
Active CAP
Inactive CAP
Allolactose
Inactive lac repressor
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
Figure 18.5b
Promoter
DNA
CAP-binding site
lacZ lacI
Operator
RNA polymerase less likely to bind
Inactive lac repressor
Inactive CAP
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
Figure 13-5
Page 262 DNA
cAMP
CAP dimer
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Examples of Hormone induced responses
mediated by cAMP
Target Tissue Hormone Major Response
Adrenal Cortex ACTH Cortisol Secretion
Ovary LH Progesterone secretion
Muscle Adrenaline Glycogen breakdown
Bone Parathyroid
hormone (PTH)
Bone reabsorption
Heart Adrenaline Increase heart rate
Liver Glucagon Glycogen breakdown
Kidney Vasopressin Water reabsorption
Fat Adrenaline,
ACTH, glucagon
Triglyceride
breakdown
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
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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
Abnormalities in gene expression can lead to
diseases including cancer
Gene expression is regulated at many stages
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Figure 18.6 Signal
NUCLEUS
Chromatin
Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation
DNA
Gene
Gene available for transcription
RNA Exon
Primary transcript
Transcription
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Translation Degradation of mRNA
Polypeptide
Protein processing, such as cleavage and chemical modification
Active protein Degradation of protein
Transport to cellular destination
Cellular function (such as enzymatic activity, structural support)
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Figure 18.6a Signal
NUCLEUS
Chromatin
Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation
DNA
Gene
Gene available for transcription
RNA Exon
Primary transcript
Transcription
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Figure 18.6b
CYTOPLASM
mRNA in cytoplasm
Translation Degradation of mRNA
Polypeptide
Protein processing, such as cleavage and chemical modification
Active protein Degradation of protein
Transport to cellular destination
Cellular function (such as enzymatic activity, structural support)
Regulation of Chromatin Structure
Genes within highly packed heterochromatin are
usually not expressed
Chemical modifications to histones and DNA of
chromatin influence both chromatin structure and
gene expression
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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
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Animation: DNA Packing
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Figure 18.7
Amino acids available for chemical modification
Histone tails
DNA double helix
Nucleosome (end view)
(a) Histone tails protrude outward from a nucleosome
Unacetylated histones Acetylated histones
(b) Acetylation of histone tails promotes loose chromatin structure that permits transcription
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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
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Histone Modifications 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
In genomic imprinting, methylation regulates
expression of either the maternal or paternal alleles
of certain genes at the start of development
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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
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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
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)
Fig. 16.9
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Figure 18.8-1
Enhancer (distal control elements)
DNA
Upstream Promoter
Proximal control elements
Transcription start site
Exon Intron Exon Exon Intron
Poly-A signal sequence
Transcription termination region
Downstream
Figure 18.8-2
Enhancer (distal control elements)
DNA
Upstream Promoter
Proximal control elements
Transcription start site
Exon Intron Exon Exon Intron
Poly-A signal sequence
Transcription termination region
Downstream Poly-A signal
Exon Intron Exon Exon Intron
Transcription
Cleaved 3 end of primary transcript
5 Primary RNA transcript (pre-mRNA)
Figure 18.8-3
Enhancer (distal control elements)
DNA
Upstream Promoter
Proximal control elements
Transcription start site
Exon Intron Exon Exon Intron
Poly-A signal sequence
Transcription termination region
Downstream Poly-A signal
Exon Intron Exon Exon Intron
Transcription
Cleaved 3 end of primary transcript
5 Primary RNA transcript (pre-mRNA)
Intron RNA
RNA processing
mRNA
Coding segment
5 Cap 5 UTR Start codon
Stop codon 3 UTR
3
Poly-A tail
P P P G AAA AAA
The Roles of Transcription Factors
To initiate transcription, eukaryotic RNA polymerase
requires the assistance of proteins called
transcription factors
General transcription factors are essential for the
transcription of all protein-coding genes
In eukaryotes, high levels of transcription of
particular genes depend on control elements
interacting with specific transcription factors
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Enhancers and Specific Transcription Factors
Proximal control elements are located close
to the promoter
Distal control elements, groupings of which
are called enhancers, may be far away from
a gene or even located in an intron
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Transcription Factors
An activator is a protein that binds to an
enhancer and stimulates transcription of a gene
Activators have two domains, one that binds
DNA and a second that activates transcription
Bound activators facilitate a sequence of
protein-protein interactions that result in
transcription of a given gene
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Enhancer Elements
Eukaryotes have enhancer elements
Regulatory proteins bind to the enhancer
elements
When the regulatory proteins are bound then
the enhancer elements increase the rate of
transcription by helping the RNA polymerase
bind and begin transcription
Fig. 16.11
Fig. 16.12
Transcription Factors
Proteins that regulate transcription in
eukaryotes are called transcription factors.
There are thousands of transcription factors,
both repressors and enhancers
Examples of common motifs found in these
proteins:
Helix-turn-helix
Zinc fingers
Leucine zipper proteins
Turn
a-helix
DNA
(a)
Finger 2
Finger 3
Finger 1
NH3+
COO-
DNA
Zinc ion
(b)
Leucine zipper
region
DNA
(c)
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Animation: Initiation of Transcription
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Figure 18.9
DNA
Activation domain
DNA-binding domain
Transcription Factors
Some transcription factors function as
repressors, inhibiting expression of a particular
gene by a variety of methods
Some activators and repressors act indirectly
by influencing chromatin structure to promote
or silence transcription
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Activators
DNA
Enhancer Distal control element
Promoter Gene
TATA box
Figure 18.10-1
Activators
DNA
Enhancer Distal control element
Promoter Gene
TATA box
General transcription factors
DNA- bending protein
Group of mediator proteins
Figure 18.10-2
Activators
DNA
Enhancer Distal control element
Promoter Gene
TATA box
General transcription factors
DNA- bending protein
Group of mediator proteins
RNA polymerase II
RNA polymerase II
RNA synthesis Transcription initiation complex
Figure 18.10-3
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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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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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
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Animation: RNA Processing
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Exons
DNA
Troponin T gene
Primary RNA transcript
RNA splicing
or mRNA
1
1
1 1
2
2
2 2
3
3
3
4
4
4
5
5
5 5
Figure 18.13
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
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Animation: mRNA Degradation
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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
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Animation: Blocking Translation
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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
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Animation: Protein Processing
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Animation: Protein Degradation
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Figure 18.14
Protein to be degraded
Ubiquitin
Ubiquitinated protein
Proteasome
Protein entering a proteasome
Proteasome and ubiquitin to be recycled
Protein fragments (peptides)
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Noncoding RNAs play multiple roles in
controlling gene expression
Only a small fraction of DNA codes for proteins, and a very small fraction of the non-protein-coding DNA consists of genes for RNA such as rRNA and tRNA
A significant amount of the genome may be transcribed into noncoding RNAs (ncRNAs)
Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration
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Effects on mRNAs by MicroRNAs and Small
Interfering RNAs
MicroRNAs (miRNAs) are small single-stranded
RNA molecules that can bind to mRNA
These can degrade mRNA or block its translation
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(a) Primary miRNA transcript
Hairpin
miRNA
miRNA
Hydrogen bond
Dicer
miRNA- protein complex
mRNA degraded Translation blocked
(b) Generation and function of miRNAs
5 3
Figure 18.15
The phenomenon of inhibition of gene expression
by RNA molecules is called RNA interference
(RNAi)
RNAi is caused by small interfering RNAs
(siRNAs)
siRNAs and miRNAs are similar but form from
different RNA precursors
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MicroRNAs and Small Interfering RNAs
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
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Figure 18.16
(a) Fertilized eggs of a frog (b) Newly hatched tadpole
1 mm 2 mm
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
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
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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
Figure 18.17a (a) Cytoplasmic determinants in the egg
Unfertilized egg
Sperm
Fertilization
Zygote (fertilized egg)
Mitotic cell division
Two-celled embryo
Nucleus
Molecules of two different cytoplasmic determinants
Cell Signaling - Induction
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
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Animation: Cell Signaling
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Figure 18.17b (b) Induction by nearby cells
Early embryo (32 cells)
NUCLEUS
Signal transduction pathway
Signal receptor
Signaling molecule (inducer)
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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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
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The Life Cycle of Drosophila
In Drosophila, cytoplasmic determinants in the
unfertilized egg determine the axes before
fertilization
After fertilization, the embryo develops into a
segmented larva with three larval stages
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Head Thorax Abdomen
0.5 mm
BODY AXES
Anterior
Left Ventral
Dorsal Right
Posterior
(a) Adult
Egg developing within ovarian follicle
Follicle cell Nucleus
Nurse cell
Egg
Unfertilized egg
Depleted nurse cells
Egg shell
Fertilization
Laying of egg
Fertilized egg
Embryonic development
Segmented embryo
Body segments
Hatching
0.1 mm
Larval stage
(b) Development from egg to larva
2
1
3
4
5
Figure 18.19
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 egg-
polarity genes because they control orientation of
the egg and consequently the fly
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Animation: Development of Head-Tail Axis in Fruit Flies
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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
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Bicoid: A Morphogen Determining Head
Structures
One maternal effect gene, the bicoid gene,
affects the front half of the body
An embryo whose mother has no functional
bicoid gene lacks the front half of its body and
has duplicate posterior structures at both
ends
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a.
b.
bicoid mRNA moves
toward anterior end
bicoid
mRNA
Movement of
maternal mRNA
Nucleus
Microtubules
Nurse
cells
Follicle
cells
nanos mRNA moves
toward posterior end
Posterior Anterior
Posterior Anterior
nanos
mRNA
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Establishment of the A/P axis
After fertilization, translation of the bicoid and
nano mRNA will create opposing gradients of
Bicoid and Nanos proteins
Fig. 19.13.a1
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.a
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Fig. 19.15.b Fig. 19.15.c
Figure 18.21
Head Tail
Tail Tail
Wild-type larva
Mutant larva (bicoid)
250 m
T1 T2 T3
A1 A2 A3 A4 A5 A6
A7
A8
A8
A7 A6 A7 A8
Figure 18.22
Bicoid mRNA in mature unfertilized egg
Bicoid mRNA in mature unfertilized egg
Fertilization, translation of bicoid mRNA
Anterior end
100 m
Bicoid protein in early embryo
Bicoid protein in early embryo
RESULTS
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
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Hunchback gene
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
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
Segmentation
Nüsslein-Volhard and Wieschaus studied segment formation
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
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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
Fig. 19.13.a2
Fig. 19.13.b2 Fig. 19.13.c2
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Fig. 19.13.d2
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
All the homeotic genes have a highly
conserved region that codes for a DNA
binding domain in the transcription factors
Therefore they are referred to as Hox
genes since they all contain the
“homeodomain”
Production of Body Plan
Drosophila HOM genes
Thorax
Antennapedia complex
Head Abdomen
Bithorax complex
Fruit fly
Fruit fly
embryo
Mouse
Hox 1
Hox 2
Hox 3
Hox 4
Mouse
embryo
a. b.
Drosophila HOM Chromosomes Mouse Hox Chromosomes
lab pb Dfd Scr Antp Ubxabd-Aabd-B
Genetic Analysis of Early Development:
Scientific Inquiry
Edward B. Lewis, Christiane Nüsslein-Volhard,
and Eric Wieschaus won a Nobel Prize in 1995
for decoding pattern formation in Drosophila
Lewis discovered the homeotic genes, which
control pattern formation in late embryo, larva,
and adult stages
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Figure 18.20
Wild type Mutant
Eye
Antenna
Leg
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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
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Types of Genes Associated with Cancer
Cancer can be caused by mutations to genes that
regulate cell growth and division
Tumor viruses can cause cancer in animals
including humans
© 2011 Pearson Education, Inc.
Proto-oncogenes
Oncogenes are cancer-causing genes
Proto-oncogenes are the corresponding
normal cellular genes that are responsible for
normal cell growth and division
Conversion of a proto-oncogene to an
oncogene can lead to abnormal stimulation of
the cell cycle
© 2011 Pearson Education, Inc.
Figure 18.23
Proto-oncogene
DNA
Translocation or transposition: gene moved to new locus, under new controls
Gene amplification: multiple copies of the gene
New promoter
Normal growth- stimulating protein in excess
Normal growth-stimulating protein in excess
Point mutation:
within a control element
within the gene
Oncogene Oncogene
Normal growth- stimulating protein in excess
Hyperactive or degradation- resistant protein
Oncogenes
Proto-oncogenes can be converted to
oncogenes by
Movement of DNA within the genome: if it ends
up near an active promoter, transcription may
increase
Amplification of a proto-oncogene: increases the
number of copies of the gene
Point mutations in the proto-oncogene or its
control elements: cause an increase in gene
expression
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Tumor-Suppressor Genes
Tumor-suppressor genes help prevent uncontrolled cell growth
Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset
Tumor-suppressor proteins
Repair damaged DNA
Control cell adhesion
Inhibit the cell cycle in the cell-signaling pathway
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23
Interference with Normal Cell-Signaling Pathways
Mutations in the ras proto-oncogene and p53 tumor-
suppressor gene are common in human cancers
Mutations in the ras gene can lead to production of
a hyperactive Ras protein and increased cell
division
© 2011 Pearson Education, Inc.
Growth factor
1
Receptor
G protein
Protein kinases (phosphorylation cascade)
NUCLEUS
Transcription factor (activator)
DNA
Gene expression
Protein that stimulates the cell cycle
Hyperactive Ras protein (product of oncogene) issues signals on its own.
(a) Cell cycle–stimulating pathway
MUTATION
Ras
GTP
P
P
P P
P
P
2
3
4
5
Ras
GTP
Figure 18.24a
P53 Gene – Tumor Suppressor Gene
Suppression of the cell cycle can be important
in the case of damage to a cell’s DNA; p53
prevents a cell from passing on mutations due
to DNA damage
Mutations in the p53 gene prevent
suppression of the cell cycle
© 2011 Pearson Education, Inc.
Figure 18.24b
(b) Cell cycle–inhibiting pathway
Protein kinases
UV light
DNA damage in genome
Active form of p53
DNA
Protein that inhibits the cell cycle
Defective or missing
transcription factor,
such as
p53, cannot
activate
transcription.
MUTATION
2
1
3
Figure 18.24c
EFFECTS OF MUTATIONS
(c) Effects of mutations
Protein overexpressed
Cell cycle overstimulated
Increased cell division
Protein absent
Cell cycle not inhibited
The Multistep Model of Cancer Development
Multiple mutations are generally needed for full-
fledged cancer; thus the incidence increases with
age
At the DNA level, a cancerous cell is usually
characterized by at least one active oncogene and
the mutation of several tumor-suppressor genes
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11/6/2011
24
Figure 18.25
Colon
Normal colon epithelial cells
Loss of tumor- suppressor gene APC (or other)
1
2
3
4
5 Colon wall
Small benign growth (polyp)
Activation of ras oncogene
Loss of tumor- suppressor gene DCC
Loss of tumor- suppressor gene p53
Additional mutations
Malignant tumor (carcinoma)
Larger benign growth (adenoma)
Inherited Predisposition and Other Factors
Contributing to Cancer
Individuals can inherit oncogenes or mutant alleles
of tumor-suppressor genes
Mutations in the BRCA1 or BRCA2 gene are found
in at least half of inherited breast cancers, and tests
using DNA sequencing can detect these mutations
© 2011 Pearson Education, Inc.
Post-transcriptional control
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
Post-transcriptional modifications
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
Post-translational Modification
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
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
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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 how eukaryotes regulate gene
transcription, including the role of TATA box,
UPEs, and enhancers, the types of transcription
factors
Know how gene expression is controlled post
transcriptionally and posttranslationally
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, Gap
genes, and homeotic genes are and what they
do.
Important Concepts
Know the basics of cell growth including the
role of growth factors, growth factor receptors
and protein kinases
What are the main types of cancer genes –
(oncogenes and tumor suppressor genes) and
what do they do, examples of each.