Upload
lenhu
View
216
Download
0
Embed Size (px)
Citation preview
Slide 1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
BiologyEighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 18
Regulation of Gene Expression
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 2
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Changes in the environment can regulate cell activity
– Natural selection has favored single-celled organisms that produce only the products needed by that cell
– The fastest way is to directly control enzyme cascades
– In bacteria it is nearly as fast to change gene expression. This is controlled by the operon model –one promoter, many genes
– Eukaryotes also have coordinately controlled gene expression – but, like everything else about them it’s more complicated (more on that later....)
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 3 Fig. 18-2
trpE gene
trpD gene
trpC gene
trpB gene
trpA gene
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
Precursor
FeedbackLoop:
Enzyme activitydirectly controlled by presence or absence of substrate
Operon:
Gene expression in a cluster of functionally related genes can be coordinated by a single on-off “switch”
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 4
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Operons: the entire stretch of DNA that includes ...
• The promoter which binds RNA polymerase.
• The operator: a regulatory “switch” in the promoter that reacts to environmental change
• All of the related genes that they control
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 5
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Negative vs. Positive Operons
• Two Types of Negative, or “Repressor”, Gene Regulation
– Operons that involve negative control of genes are switched off by the active form of a repressor
– 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
• One Type of Positive, or “Activator”, Gene Regulation
– An activator of transcription is a stimulatory protein that acts to bind RNA polymerase much like an eukaryotic transcription factor
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 6
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product
– The trp operon is a repressible operon: By default it is on and the genes for tryptophan synthesis are transcribed
– Tryptophan is a corepressor and when it is present, it binds to the trp repressor protein, which turns the operon off
Example 1 of Negative, or “Repressor”, Gene Regulation
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 7 Fig. 18-3a
Polypeptide subunits that make upenzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
DNA
mRNA 5′
Protein Inactiverepressor
RNApolymerase
Regulatorygene
Promoter Promoter
trp operon
Genes of operon
OperatorStop codonStart codon
mRNA
trpA
5′
3′
trpR trpE trpD trpC trpB
ABCDE
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 8 Fig. 18-3b-1
Tryptophan present, repressor active, operon off
Tryptophan(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 9 Fig. 18-3b-2
Tryptophan present, repressor active, operon off
Tryptophan(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 10
• Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal
– The lac operon is an inducible operon and by itself, the lac repressor is active and switches the lac operon off
– Lactose is an inducer and inactivates the repressor to turn the lac operon on
Example 2 of Negative, or “Repressor”, Gene Regulation
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 11 Fig. 18-4a
Lactose absent, repressor active, operon off
DNA
Protein Activerepressor
RNApolymerase
Regulatorygene
Promoter
Operator
mRNA5′
3′
NoRNAmade
lacI lacZ
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 12 Fig. 18-4b
Lactose present, repressor inactive, operon on
mRNA
Protein
DNA
mRNA 5′
Inactiverepressor
Allolactose(inducer)
5′
3′RNApolymerase
Permease Transacetylase
lac operon
β-Galactosidase
lacYlacZ lacAlacI
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 13
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Positive Gene Regulation
• Positive secondary control of the speed of an operon
• Sometimes lactose is present but glucose is scarce
• Catabolite Activator Protein (CAP) is activated by binding with cyclic AMP
• Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription
• When glucose levels increase, CAP detaches from the lacoperon, and transcription returns to a normal rate
• CAP helps regulate other operons that encode enzymes used in catabolic pathways
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 14 Fig. 18-5
Lactose present, glucose present (cAMP level low):
less lac mRNA synthesized
cAMP
DNA
Inactive lacrepressorInactive
CAP
lacICAP-binding site
Promoter
ActiveCAP
Operator
lacZ
RNA polymerase bindsand transcribes
Inactive lacrepressor
lacZ
OperatorPromoter
DNA
CAP-binding site
lacI
RNA polymerase lesslikely to bind
InactiveCAP
Lactose present, glucose scarce (cAMP level high):
abundant lac mRNA synthesized
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 15
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Differential gene expression in multicellular development and cell specialization
– Regulation of Chromatin Structure
– Regulation of Transcription Initiation
– Mechanisms of Post-Transcriptional Regulation
– Mechanisms of Post-Translational Regulation
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 16 Fig. 18-6a
DNA
Signal
Gene
NUCLEUS
Chromatin modification
Chromatin
Gene availablefor transcription
Exon
Intron
Tail
RNA
Cap
RNA processing
Primary transcript
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Transcription
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 17 Fig. 18-6b
mRNA in cytoplasm
Translation
CYTOPLASM
Degradationof mRNA
Protein processing
Polypeptide
Active protein
Cellular function
Transport to cellulardestination
Degradationof protein
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 18
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Regulation of Chromatin Structure
• Genes within highly packed heterochromatin are usually not expressed
• Chemical modifications to histones and DNA influence chromatin structure and gene expression by making a region of DNA either more or less able to bind the transcription machinery
• The histone code hypothesis proposes that specific combinations of modifications help determine chromatin configuration and influence transcription
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 19 Fig. 18-7
Histonetails
DNAdouble helix
(a) Histone tails protrude outward from anucleosome
Acetylated histones
Aminoacidsavailablefor chemicalmodification
Unacetylated histones
In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails
This process loosens chromatin structure, thereby initiating 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
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 20
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 21
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 22 Fig. 18-8-1
Enhancer(distal control elements)
Proximalcontrol elements
Poly-A signalsequence
Terminationregion
DownstreamPromoterUpstreamDNA
ExonExon ExonIntron Intron
Organization of a Typical Eukaryotic Gene
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 23 Fig. 18-8-2
Enhancer(distal control elements)
Proximalcontrol elements
Poly-A signalsequence
Terminationregion
DownstreamPromoterUpstreamDNA
Exon Exon ExonIntronIntron Cleaved 3′ endof primarytranscript
Primary RNAtranscript
Poly-Asignal
Transcription
5′
ExonExon ExonIntron Intron
Organization of a Typical Eukaryotic Gene
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 24 Fig. 18-8-3
Enhancer(distal control elements)
Proximalcontrol elements
Poly-A signalsequence
Terminationregion
DownstreamPromoterUpstreamDNA
ExonExon ExonIntron Intron
Exon Exon ExonIntronIntron Cleaved 3′ endof primarytranscript
Primary RNAtranscript
Poly-Asignal
Transcription
5′RNA processing
Intron RNA
Coding segmentmRNA
5′ Cap 5′ UTRStart
codonStop
codon 3′ UTR Poly-Atail
3′
Organization of a Typical Eukaryotic Gene
What do the GTP Cap and Poly-A Tail do again?
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 25 Fig. 18-9-1
Enhancer TATAbox
PromoterActivatorsDNA
Gene
Distal controlelement
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 26 Fig. 18-9-2
Enhancer TATAbox
PromoterActivatorsDNA
Gene
Distal controlelement
Tissue-specifictranscription factors
DNA-bendingprotein
Generaltranscriptionfactors
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 27 Fig. 18-9-3
Enhancer TATAbox
PromoterActivatorsDNA
Gene
Distal controlelement
Tissue-specifictranscription factors
DNA-bendingprotein
Generaltranscriptionfactors
RNApolymerase II
RNApolymerase II
Transcriptioninitiation complex RNA synthesis
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 28 Fig. 18-10
Controlelements
Enhancer
Availableproteins
Albumin gene
(b) Lens cell
Crystallin geneexpressed
Availableproteins
LENS CELLNUCLEUS
LIVER CELLNUCLEUS
Crystallin gene
Promoter
(a) Liver cell
Crystallin genenot expressed
Albumin geneexpressed
Albumin genenot expressed
A particular combination of control elements can activate transcription only when the appropriate activator proteins and transcription factors are present
Unlike prokaryotes, coordinately controlled eukaryotic genes can be scattered over different chromosomes, but each has the same combination of control elements
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 29 Fig. 18-11
or
RNA splicing
mRNA
PrimaryRNAtranscript
Troponin T gene
Exons
DNA
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 30 Fig. 18-12
Proteasomeand ubiquitinto be recycledProteasome
Proteinfragments(peptides)Protein entering a
proteasome
Ubiquitinatedprotein
Protein tobe degraded
Ubiquitin
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
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 31
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Noncoding RNAs can control gene expression through RNA Interference (RNAi).
• Only a small fraction of DNA codes for proteins, rRNA, and tRNA. A significant amount of the genome may be transcribed into noncoding RNAs
• MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA and degrade it or block its translation
• siRNAs and miRNAs are similar but form from different RNA precursors
• siRNAs play a role in heterochromatin formation and can block large regions of the chromosome
• Small RNAs may also block transcription of specific genes
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 32 Fig. 18-UN5
Chromatin modification
RNA processing
TranslationmRNAdegradation
Protein processingand degradation
mRNA degradation
• miRNA or siRNA can target specific mRNAsfor destruction.
• miRNA or siRNA can block the translationof specific mRNAs.
Transcription
• Small RNAs can promote the formation ofheterochromatin in certain regions, blocking transcription.
Chromatin modification
Translation
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 33 Fig. 18-13
miRNA-proteincomplex(a) Primary miRNA transcript
Translation blocked
Hydrogenbond
(b) Generation and function of miRNAs
Hairpin miRNA
miRNA
Dicer
3′
mRNA degraded
5′
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 34 Fig. 18‐14
Fertilized eggs are single cells This tadpole has neurons, muscle, gills, fins, etc.
The differentiation of cell types in a multicellular organism results from orchestrated changes in gene expression
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 35 Fig. 18‐15a
Two differentcytoplasmicdeterminants
Unfertilized egg cell
Sperm
Fertilization
Zygote
Mitoticcell division
Two‐celledembryoshave twodifferentcell types
Nucleus
An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 36 Fig. 18‐15b
Signalmolecule(inducer)
Signaltransductionpathway
Early embryo(32 cells)
NUCLEUS
Signalreceptor
In the process called induction, signal molecules from nearby cells cause transcriptional changes in embryonic target cells
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 37 Fig. 18‐17b
Follicle cellNucleus
Eggcell
Nurse cell
Egg celldeveloping withinovarian follicle
Unfertilized egg
Fertilized egg
Depletednurse cells
Eggshell
FertilizationLaying of egg
Bodysegments
Embryonicdevelopment
Hatching
0.1 mm
Segmentedembryo
Larval stage
1
2
3
4
5
Ovary cells can tell the unfertilized egg which end is which
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 38 Fig. 18‐19c
bicoid mRNA
Nurse cells
Egg
Developing egg Bicoid mRNA in matureunfertilized egg
Bicoid proteinin early embryo
Bicoid: Nurse cells help localize the mRNA for a protein that is required to make head structures
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 39 Fig. 18‐19a
T1 T2T3
A1 A2 A3 A4 A5 A6A7
A8
A8A7 A6 A7
Tail
TailTail
Head
Wild‐type larva
Mutant larva (bicoid)
EXPERIMENT
A8
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 40 Fig. 18‐16‐1
Embryonicprecursor cell
Nucleus
OFF
DNA
Master regulatory gene myoD Other muscle‐specific genes
OFF
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 41 Fig. 18‐16‐2
Embryonicprecursor cell
Nucleus
OFF
DNA
Master regulatory gene myoD Other muscle‐specific genes
OFF
OFFmRNA
MyoD protein(transcriptionfactor)
Myoblast(determined)
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
Slide 42 Fig. 18‐16‐3
Embryonicprecursor cell
Nucleus
OFF
DNA
Master regulatory gene myoD Other muscle‐specific genes
OFF
OFFmRNA
MyoD protein(transcriptionfactor)
Myoblast(determined)
mRNA mRNA mRNA mRNA
Myosin, othermuscle proteins,and cell cycle–blocking proteinsPart of a muscle fiber
(fully differentiated cell)
MyoD Anothertranscriptionfactor
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________
___________________________________