Transcript ch18 - Homework Market

Chapter 18
Regulation of Gene
Expression
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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The Bacterial Genome and Its Replication
• Bacterial chromosome
– Circular DNA molecule
– Few associated proteins
• Binary Fission
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LE 18-14
Replication fork
Origin of
replication
Termination
of replication
Concept 18.4: Individual bacteria respond to
environmental change by regulating their gene expression
• Bacterial metabolism can change with changing
environment and food sources
• Metabolic control on two levels:
– Adjusting activity of metabolic enzymes
– Regulating genes that encode metabolic
enzymes
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Bacterial Genome: Operons:
• Genes clustered into Operons
– Promoter
– Operator = “on-off” switch
– Genes
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bacterial Genome: Operons:
• Genes clustered into Operons
– Operator = “on-off” switch
– Promoter
– Genes
• Repressor- Protein that can switch off Operator
• Inducer- Small molecule that cooperates with a
repressor to switch an Operon off
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Lac operon = inducible (usually OFF)
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Repressible vs. Inducible Operons
• Inducible operon = usually off
• Inducer inactivates the repressor  turns on
transcription
• Ex: lac operon
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Repressible vs. Inducible Operons
• Inducible operon = usually off
• Inducer inactivates the repressor  turns on
transcription
• Ex: lac operon
• Repressible operon = usually on
• binding of a repressor to the operator shuts off
transcription
• Ex: trp operon
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• Chromatin = DNA + proteins
– Euchromatin- chromatin in non-condensed
state (~packaging level 2)
– Heterochromatin- chromatin in highly
condensed state (always packaging level 4)
• What DNA would be found in this state?
– Skin vs. Nerve cell
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Concept 19.2: Regulation of gene expression
• Each cell type expresses only a fraction of its
genes (skin vs. muscle cells)
• All organisms regulate which genes are expressed
HOW?
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Regulation of Gene expression:
1. Chromatin structure
2. Transcription initiation
3. Post-transcriptional/translational
(mRNA or polypeptide)
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1. Chromatin Structure: Histone Modification
• Chemical modification of histone tails:
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• Histone acetylation-
Add acetyl group (-COCH3)
Positive charge of histone neutralized
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• Histone acetylation-
Add acetyl group (-COCH3)
Positive charge of histone neutralized
Loosen chromatin structure transcription
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• DNA Methylation
• Add methyl groups (-CH3) to certain DNA bases
• = Condenses chromatin
• EX: Inactivated X chromosome in females
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Regulation of Gene expression:
1. Chromatin structure
2. Transcription initiation
3. Post-transcriptional/translational
(mRNA or polypeptide)
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2. Regulation of Transcription Initiation
• Control elements –non-coding DNA, bind
proteins
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Enhancers and Specific Transcription Factors
• Proximal control elements
= close to the promoter
• Distal control elements = enhancers
– May be far away from a gene or even in
an intron
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• To initiate transcription
– Eukaryotic RNA polymerase requires the
assistance of transcription factors
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• To initiate transcription
– Eukaryotic RNA polymerase requires the
assistance of transcription factors
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• An activator
= protein that binds to an enhancer and
stimulates transcription of a gene
Figure 19.6
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#1
#2
1. Activators bind to enhancer
2. DNA bending protein, mediator proteins and
Transcription factors recruited
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#1
3. Transcription
factors and mediator
proteins bind to
activators
#2
-Transcription factors
bind to promoter
#3
-RNA polymerase
binds to promoter
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• Repressors
= Inhibit expression of a gene
Repressors
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Regulation of Gene expression:
• Chromatin structure
• Transcription initiation
• Post-transcriptional (mRNA)
Post-translational (polypeptide)
Modifications
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• 3. Post-transcriptional modification:
• “After transcription” Splicing
Figure 19.8
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• Post-Transcriptional: mRNA Degradation
• Life span of mRNA molecules short
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• Post-Translational modifications:
• “After translation”
– Cleavage of certain a.a.
– Addition of chemical groups
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• Post-translational: Proteasomes
– Giant protein complexes that bind protein
molecules and degrade them
3 Enzymatic components of the
1 Multiple ubiquitin molecules are attached to a protein
by enzymes in the cytosol.
2 The ubiquitin-tagged protein
is recognized by a proteasome,
which unfolds the protein and
sequesters it within a central cavity.
Figure 19.10
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proteasome cut the protein into
small peptides, which can be
further degraded by other
enzymes in the cytosol.
• Ch. 18 Bacterial genomes
• Basic replication
• Lac Operon
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