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PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 18

Regulation of Gene Expression

The Bacterial Genome and Its Replication

Bacterial chromosome

Circular DNA molecule

Few associated proteins

Binary Fission

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

LE 18-14

Origin of

replication

Replication fork

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

Bacterial Genome: Operons:

Genes clustered into Operons

Promoter

Operator = “on-off” switch

Genes

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

Lac operon = inducible (usually OFF)

Repressible vs. Inducible Operons

Inducible operon = usually off
Inducer inactivates the repressor  turns on transcription
Ex: lac operon

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

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

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?

Regulation of Gene expression:

Chromatin structure

Transcription initiation

Post-transcriptional/translational

(mRNA or polypeptide)

1. Chromatin Structure: Histone Modification

Chemical modification of histone tails:

Histone acetylation-

Add acetyl group (-COCH3)

Positive charge of histone neutralized

Histone acetylation-

Add acetyl group (-COCH3)

Positive charge of histone neutralized

Loosen chromatin structure transcription

DNA Methylation
Add methyl groups (-CH3) to certain DNA bases
= Condenses chromatin

EX: Inactivated X chromosome in females

Regulation of Gene expression:

Chromatin structure

Transcription initiation

Post-transcriptional/translational

(mRNA or polypeptide)

2. Regulation of Transcription Initiation

Control elements –non-coding DNA, bind proteins

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

To initiate transcription

Eukaryotic RNA polymerase requires the assistance of transcription factors

To initiate transcription

Eukaryotic RNA polymerase requires the assistance of transcription factors

An activator

= protein that binds to an enhancer and stimulates transcription of a gene

Figure 19.6

Activators bind to enhancer
DNA bending protein, mediator proteins and Transcription factors recruited

3. Transcription factors and mediator proteins bind to activators

-Transcription factors bind to promoter

-RNA polymerase binds to promoter

Repressors

= Inhibit expression of a gene

Repressors

Regulation of Gene expression:

Chromatin structure

Transcription initiation

Post-transcriptional (mRNA)

Post-translational (polypeptide)

Modifications

3. Post-transcriptional modification:
“After transcription” Splicing

Figure 19.8

Post-Transcriptional: mRNA Degradation
Life span of mRNA molecules short

Post-Translational modifications:
“After translation”

Cleavage of certain a.a.

Addition of chemical groups

Post-translational: Proteasomes

Giant protein complexes that bind protein molecules and degrade them

Figure 19.10

Multiple ubiquitin mol-

ecules are attached to a protein

by enzymes in the cytosol.

The ubiquitin-tagged protein

is recognized by a proteasome,

which unfolds the protein and

sequesters it within a central cavity.

Enzymatic components of the

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