Anticipatory Questions

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Anticipatory Questions. 1.What might happen if an organism had its cells expressing all genes within the genome all the time? 2.At what levels can control of cellular activities/pathways be controlled? - PowerPoint PPT Presentation

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video slideAnticipatory Questions
1. What might happen if an organism had its cells expressing all genes within the genome all the time?
2. At what levels can control of cellular activities/pathways be controlled?
3. Based on our discussions up to this point, what do you think the term “negative feedback” means?
4. What steps are involved in the initiation of prokaryotic transcription?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Learning Objectives
understand that regulation of gene expression is a means by which to control timing and rate of generation regarding functional gene product (either RNA or polypeptide/protein).
explain the concept of an operon in terms of components’ functions (promoter, operator, repressor, co-repressor, inducer, gene cluster, polycistronic transcript).
compare and contrast repressible and inducible operon systems/pathways.
compare and contrast negative versus positive regulation of operons
apply the operon concept to gene expression as it relates to genetic engineering (specifically, our cloning and expression of the “tomato” gene).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Individual bacteria respond to environmental change by regulating their gene expression
A bacterium can tune its metabolism to the changing environment and food sources
This metabolic control occurs on two levels:
Adjusting activity of metabolic enzymes
Regulating genes that encode metabolic enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Operons: The Basic Concept
In bacteria, genes are often clustered into operons, composed of
An operator, an “on-off” switch
A promoter
Genes for metabolic enzymes
An operon can be switched off by a protein called a repressor
A corepressor is a small molecule that cooperates with a repressor to switch an operon off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Polycistronic*
mRNA
trpE
trpD
trpC
trpB
trpA
Operator
D
C
B
A

* = mRNA carries the information of several genes, which are translated into several proteins
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
mRNA
Active
repressor
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
mRNA
Active
repressor
Trp Operon Animation
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
The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
lacZ
lacY
lacA
RNA
polymerase
Polycistronic
mRNA
Protein
Allolactose
(inducer)
Inactive
repressor
-Galactosidase
Permease
Transacetylase

Lac Operon Animation
Inducible enzymes usually function in catabolic pathways
Repressible enzymes usually function in anabolic pathways
Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Positive Gene Regulation
Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP)
When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP
When glucose levels increase, CAP detaches from the lac operon, turning it off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
mRNA synthesized
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
mRNA synthesized
Catabolite Activator Protein Mechanism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Arabinose Operon - A Composite of Negative & Positive Regulation
a) In the presence of arabinose:
CAP-cAMP complex and araC-arabinose complex bind to initiator region
this allows RNA polymerase to bind to the promoter
transcription begins
acts as a repressor
forms a loop
Regulatory gene
araC regulatory gene
Tomato gene