1. The Chapter 15 Homework is due Wednesday, January 30 at 11:59 pm.
The Chapter 15 Test is Thursday, January 31
There will be a POGIL Quiz tomorrow

2. Overview of Prokaryotic Gene Regulation
Chapter 15
Overview of Prokaryotic Gene Regulation

3. You Must Know
A regulatory gene is a sequence of DNA that codes for a regulatory protein such as a repressor protein.
How the components of an operon function to regulate gene expression.

4. Prokaryotic Gene Regulation
Natural selection has favored bacteria that produce only the gene products needed by the cell.

5. A cell can regulate the production of enzymes by feedback inhibition or by gene regulation.

6. Polypeptide subunits that make up enzymes for doing a particular task
Gene expression in bacteria is controlled by operons.
Operons are a group of functionally related genes that can be coordinately controlled by a single “on-off switch”. The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter.
operon
Promoter
Operator
RNA
polymerase
Stop codon
An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control. .
Start codon E D C B A
Polypeptide subunits that make up
enzymes for doing a particular task 6

7. Polypeptide subunits that make up enzymes for doing a particular task
The operon can be switched off by a protein repressor.
Promoter
Regulatory
gene
Polypeptide subunits that make up
enzymes for doing a particular task E D C B A
RNA
polymerase
mRNA
Promoter
operon
Genes of operon
Operator
mRNA
Protein
Inactive
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. 7

8. The repressor can be in an active or inactive form, depending on the presence of other molecules.
DNA
mRNA
Protein
Active
repressor
No RNA
made
(corepressor)
A corepressor is a molecule that cooperates with a repressor protein to switch an operon off. 8

9. Tryptophan Tryptophan Operon Promoter Genes of operon DNA Regulatory
mRNA
Protein
RNA
polymerase
Inactive
repressor
Operator
Start codon
Stop codon
mRNA 5
trpE
trpD
trpC
trpB
trpA E D C B A
(a) Tryptophan absent, repressor inactive, operon on
Polypeptide subunits that make up
enzymes for tryptophan synthesis 5 3
trpR
Promoter
Genes of operon
RNA
polymerase
Operator
Start codon
Stop codon
mRNA 5
trpE
trpD
trpC
trpB
trpA E D C B A
Polypeptide subunits that make up
enzymes for tryptophan synthesis
trp operon
DNA
mRNA
Protein
Active
repressor
No RNA
made
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
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 then 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.
Tryptophan 9