1. Gene Expression
AP Biology

2. Questions to Ponder…..
How do your cells “know” what kind of cell they are?
How do your cells “know” when to make a particular protein? When to stop making it?
How does the environment affect your cells?
ANSWER: Gene Expression

3. What makes cells from the same individual look different?
Stem Cells
Liver Cells
Red Blood Cells
Cartilage Cells
DNA sequence in each cell is the same, but different cell types have different “GENE EXPRESSION PATTERNS”

4. Each cell type has a unique gene expression profile. Insulin DNA?
When a gene is “on” and its protein or RNA product is being made, scientists say that the gene is being EXPRESSED.
The on and off states of all of a cell’s genes is known as a GENE EXPRESSION PROFILE.
Each cell type has a unique gene expression profile.
Insulin
DNA?
Protein?
Muscle Cell X
Pancreatic Cell
Slide adapted from Genetic Science Learning Center, University of Utah  2013

5. Gene Expression in Bacteria
Bacteria are single-celled organisms who are surrounded on all sides by their environment.
They must be able to regulate expression of their genes in response to environmental changes.

6. Bacteria Respond by Regulating Transcription
Bacteria cells that can conserve resources and energy have a selective advantage over cells that cannot do so.
Natural selection has favored bacteria that express only the genes they need.

7. E. Coli Regulation of Tryptophan
An individual E. coli cell living in the erratic environment of the human colon, is dependent for its nutrients on the whimsical eating habits of its host—you!
If the environment is lacking in the amino acid tryptophan, which the bacterium needs to survive, it responds by activating a metabolic pathway that makes tryptophan from another compound.
If tryptophan becomes available, it shuts down this pathway.

8. Regulation of a Metabolic Pathway
In the pathway for tryptophan synthesis, an abundance of tryptophan can both inhibit the activity of the first enzyme (a rapid response) OR repress expression of the genes encoding the enzymes in the pathway (a longer response).
This is an example of feedback inhibition. It allows for a cell to adapt to short-term fluctuation in the supply of a substance it needs.

9. (a) Regulation of enzyme activity (b) Regulation of enzyme production
Fig. 18-2
Precursor
Feedback
inhibition
trpE gene
Enzyme 1
trpD gene
Regulation
of gene
expression
Enzyme 2
trpC gene
trpB gene
Figure 18.2 Regulation of a metabolic pathway
Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
(b) Regulation of enzyme
production

10. Gene Expression Controls Which Enzymes are Made and When
In many cases, this occurs in the process of transcription.
Many genes may be switched on or off by changes in the metabolic status of the cell.
One example was discovered in 1961 by Francois Jacob and Jacques Monod at the Pasteur Institute in Paris. This method is called the Operon Model.

11. Operons: The Basic Concept
A cluster of functionally related genes that can be under coordinated control by a single on-off “switch”.
The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter.
An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control.

12. The operon can be switched off by a protein 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

13. The repressor can be in an active or inactive form, depending on the presence of other molecules.
A corepressor is a molecule that cooperates with a repressor protein to switch an operon off.

14. Bacteria can synthesize tryptophan by utilizing the trp operon.
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 turns the operon off
The repressor is active only in the presence of its co-repressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high

15. Polypeptide subunits that make up enzymes for tryptophan synthesis
Fig. 18-3a
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
trpE
trpD
trpC
trpB
trpA
Regulatory
gene
Operator
Start codon
Stop codon 3
mRNA 5
mRNA
RNA
polymerase 5 E D C B A
Protein
Inactive
repressor
Polypeptide subunits that make up
enzymes for tryptophan synthesis
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes
(a) Tryptophan absent, repressor inactive, operon on

16. (b) Tryptophan present, repressor active, operon off
Fig. 18-3b-1
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes
(b) Tryptophan present, repressor active, operon off

17. (b) Tryptophan present, repressor active, operon off
Fig. 18-3b-2
DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes
(b) Tryptophan present, repressor active, operon off

18. Video for Gene Expression

19. Different Types of Operons
A repressible operon is one that is usually ON—binding a repressor to the operator turns off transcription. (The trp operon is like this)
An inducible operon is one that is usually OFF—a molecule called an inducer inactivates the repressor and starts transcription. (The lac operon is this type)

20. The lac Operon
The lac operon is an inducible operon (usually off) and contains genes that code for enzymes that break down the sugar lactose (found in dairy products)
By itself, the lac repressor is active and therefore shuts the lac operon off most of the time.
A molecule called an inducer inactivates this repressor which turns the lac operon on.

21. (a) Lactose absent, repressor active, operon off
Fig. 18-4
Regulatory
gene
Promoter
Operator
DNA
lacI
lacZ No
RNA
made 3
mRNA
RNA
polymerase 5
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off
lac operon
DNA
lacI
lacZ
lacY
lacA
RNA
polymerase
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
For the Cell Biology Video Cartoon Rendering of the lac Repressor from E. coli, go to Animation and Video Files. 3
mRNA
mRNA 5 5
-Galactosidase
Permease
Protein
Transacetylase
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on

22. (a) Lactose absent, repressor active, operon off
Fig. 18-4a
Regulatory
gene
Promoter
Operator
DNA
lacI
lacZ No
RNA
made 3
mRNA
RNA
polymerase 5
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off

23. (b) Lactose present, repressor inactive, operon on
Fig. 18-4b
lac operon
DNA
lacI
lacZ
lacY
lacA
RNA
polymerase 3
mRNA
mRNA 5 5
-Galactosidase
Permease
Transacetylase
Protein
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on

24. Inducible Enzymes
Inducible enzymes (such as those found in the lac operon) are usually catabolic enzymes, which means they break things apart.
Their synthesis is usually induced by some kind of signal.
In the lac operon, the signal is the presence of the lactose sugar molecule.

25. Repressible Enzymes
Repressible enzymes (such as those in the trp operon) usually function in anabolic pathways which build things or put things together.
Since these are almost always ON, they are repressed (shut down) when there are high levels of the end-product present.