1. Gene Expression

2. Gene Expression
The overall process by which genetic information flows from gene to protein
Genotype  Phenotype
Gene expression is regulated by the turning on and off of transcription
The control of gene expression makes it possible for cells to produce specific proteins when and where they are needed.

3. Just a reminder…

4. Gene expression in prokaryotes

5. Bacterial metabolism
Bacteria need to respond quickly to changes in their environment
if they have enough of a product, need to stop production
why? waste of energy to produce more
how? stop production of enzymes for synthesis
if they find new food/energy source, need to utilize it quickly
why? metabolism, growth, reproduction
how? start production of enzymes for digestion
STOP GO

6. Genes in prokaryotes are controlled by regulatory units called operons
Operon: cluster of related genes with on/off switch
example: all enzymes in a metabolic pathway
Operons consist of:
Genes:
Entire gene segment/segments
Promoter
Site where RNA polymerase binds and begins transcription
Operator
Acts as the switch…on or off. Either blocks RNA poly or allows access
Operons only exist in prokaryotes
Adjacent to the gene sequences are two control sequences, short sections of DNA that help control the enzyme gnees.
One is the promoter- site where transcription enzyme, RNA polymerase binds and initiates transcription
Between the promoter and genes is a DNA segment called the operator which acts as a switch. The operator determines whether RNA polymerase can attach to the promoter and start transcribing the genes.

7. So how can these genes be turned off?
Repressor protein
binds to DNA at operator site
blocking RNA polymerase
blocks transcription

8. Regulatory proteins turn operons ON or OFF
Regulatory proteins bind to control sequences in the DNA and turn operons on or off in response to environmental changes
Ex. E.coli changes its activities in response to changes in its environment.
Picture an e.coli living in your intestine. If you only eat a doughnut for breakfast, the bacterium will be bathed in sugars glucose and fructose and the digestion products of fats.
Later, if you have a glass of fat-free milk and salad for lunch, E.coli’s environment will change drastically.
VS.

9. The LAC operon: Inducible Operon
When lactose is plentiful in the environment, E.coli will turn on the gene for synthesizing a protein that absorbs the sugar as a fuel source
E. coli can turn the lactose enzyme on or off depending on whether lactose is present
When not present, E.coli doesn’t waste energy making the unnecessary enzymes
Inducible Operon
Normally off
When food available, turns on and breaks it down for energy
Inducible- catabolic- break down food for energy

10. The lac operon off
OPERON
Regulatory gene
Promoter
Operator
Lactose-utilization genes
DNA
mRNA
RNA polymerase cannot attach to promoter
Protein
Active repressor
OPERON TURNED OFF (lactose absent)
When lac operon is in the “off” mode, transcription is turned off by a repressor
protein that binds to the operator, blocking the attachment of RNA polymerase
Repressors are made from the regulatory gene located outside of the operon

11. Enzymes for lactose utilization
How do we turn it on?
When lactose is IN the environment, it will attach to the repressor, changing its shape.
With its new shape, the repressor cannot bind to the operator and the operatory switch remains ON
RNA polymerase binds to the promoter and the enzymes to utilize lactose are made
OPERON TURNED OFF (lactose absent)
OPERON
Promoter
Regulatory gene
Operator
Lactose-utilization genes
DNA
RNA polymerase bound to promoter
mRNA
Protein
Inactive repressor
Lactose
Enzymes for lactose utilization
OPERON TURNED ON (lactose inactivates repressor)

12. The (trp) Operon: Repressible operon
Tryptophan is a molecule needed in the body.
However, if it accumulates in your body, then it is a waste to create more.
Repressible Operon
Normally on and builds organic molecules
When tryptophan is in environment, shuts down operon because why make more?
Co-repressor binds to the allosteric site on the repressor and activates it
-repressor binds to DNA, stopping all transcription
-co-repressor is often the product of the enzymatic activity.

13. trp operon When Tryptophan is absent -repressor inactive -operon is ON
What happens when tryptophan is present?
Don’t need to make tryptophan-building enzymes

14. trp operon What happens when tryptophan is present?
Don’t need to make tryptophan-building enzymes

15. Operon summary Repressible operon (trp) Inducible operon (lac)
usually functions in creating products
when end product is present in excess, cell allocates resources to other uses
Inducible operon (lac)
usually functions in digesting products,
produce enzymes only when nutrient is available
cell avoids making proteins that have nothing to do, cell allocates resources to other uses

16. Repressible Operon (trp) (ON  OFF)
Inducible Operon (lac) (OFF  ON)

17. Eukaryotic Gene Expression
In eukaryotes, gene expression is complex and control involves regulatory genes, regulatory elements and transcription factors that act in concert.

18. Then why are cells different?
In general, all somatic cells of a multicellular organism have the same genes
Then why are cells different?
For example:
Skin cell and nerve cell have the same genes, yet they are so different…why?
Each cell type is expressing certain genes that are present in, but not expressed in, the other cell type.

19. Cell Differentiation= regulation of genes

20. DNA packaging in eukaryotic chromosomes helps regulate gene expression
DNA double helix (2-nm diameter)
DNA packaging in eukaryotic chromosomes helps regulate gene expression
Nucleosomes
1st level of DNA packing
histone proteins
8 protein molecules
positively charged amino acids
bind tightly to negatively charged DNA
Histones
“Beads on a string”
Nucleosome (10-nm diameter)
Other proteins in the nucleus control how tightly the histones bind to the DNA
The packing and unpacking of DNA make a region of DNA either more or less available for transcription.
Tight helical fiber (30-nm diameter)
Supercoil (200-nm diameter)

21. DNA packing as gene control
When DNA is tightly coiled with histones, it prevents gene expression by preventing RNA polymerase from contacting the DNA
For a gene to be transcribed, the histones must loosen their grip on DNA

22. Transcription initiation
Control regions on DNA
Promoter
nearby control sequence on DNA
Contains TATA box
Transcription factors will bind to the TATA box and attract RNA polymerase to begin transcription
Enhancers
distant control sequences on DNA
binding of activator proteins
Act as the GO SIGNAL!
transcription factors (regulatory proteins) must assemble on the chromosome before RNA polymerase can bind to the promoter.

23. Model for Enhancer action
Enhancer DNA sequences
distant control sequences
Activator proteins
bind to enhancer sequence & stimulates transcription
When activator proteins are bound to enhancer, DNA bends and a complex is made at the promoter region = begin transcription
Each gene has its own promoter…default state for most genes is off
A typical animal or plant cell needs to turn on only a small percentage of its genes, those required for the cell’s specialized structure and function.
Activators bind to DNA sequences called enhancers, DNA bends as a response and activators interact with other proteins = complex at promoter = start of transcription
Silencer proteins are, in essence, blocking the positive effect of activator proteins, preventing high level of transcription.
Negative regulatory sequences or silencer sequences turn off transcription by binding repressor proteins.
Transcription video

24. Post-transcriptional control
Alternative RNA splicing
variable processing of exons creates different mRNA molecules
Proteins manufactured are related
Ex: muscle cell producing two proteins actin and myosin