1. Regulation of Gene Expression
SBI4U1
Regulation of Gene Expression

2. Gene Regulation: control of the level of gene expression -whether a gene is active or inactive -determines the level of activity and the amount of protein that is available. Constitutive Genes: genes that are always active and expressed at constant levels “ housekeeping genes”

3. Transcription * most common during initiation Translation
Regulation of gene expression in prokaryotes occurs at three different levels
Transcription * most common during initiation
Translation
After protein has been synthesized

4. Recall? Promoter region is where RNA polymerase binds to DNA to initiate trancription
What is an Operon?
A cluster of genes under the control of 1 promoter
-these genes are all transcribed together
Let’s use E. Coli as an example…they use different sugars as a source of energy

5. Genes that encode the enzymes needed to break down the sugar lactose
The “Lac Operon”
Genes that encode the enzymes needed to break down the sugar lactose
Composed of 2 regions:
regulatory region & coding region
Enzymes which break down lactose
Promoter
Operator Z Y A
Controls whether gene in transcribed or not
RNA polymerase binds here to start making mRNA copy

6. Here’s how it works…
In the absence of lactose, the lac repressor protein binds to the operator, preventing RNA polymerase from binding to the promoter to initiate transcription.
In the presence of lactose, an activator binds to the repressor so it can no longer bind to the operator. Transcription of the genes can therefore occur.

7. If tryptophan is present; no transcription
The “Trp Operon”
If tryptophan is present; no transcription
Genes synthesize tryptophan. The operon is transcribed until sufficient tryptophan is present.
Promoter
Operator E D C B A
Trp repressor binds to promoter and inhibits transcription

8. “Trp Operon” contd..
The trp (tryptophan) operon, however, is normally active until a repressor turns it off.
It contains:
 a coding region with five genes for enzymes required for tryptophan synthesis
a regulatory region with a promoter and an operator

9. The trp operon contains five genes that are involved in the synthesis of tryptophan. This operon is normally transcribed until the cell has sufficient tryptophan. Then the trp repressor binds to the promoter and inhibits transcription.

10. Regulation in Eukaryotes
5 levels of regulation:
Pre-transcriptional
Transcriptional
Post-transcriptional
Translational
Post-translational

11. Pre-Transcriptional/Transcriptional
DNA in highly condensed chromatin is not transcribed
Chromatin is physical barrier to proteins needed to synthesize pre-mRNA
Can be loosened so transcription can occur via chromatin remodelling complexes
Transcription factors are a set of proteins needed to RNA polymerase to bind to promoter
Activators bind to transcription factors, RNA polymerase, enhancers

12. Post-Transcriptional/Translation
mRNA level
5’ cap/poly-A tail may not be added
mRNA does not leave nucleus
Or is degraded in cell
Small RNA molecules control gene expression via RNA interference
Micro RNA (miRNA) and small interfering RNA (siRNA) turn off gene expression

13. Post-Translational Polypeptide level
They must be activated by modifications
E.g. Insulin is folded into 3D structure
Removal of amino acids activates it
Leaves two polypeptide chains
Regulating length of protein in cell also regulates the gene
Adding a chain of
molecules can signal protein degradation

14. Learning Expectations...
What is gene regulation?
Where does it occur in a cell?
How do the Lac and Trp operons work?
Compare/contrast
Difference in gene regulation in prokaryotes and eukaryotes