Download Regulation of Gene Expression

SBI4U1
Regulation of Gene Expression
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”
Regulation of gene expression in prokaryotes
occurs at three different levels
• Transcription * most common during initiation
• Translation
• After protein has been synthesized
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
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
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.
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
Trp repressor binds to promoter
and inhibits transcription
B
A
“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
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.
Regulation in Eukaryotes
• 5 levels of regulation:
– Pre-transcriptional
– Transcriptional
– Post-transcriptional
– Translational
– Post-translational
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
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
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
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