Control of Transcription
DNA has “on” and “off” switches
Activator –protein that binds near gene’s promoter region
- allows RNA polymerase to transcribe (allows it to fit)
Repressor – protein that binds to DNA and prevents RNA polymerase from binding
-coded for by “regulator” gene
The Operon Model
Operon – region of DNA with group of genes for proteins with related functions
(see diagram)
-All of the genes in the operon are controlled by activity at the promoter & the operator
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Promoterfor I gene
Gene forrepressor protein
Regulatory region Coding region
CAP-bindingsite
Gene forpermease
Operator
Promoter forlac operon
Gene for-galactosidase
Gene fortransacetylase
PI CAP O
ZY A
PlacI
lac control system
The “Lac” Operon-in E. coli bacteria-genes for enzymes to break down lactose sugar
1. Regulator gene codes for Lac repressor protein2. Repressor binds with operator region
If no lactose present:3. RNA polymerase can not bind to promoter, no enzyme made
and operon is “off”If lactose is present:
3. Lactose binds with repressor, repressor no longer binds to operator
4. RNA polymerase transcribes the structural genes• Translation occurs, enzymes are made• Lactose is metabolized
*Conservation of resources – enzymes only made when needed*
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RepressorCAP
CAP
Promoter OperatorcAMP
RNApolymerase
OY
A
IZ
lac operon is "repressed"
Plac
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lac operon is "induced"
CAP
CAP
Promoter Operator
Allolactose(inducer)
cAMP
Plac
RNApolymerase
OY
A
IZ
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mRNA synthesis
CAP-bindingsite
RNA-polymrasebinding site(promoter) Operator
lacZ gene
Operon ? because CAP is not bound
Operon ? both because lac repressor is bound and CAP is not
Operon ? because lac repressor is bound
Operon ? because CAP is bound and lacrepressor is not
RNA polymerase
Repressor
RNA polymerase
CAP
CAP
+
+
+
+G
luco
se
Lac
tose
–
––
–
RNA in Gene regulation“Small RNA’s”
Process of RNA interference (RNAi) or “RNA silencing”-double stranded RNA is cut into small pieces by enzyme called dicer-pieces then unravel into single strands
1. miRNA – micro RNA-binds to a specific RNA thus blocking it from
being translated-reversible2. siRNA – small interfering RNA-binds to specific mRNA, destroys it
Purposes: (evolutionary origins)1. Inactivate viral DNA transcription2. Inactivate transposons – renegade “jumping genes”3. Regulation
Nucleosomes
X- Inactivation