Transcript Operons-Prokaryotes - Solon City Schools

Horizontal gene transfer

Transmission of DNA between species
Transformation
 Transduction
 Conjugation

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Transposons
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Transposable elements
Can move around within a cells
genome
Jumping genes
Contribute to genetic shuffling in
bacteria
Contribute to antibiotic resistnace
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Bacterial metabolism

Bacteria need to respond quickly to
changes in their environment

if they have enough of a product,
need to stop production

STOP

GO
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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
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why? metabolism, growth, reproduction
how? start production of enzymes for digestion
Different way to Regulate Metabolism
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Gene regulation
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instead of blocking
enzyme function,
block transcription
of genes for all
enzymes in
tryptophan pathway
 saves energy by
not wasting it on
unnecessary
protein synthesis
-
= inhibition
-
-
Bacteria group genes together
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Operon
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genes grouped together with related functions
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promoter = RNA polymerase binding site
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example: all enzymes in a metabolic pathway
single promoter controls transcription of all genes in operon
transcribed as one unit & a single mRNA is made
operator = DNA binding site of repressor protein (on/off
switch)
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So how can these genes be turned off?
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Repressor protein
binds to DNA at operator site
 blocking RNA polymerase
 blocks transcription

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Operon model
Operon:
operator, promoter & genes they control
serve as a model for gene regulation
RNA
polymerase
RNA
repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
DNA
operator
Repressor protein turns off gene by
blocking
AP BiologyRNA polymerase binding site.
repressor
= repressor protein
Repressible operon: tryptophan
Synthesis pathway model
When excess tryptophan is present,
it binds to tryp repressor protein &
triggers repressor to bind to DNA
RNA
polymerase

RNA
trp repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
blocks (represses) transcription
DNA
trp
operator
trp
trp
repressor
repressor protein
trp
trp
trp
trp
trp
trp
conformational change in
AP Biologyprotein!
repressor
trp
repressor
tryptophan
trp
tryptophan – repressor protein
complex
Tryptophan operon
What happens when tryptophan is present?
Don’t need to make tryptophan-building
enzymes
Tryptophan
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is allosteric regulator of repressor protein
Inducible operon: lactose
lac
lac
RNA
polymerase
lac
Digestive pathway model
lac
When lactose is present, binds to
lac repressor protein & triggers
repressor to release DNA
lac
lac
lac
RNA
lac repressor
TATA
polymerase
induces transcription
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter

operator
repressor
lac
conformational change in
AP Biologyprotein!
repressor
lac
repressor
DNA
repressor protein
lactose
lactose – repressor protein
complex
Lactose operon
What happens when lactose is present?
Need to make lactose-digesting enzymes
Lactose is allosteric regulator of repressor protein
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cAMP and CAP
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cAMP accumulates when glucose is
scarce
CAP activates and binds to DNA
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Stimulates transcription
cAMP and CAP
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High glucose levels  concentration of
cAMP goes down
Without cAMP, CAP detaches from
operon
CAP becomes inactive and polymerase
binds less

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Low transcription rate
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Cellular
Condition
Effector
Molecule
Regulatory
Protein
Glucose
absent; lactose
used
High cAMP
Catabolic
Activation of
activator protein CAP
(CAP)
Binds to CAP
site,
transcription of
lac genes
Glucose
Low cAMP
present; lactose
not needed
CAP
Inactivation of
CAP
Cannot bind to
CAP site, no
transcription of
genes
No lactose
Lac repressor
Activation of lac
repressor
Binds to
operator site,
prevents
transcription
Lac repressor
Inactivation of
lac repressor
Prevents
repressor from
binding,
transcription of
lac genes
No allolactose
Lactose present Allolactose
protesent
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Result
Outcome
Operon summary
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Repressible operon
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usually functions in anabolic pathways
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synthesizing end products
when end product is present in excess,
cell allocates resources to other uses
Inducible operon
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usually functions in catabolic pathways,
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
produce enzymes only when nutrient is
available
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digesting nutrients to simpler molecules
cell avoids making proteins that have nothing to do,
cell allocates resources to other uses