AP Biology
Figure 18.1 How can this fish’s eyes see equally well in both air and
water?Aka. Quatro ojas
Orchestrating proper gene expression by cells
is crucial to the function of life in (prokaryotes and eukaryotes)
AP Biology
Regulation of Gene Expression:n Prokaryotes and eukaryotes precisely regulate
gene expression in response to environmental conditions
n In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types, e.g., embryonic development
n RNA molecules play many roles in regulating gene expression in eukaryotes
Oh, so that explains it
AP Biology
Bacterial metabolism:n Bacteria need to respond quickly to changes in
their environment (relatively unstable)u if they have enough of a product,
need to stop productionn why? waste of energy to produce moren how? stop production of enzymes for synthesis
u if they find new food/energy source, need to utilize it quicklyn why? metabolism, growth, reproductionn how? start production of enzymes for digestion
STOP
GO
AP Biology
Remember Regulating Metabolism?n Feedback inhibition
u product acts as an allosteric inhibitor of 1st enzyme in tryptophan pathway
u but this is wasteful production of enzymes
= inhibition-
-Oh, I
remember thisfrom our
Metabolism Unit!
AP Biology
A schematic representation of a Generalized Feedback Loop
(two classifications)
Negative Feedback Loop = the effect causes a reversal of the
initial condition. “stimulatory-inhibitory
response”
Positive Feedback Loop = the effect intensifies initial
condition.“stimulatory-stimulatory
response”
AP Biology
n Gene regulation u instead of blocking
enzyme function, block transcription of genes for all enzymes in tryptophan pathwayn saves energy by
not wasting it on unnecessary protein synthesis
= inhibition-
Now, that’s a good idea from a lowly bacterium!
--
A different way to Regulate Metabolism:
AP Biology
Gene regulation in bacteria:n Cells vary amount of specific enzymes by
regulating gene transcriptionu turn genes on or turn genes off
n turn genes OFF exampleif bacterium has enough tryptophan then it doesn’t need to make enzymes used to buildtryptophan
n turn genes ON exampleif bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose
STOP
GO
AP Biology
n Natural selection has favored bacteria that produce only the products needed by that cell
n A cell can regulate the production of enzymes by feedback inhibition or by gene regulation
n One mechanism for control of gene expression in bacteria is the operon model
Bacteria often respond to environmental change by regulating transcription:
Discuss how the processes shown in Figure 18.2 are examples of feedback mechanisms regulating biological systems in bacterial cells.
AP Biology
Regulation of a metabolic pathway in E. coli for tryptophan synthesis (anabolic). An abundance of Trp can both:
Precursor
(typical response in synthesis pathways)
Tryptophan(b) Regulation of enzyme production.
(at the level of transcription)repress expression of the gene encoding all subunits of enzymes in the pathway;
long-term response
(a) Regulation of enzyme activity.(allosteric regulation), inhibit the
activity of the 1st enzymein the pathway; rapid response
Regulationof gene
expression
trpE
trpD
trpC
trpB
trpA
Enzyme 1
Enzyme 2
Enzyme 3
Figure 18.2 Feedbackinhibition
AP Biology
Bacteria group genes together:n Operon
u genes grouped together with related functions n example: all enzymes in a metabolic pathway
u promoter = RNA polymerase binding siten single promoter controls transcription of all genes in
operonn transcribed as one unit & a single mRNA is made
u operator = DNA binding site of repressor protein
AP Biology
n Repressor proteinu binds to DNA at operator site u blocking RNA polymeraseu blocks transcription
So how can these genes be turned off?
AP Biology
operatorpromoter
Operon model:
DNA
TATA
RNApolymerase
repressor
repressor = repressor protein
Operon: operator, promoter & genes they controlserve as a model for gene regulation
gene1 gene2 gene3 gene4RNApolymerase
Repressor protein turns off gene by blocking RNA polymerase binding site.
1 2 3 4mRNA
enzyme1 enzyme2 enzyme3 enzyme4
AP Biology
mRNA
enzyme1 enzyme2 enzyme3 enzyme4operatorpromoter
Repressible operon: tryptophan:
DNATATA
RNApolymerase
tryptophan
repressor repressor protein
repressor tryptophan – repressor proteincomplex
Synthesis pathway modelWhen excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNAu blocks (represses) transcription
gene1 gene2 gene3 gene4
conformational change in repressor protein!
1 2 3 4
repressortrpRNA
polymerase
trp
trp
trp trp
trp trp
trptrp
trptrp
trp
AP Biology
Tryptophan operon:What happens when tryptophan is present?
Tryptophan is allosteric regulator of repressor protein
Don’t need to make tryptophan-building enzymes
AP Biology
mRNA
enzyme1 enzyme2 enzyme3 enzyme4operatorpromoter
Inducible operon: lactose
DNA
TATARNA
polymerase
repressor repressor protein
repressor lactose – repressor proteincomplex
lactose
lac repressor gene1 gene2 gene3 gene4
Digestive pathway modelWhen lactose is present, binds to lac repressor protein & triggers repressor to release DNA
u induces transcription
RNApolymerase
1 2 3 4
lac lac
laclac
laclac
lac
conformational change in repressor protein!
lac
lac
AP Biology
Lactose operon:What happens when lactose is present?Need to make lactose-digesting enzymes
Lactose is allosteric regulator of repressor protein
AP Biology
Jacob & Monod: lac Operonn Francois Jacob & Jacques Monod
u first to describe operon systemu coined the phrase “operon”
1961 | 1965
Francois JacobJacques Monod
AP Biology
Operon summary:n Repressible operon
u usually functions in anabolic pathwaysn synthesizing end products
u when end product is present in excess,cell allocates resources to other uses
n Inducible operonu usually functions in catabolic pathways,
n digesting nutrients to simpler molecules
u produce enzymes only when nutrient is availablen cell avoids making proteins that have nothing to do, cell
allocates resources to other uses
Top Related