Post on 03-Apr-2018
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LOGO
4. Gene expression control
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Bacterial metabolism
Bacteria need to respond quickly tochanges in their environment
if have enough of a product,
need to stop production why? waste of energy to produce more
how? stop production of synthesis enzymes
if find new food/energy source,
need to utilize it quickly
why? metabolism, growth, reproduction
how? start production of digestive enzymes
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Reminder: Regulation of metabolism
Feedback inhibition product acts
as an allosteric
inhibitor of
1st enzyme in
tryptophan pathway
= inhibition-
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Another way to regulate metabolism
Gene regulation block transcription of
genes for all
enzymes in
tryptophan pathway saves energy by
not wasting it on
unnecessary protein
synthesis
= inhibition-
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Gene regulation in bacteria
Cells vary amount of specific enzymesby regulating gene transcription
turn genes on or turn genes off
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So how can genes be turned off?
First step in protein production? transcription
stop RNA polymerase!
Repressor protein binds to DNA near promoter region blocking
RNA polymerase
binds to operatorsite on DNA blocks transcription
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Jacob & Monod: lac Operon
Francois Jacob & Jacques Monod
first to describe operon system
coined the phrase operon
1961 | 1965
Francois JacobJacques Monod
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Genes grouped together
Operon genes grouped together with related functions
ex. enzymes in a synthesis pathway
promoter = RNA polymerase binding site
single promoter controls transcription of all genes in operon transcribed as 1 unit & a single mRNA is made
operator = DNA binding site of regulator protein
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operatorpromoter
Repressible operon: tryptophan
DNATATA
RNApolymerase
repressor
tryptophan
repressor repressor protein
repressortryptophan repressor protein
complex
Synthesis pathway modelWhen excess tryptophan is present,
binds to tryp repressor protein &
triggers repressor to bind to DNA
blocks (represses) transcription
gene1 gene2 gene3 gene4RNA
polymerase
conformational change inrepressor protein!
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operatorpromoter
Inducible operon: lactose
DNATATARNA
polymerase
repressor repressor protein
repressorlactose repressor protein
complex
lactose
repressor gene1 gene2 gene3 gene4
Digestive pathway modelWhen lactose is present, binds to
lacrepressor protein & triggers
repressor to release DNA
induces transcription
RNApolymerase
conformational change inrepressor protein!
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Operon summary
Repressible operon usually functions in anabolic pathways
synthesizing end products
when end product is present in excess,
cell allocates resources to other uses
Inducible operon
usually functions in catabolic pathways,
digesting nutrients to simpler molecules
produce enzymes only when nutrient is available
cell avoids making proteins that have nothing to do, cell
allocates resources to other uses
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The BIG Questions
How are genes turned on & off in eukaryotes?
How do cells with the same genes differentiate to
perform completely different, specialized
functions?
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Why turn genes on & off?
Specialization each cell of a multicellular eukaryote expresses
only a small fraction of its genes
Development
different genes needed at different points in lifecycle of an organism
afterwards need to be turned off permanently
Responding to organisms needs
homeostasis cells of multicellular organisms must continually
turn certain genes on & off in response to signalsfrom their external & internal environment
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Prokaryote vs.eukaryote genome
Prokaryotes small size of genome
circular molecule ofnaked DNA DNA is readily available to RNA polymerase
control of transcription by regulatory proteins
operon system
most of DNA codes for protein or RNA no introns, small amount of non-coding DNA
regulatory sequences: promoters, operators
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Prokaryote vs. eukaryote genome
Eukaryotes much greater size of genome
how does all that DNA fit into nucleus?
DNA packaged in chromatin fibers
regulates access to DNA by RNA polymerase
cell specialization need to turn on & off large numbers of genes
most of DNA does not code for protein
97% junk DNA in humans
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Points of control
The control of gene expressioncan occur at any step in thepathway from gene to functionalprotein
unpacking DNA transcription
mRNA processing
mRNA transport
out of nucleus through cytoplasm
protection from degradation
translation
protein processing
protein degradation
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DNA packing
How do you fit allthat DNA intonucleus? DNA coiling &
folding double helix
nucleosomes
chromatin fiber
looped domains
chromosome
from DNA double helix to
condensed chromosome
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Unpacking DNA control
Degree of packing of DNA regulates transcription tightly packed = no transcription
= genes turned off
darker DNA (H) = tightly packed
lighter DNA (E) = loosely packed
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Unpacking DNA control : histone acetylation
Acetylation ofhistones unwinds DNA loosely packed = transcription
= genes turned on
attachment of acetyl groups (COCH3) to histones
conformational change in histone proteins
transcription factors have easier access to genes
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Unpacking DNA control : DNA methylation
Methylation ofDNA blocks transcription factors attachment of methyl groups (CH3) to cytosine
C = cytosine
no transcription = genes turned off
nearly permanent inactivation of genes
ex. inactivated mammalian X chromosome
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Transcription control : initiation step
Control regions on DNA promoter
nearby control sequence on DNA
binding of RNA polymerase & transcription factors
base rate of transcription
enhancers distant control
sequences on DNA
binding of activatorproteins
enhanced rate (high level)of transcription
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Model for Enhancer action
Enhancer DNA sequences distant control sequences
Activator proteins bind to enhancer sequence &
stimulates transcription
Silencer proteins bind to enhancer sequence &
block gene transcription
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mRNA processing control
Alternative RNA splicing variable processing of exons creates a family
of proteins
RNA t t t l
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mRNA transport control :
mRNA degradation
Life span of mRNA determines pattern ofprotein synthesis
mRNA can last from hours to weeks
RNA t t t l
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mRNA transport control :
RNA interference
Small RNAs (sRNA) short segments of RNA (21-28 bases)
bind to mRNA
create sections of double-stranded mRNA
death tag for mRNA triggers degradation of mRNA
cause gene silencing even though post-transcriptional control, still turns
off a gene
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RNA interference
Small RNAs
double-stranded RNA
sRNA + mRNA
mRNA
mRNA degraded
functionally turns
gene off
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Translation control
Block initiation stage regulatory proteins attach to
5 end of mRNA
prevent attachment of ribosomal subunits & initiator
tRNA
block translation of mRNA to protein
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Protein processing & degradation control
Protein processing folding, cleaving, adding sugar groups,
targeting for transport
Protein degradation
ubiquitin tagging
proteosome degradation
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Ubiquitin
Death tag mark unwanted proteins with a label
76 amino acid polypeptide, ubiquitin
labeled proteins are broken down rapidly in"waste disposers"
proteasomes
1980s | 2004
Aaron Ciechanover
Israel
Avram Hershko
Israel
Irwin Rose
UC Riverside
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Proteasome
Protein-degrading machine cells waste disposer
can breakdown all proteins
into 7-9 amino acid fragments