regulation of gene expression prokaryotes vs. eukaryotes
TRANSCRIPT
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REGULATION OF GENE EXPRESSION
Prokaryotes vs. Eukaryotes
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Gene Expression
is the process by which information from a gene is used in the synthesis of a functional protein or other gene product such as tRNA or rRNA.
As you can imagine, not all genes are expressed at the same time in an organism.
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Regulation of Gene Expression Both prokaryotic and eukaryotic cells,
have the ability to turn gene expression on or off depending on what the cell or organism needs at any given time.
Prokaryotic cells have evolved to be able to regulate gene expression in response to different environmental conditions.
Eukaryotes regulate gene expression to maintain different cell types, in embryonic development, etc.
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Why Regulate Gene Expression? Bacterial cells that can conserve energy
have the selective advantage over cells that are unable to do so.
Natural selection has favored bacteria who express genes only when their products are needed by the cell.
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Why regulate gene expression? E. coli can adjust their metabolism to
changing environments i.e. They can adjust the activity of enzymes
already present in the cell by Feedback inhibition where the product inhibits the first enzyme in a metabolic pathway.
Cells can adjust the production level of the enzyme and only produce it as needed.
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The operon model was dicovered in 1961 The mechanism by which bacterial cells
can switch genes on or off Synthesis or metabolism of molecules
(metabolism) occurs in steps (pathway) Each reaction in a pathway is catalyzed
by a specific enzyme. The genes for the enzymes that work
together are usually clustered on the same chromosome.
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The Lac Operon
Promoter – a DNA segment that signals the beginning of a gene (RNA polymerase attaches)
Operator – a DNA segment that controls the access of RNA polymerase to the genes
Operon – a DNA segment that includes a cluster of genes, the promoter and the operator
Repressor – a protein that binds to a specific operator, blocking attachment of RNA polymerase & thus turning the operon off
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The Lac Operon
Regulatory genes – code for repressor proteins
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The Lac Operon
Genes LacI – regulatory gene, codes for the lac
repressor LacZ – codes for βgalactosidase (hydrolysis
lactose) LacY – codes for permease (membrane
protein that transport lactose into the cell) LacA – codes for transacetylase ( transfers
an acetyl group from acetyl CoA to βgalactosidase but function unclear)
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How it works
The dissacharide lacose is available to E.coli in the human colon if the host drinks milk.
Lactose metabolism begins with hydrolysis into ___ and _____.
This reaction is catalyzed by β-galactosidase.
In the absence of glucose, only a few molecules are present in the E.coli cell.
If lactose is added to the environment, the number of molecules of enzyme increase a thousandfold
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E.coli uses 3 enzymes to metabolize lactose. The genes for these enzymes are all clustered together on a molecule of DNA in the lac operon .
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How it Works
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How it Works
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Matching
1. β-galactosidase2. Allolactose3. Operator4. Promoter5. Regulator gene6. Repressor7. Gene in operon
A. Is inactivated when attached to allolactose
B. Codes for repressor protein
C. Hydrolyzes lactose
D. Repressor attaches here
E. RNA polymerase attaches here
F. Acts as an inducer that inactivates repressor
G. Usually codes for an enzyme
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Prokaryotes vs. Eukaryotes
Control expression by transcription.
No Nucleus to separate transcription and translation
Nucleus provides opportunity for post transcription control.
Transcription RNA processing mRNA transport mRNA translation mRNA degradation Protein degradation
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FIGURE 18.6Signal
NUCLEUSChromatin
Chromatin modification:DNA unpacking involvinghistone acetylation and
DNA demethylationDNA
Gene
Gene availablefor transcription
RNA ExonPrimary transcript
Transcription
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
TranslationDegradationof mRNA
Polypeptide
Protein processing, suchas cleavage and
chemical modification
Active proteinDegradation
of proteinTransport to cellular
destination
Cellular function (suchas enzymatic activity,structural support)
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Control Points of Gene Expression Chromatin Modification Transcription RNA Processing (Post transcriptional) mRNA degradation (Post transcriptional) Translation Protein Degradation (Post translational)
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Chromatin modification
Chromatin is the DNA – protein complex in eukaryotic cells
Each chromosome consists of 1 long DNA strand bound and wound around positively charges proteins called histones.
DNA is wound around 4 histones to form a nucleosome
Some areas of chromatin are coiled more tightly than others
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FIGURE 18.7
Amino acidsavailablefor chemicalmodification
Histone tails
DNA double helix
Nucleosome(end view)
(a) Histone tails protrude outward from a nucleosome
Unacetylated histones Acetylated histones
Acetylation of histone tails promotes loose chromatinstructure that permits transcription
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Genes within regions where chromatin is tightly coiled (heterochromatin) are usually not transcribed or expressed.
Histones can be modified to regulate gene expression
DNA can be modified to regulate gene expression
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Histone tails protrude from the nucleosome and are accessible to enzymes that catalyse the addition or removal of certain chemical groups.
Acetyl, methyl, phosphate groups Acetylation – promotes transcription
(loosens chromatin) Methylation – prevents transcription
(tightens chromatin)
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Epigenetic inheritance
Once methylated, genes usually stay that way through successive generations
Enzymes methylate daughter strands according to the template strand in each round of replication.
Chromatin modification such as methylation can be passed from generation to generation.
Epigenetics is the inheritance of traits transmitted by mechanisms not involving the nucleotide sequence.
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Changes in nucleotide sequences (mutations)are permanent.
Changes in chromatin (methylation, acetylation) is reversible.
Alterations in normal patterns of DNA methylation are seen in some cancers (inappropriate gene expression)
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Transcriptional Control
Includes transcription factors Enhancers Promoters Hormones
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A Typical Eukaryotic Gene
Consists of a promoter sequence – a sequence where RNA polymerase binds and starts transcription
Introns Exons
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Upstream from the promoter sequence are: Enhancer (distal control elements) Proximal Control Elements
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RNA polymerase attaches to the promoter sequence
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FIGURE 18.10-3
ActivatorsDNA
EnhancerDistal controlelement
PromoterGene
TATA box
Generaltranscriptionfactors
DNA-bendingprotein
Group of mediator proteins
RNApolymerase II
RNApolymerase II
RNA synthesisTranscriptioninitiation complex