Control of Gene Expression. Steps of gene expression Transcription – DNA is read to make a mRNA in...
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Transcript of Control of Gene Expression. Steps of gene expression Transcription – DNA is read to make a mRNA in...
Control of Gene Expression
Steps of gene expressionSteps of gene expression
TranscriptioTranscription – n – DNA is DNA is read to make read to make a mRNA in a mRNA in the nucleus of the nucleus of our cellsour cells
Translation Translation – – Reading the Reading the mRNA to mRNA to make a make a protein in the protein in the cytoplasmcytoplasm
Mainly controlled at the level of transcription
Prokaryotic and eukaryotic gene organization
Prokaryotic transcriptional
regulatory regions
(promoters and operators) lie close to the
transcription start siteFunctionally
related genes are frequently
located near each other These “operons”
are transcribed into a single mRNA with
internal translation
initiation sites
Prokaryotic Gene Prokaryotic Gene ExpressionExpression
PromoterCistron1Cistron2CistronNTerminator
Transcription RNA Polymerase
mRNA 5’ 3’
TranslationRibosome, tRNAs,Protein Factors
1 2 N
Polypeptides
NC
NC N
C
1 2 3
Expression mainly by controlling transcription
OperonsOperons A cluster of related genes often coding for enzymes in a metabolic pathway, which are under the control of a single
promoter regulatory region Genes that work together are located together
A promoter plus a set of adjacent genes whose gene products function together.
They are controlled as a unit They usually contain 2 –6 genes (up to 20 genes)
These genes are transcribed as a polycistronic transcript. It is relatively common in prokaryotes
It is rare in eukaryotes
Operon SystemOperon System
Structural genes : DNA that code for a specific polypeptide (protein)
Promoter : DNA segment that recognizes RNA polymerase
Operator : Element that serves as a binding site for an inhibitor protein (modulator) that controls transcription
Repressor : Protein which binds to a specific DNA sequences to determine the transcription of a particular geneRegulatory gene : Gene encode for repressor protein
Regulatory elements of transcription
Regulatory gene:Organization of operon
Operons• The Tryptophan Operon (Repressible and
attenuation) Repressor does not bind to operator unless it interacts with co repressorBiosynthetic pathways
• The Lactose Operon (Induction and catabolite
repression)Repressor is bound to operator unless molecule to be metabolized is present (inducer)Catabolic pathways
A repressible operon
Inducible Operon
Lactose Operon• It codes for the enzymes responsible for lactose catabolism• Within the operon, there are three genes that code for
proteins (structural protein) and an upstream control region including promoter and a regulatory site called the operator
• Laying outside the operon is the repressor gene, which codes for a protein (lac repressor) that binds to the operator site and is responsible for the suppression of the operon by blocking the binding of RNA polymerase
• Transcribed mRNA may contain information for more than one protein (a polycistronic mRNA)
• The synthesis of these mRNA is regulated in accordance with the needs of the cells at any time thus enable the cell to adapt quickly to changing environmental conditions
The lactose (lac) The lactose (lac) operonoperon
• Contains several elementsContains several elements– laclacZ gene = Z gene = ββ-galactosidase-galactosidase– laclacY gene = galactosidase permeaseY gene = galactosidase permease– laclacA gene = thiogalactoside transacetylaseA gene = thiogalactoside transacetylase– laclacI gene = I gene = lac lac repressorrepressor
– PPii = promoter for the = promoter for the laclacI geneI gene– P = promoter for P = promoter for laclac-operon-operon– QQ11 = main operator = main operator– QQ22 and Q and Q33 = secondary operator sites (pseudo- = secondary operator sites (pseudo-
operatorsoperators))
Pi P Z Y A I Q3 Q1 Q2
Regulation of the lac operonRegulation of the lac operon
Pi P Z Y A I Q3 Q1 Q2
Inducer molecules→ Allolactose: - natural inducer, degradable IPTG (Isopropylthiogalactoside)- synthetic inducer, not metabolized
lacI repressor
Pi P Z Y A I Q3 Q1 Q2
LacZ LacY LacA
The lac operon: model for gene expression
Includes three protein synthesis coding region--sometimes called "genes" as well as region of chromosome that controls transcription of genes Genes for proteins involved in the catabolism or breakdown of lactose When lactose is absent, no transcription of gene since no need for these proteinsWhen lactose is present, transcription of genes takes place so proteins are available to catalyze breakdown of lactose
Eukaryotic geneEukaryotic gene
Eukaryotic gene ExpressionEukaryotic gene Expression
1.Transcripts begin and end beyond the coding region
2.The primary transcript is processed by:5’ capping3’ formation / polyA
splicing
3.Mature transcripts are transported to the cytoplasm for translation
Control of Gene Expression
Regulation of gene expression Gene expression is regulated—not all genes Gene expression is regulated—not all genes
are constantly active and having their protein are constantly active and having their protein producedproduced
The regulation or feedback on gene The regulation or feedback on gene expression is how the cell’s metabolism is expression is how the cell’s metabolism is controlled. controlled.
This regulation can happen in different ways:This regulation can happen in different ways:1. Transcriptional control (in nucleus):1. Transcriptional control (in nucleus):
e.g. chromatin density and transcription factorse.g. chromatin density and transcription factors
2. Posttranscriptional control (nucleus)2. Posttranscriptional control (nucleus)e.g. mRNA processinge.g. mRNA processing
3. Translational control (cytoplasm)3. Translational control (cytoplasm)e.g. Differential ability of mRNA to bind ribosomese.g. Differential ability of mRNA to bind ribosomes
4. Posttranslational control (cytoplasm)4. Posttranslational control (cytoplasm)e.g. changes to the protein to make it functionale.g. changes to the protein to make it functional
– Regulatory proteins that bind to control sequences– Transcription factors promote RNA
polymerase binding to the promoter– Activator proteins bind to DNA enhancers
and interact with other transcription factors– Silencers are repressors that inhibit
transcription– Control sequences
– Promoter– Enhancer
– Related genes located on different chromosomes can be controlled by similar enhancer sequences
Enhancers
Otherproteins
DNA
Transcriptionfactors
Activatorproteins
RNA polymerase
Promoter Gene
Bendingof DNA
Transcription
Transcription control• Transcription factors• Proximal activators• Distal control elements
(enhancers)– DNA binding domain– Activation domains
bind to other proteins– These are cell-specific– A few common
structures, but found in different combinations in different cells
Eukaryotic gene expression
Gene regulation of the transcription
Chr. I
Chr. II
Chr. III
Condition 1
“turned on”
“turned off”
Condition 2
“turned off”
“turned on”
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26
constitutively expressed gene
induced gene
repressedgene
inducible/ repressible genes
Gene regulationGene regulation
constitutively expressed gene
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26
Condition 3 Condition 4 upregulated gene expression
down regulated gene expression
Post-Transcriptional Modification in EukaryotesPost-Transcriptional Modification in Eukaryotes
Primary transcriptPrimary transcript formed firstformed first Then processed (3 steps) to form mature mRNA Then processed (3 steps) to form mature mRNA Then transported to cytoplasmThen transported to cytoplasm
Step 1: 7- methyl-guanosine “5’-cap” added to 5’ endStep 2: introns spliced out; exons link up
Step 3: Poly-A tail added to 3’ end
mature mRNA5’-cap- exons -3’ PolyA tail
Alternative picture: co-transcriptional pre-mRNA processing
Cap Functions
1. Protection of the mRNA from degradation (Protection from 5 exoribonucleases)
2. Enhances translation in the cytoplasm (Enhancement of the mRNA’s translatability)
3. Enhances transport from the nucleus4. Proper splicing of the pre-mRNA (Enhances
splicing of the first intron (for some pre-mRNAs))
The attachment of 7Me-GTP to the 5’ end of a nascent mRNA with a 5’ to 5’
phospho-ester linkage
Intron Splicing
•Exons : coding regions•Introns : noncoding regions
Step by step removal of pre-mRNA and joining of remaining exons
Removing intron from pre-mRNA
PolyadenylationThe process of adding poly(A) to RNA
Synthesis of the poly (A) tail involves cleavage of its 3’end and then the addition of about 40-200 adenine residues to form a poly (A) tail
Function - Poly(A) enhances both the lifetime and translatability of mRNA
End ProductEnd Product
The end products of protein synthesis is a The end products of protein synthesis is a primary structure of a proteinprimary structure of a protein..
A sequence of A sequence of amino acid amino acid bonded together bonded together by by peptide bondspeptide bonds..
aa1
aa2 aa3 aa4aa5
aa200
aa199
PolyribosomePolyribosome
incominglarge
subunit
incomingsmall subunit polypeptide
mRNA1 2 3 4 5 6 7
Groups of ribosomes reading same mRNA simultaneously producing many proteins
(polypeptides).
TYPES OF PROTEINSTYPES OF PROTEINS
Enzymes (Helicase)Enzymes (Helicase)Carrier (Haemoglobine)Carrier (Haemoglobine)
Immunoglobulin (Antibodies)Immunoglobulin (Antibodies)Hormones (Steroids)Hormones (Steroids)Structural (Muscle)Structural (Muscle)
Ionic (K+,Na+)Ionic (K+,Na+)
Coupled transcription and translation in bacteria
VALINE
HISTIDINE
LEUCINE
PROLINE THREONINE
GLUTAMATE
VALINE
original base triplet in a DNA strand
As DNA is replicated, proofreadingenzymes detect the mistake and
make a substitution for it:
a base substitution within the triplet (red)
One DNA molecule carries the original, unmutated sequence
The other DNAmolecule carries a gene mutation
POSSIBLE OUTCOMES:
OR
A summary of transcription and translation in a eukaryotic cellA summary of transcription and translation in a eukaryotic cell