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Control MechanismsThere are 42 000 genes that code for proteins in
humansHowever, not all proteins are required at all times.
E.g. Insulin is only required in a cell when glucose levels are high. It would be inefficient and wasteful for a cell to transcribe,
translate the insulin gene when glucose levels are low
Regulation is therefore vital to an organism’s survivalRegulation – the turning on or off of specific genes
depending on the requirements of an organism
Why Turn Genes On and Off?Cell Specialization
each cell of a multicellular eukaryote expresses only a small fraction of its genes
Developmentdifferent genes needed at different points in life cycle
of an organism afterwards need to be turned off permanently
Responding to organism’s needscells of multicellular organisms must continually turn
certain genes on & off in response to signals from their external & internal environment
The control of gene expression can occur at any step in the pathway from gene to functional protein
The focus of today’s lesson will be on the regulation of gene expression at the transcriptional level.
Figure 19.3
Signal
NUCLEUS
Chromatin
Chromatin modification:DNA unpacking involvinghistone acetylation and
DNA demethlation
Gene
DNAGene availablefor transcription
RNA Exon
Transcription
Primary transcript
RNA processing
Transport to cytoplasm
Intron
Cap mRNA in nucleus
Tail
CYTOPLASM
mRNA in cytoplasm
Degradationof mRNA
Translation
Polypetide
CleavageChemical modificationTransport to cellular
destination
Active protein
Degradation of protein
Degraded protein
Gene ExpressionGene expression for all genes falls into one of two
categories. (prokaryotes and eukaryotes)
1. constitutive expression – genes which are always turned on
known as housekeeping genes
2. induced expression – genes which are only turned on as needed
lac Operonlac - lactose
operon – several genes in a sequence all controlled by a single promoter (mainly in prokaryotes; some eukaryotes). It also includes an operator which is the “on” and “off” switch.
promoter gene 1 gene 2 gene 3
operator
lac OperonCells mainly use glucose as a source of energy.
The lac operon is only turned on when glucose is absent, but lactose is present.
lac Operon Animation
lac Operon DetailsOperon codes for 3 enzymes – found in E. coli
1. beta-galactosidase (lacZ gene) enzyme which breaks down lactose
2. permease (lacY gene) protein transporter which brings lactose into cell
3. transacetylase (lacA gene) adds acetyl group to galactose
Repressor Proteinrepressor protein (lacI gene)
transcribed by a different gene from the lac operonbinds to the operator portion in the presence of glucoseprevents RNA polymerase from transcribing genes
when bound to operator Lactose is not needed as an energy source because glucose
is present
When do you want the repressor to bind / not bind to the operon?
When lactose is absent:enzymes are not needed to metabolise lactoserepressor binds to the operator to inhibit transcription
Figure 18.22a
DNA
mRNA
ProteinActiverepressor
RNApolymerase
NoRNAmade
lacZlacl
Regulatorygene
Operator
Promoter
Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.
(a)
5’
3’
When lactose is present:Lactose (or allactose) binds to the repressor proteinlactose-repressor complex cannot bind to the operatortranscription can proceed
mRNA 5'
DNA
mRNA
Protein
Allolactose(inducer)
Inactiverepressor
lacl lacz lacY lacA
RNApolymerase
Permease Transacetylaseβ-Galactosidase
5’
3’
(b)Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
mRNA 5’
lac operon
Figure 18.22b
Effector MoleculesSince lactose is the molecule that determines when
the operon is turned on or off, it is known as an effector molecule.
effector molecule – any molecule that can regulate the activity of a protein
inducer – effector molecule that binds repressor protein to cause it to fall off operator
DNA
mRNA
ProteinActiverepressor
RNApolymerase
NoRNAmade
lacZlacl
Regulatorygene
Operator
Promoter
Lactose absent, repressor active, operon off. (a)
5’
3’
mRNA 5'
DNA
mRNA
Protein
Allolactose(inducer)
Inactiverepressor
lacl lacz lacY lacARNApolymerase
5’3’
(b) Lactose present, repressor inactive, operon on.
mRNA 5’
lac operon
lac Operon Animation
lac Operon Animation
trp Operontrp – tryptophan
The genes of the trp operon are used to make the amino acid tryptophan.
It is turned off when enough tryptophan is in the cell.
Tryptophan is the effector molecule.
trp Operon
Operon codes for 5 genes – found in E. coli
Five polypeptides combine to make three enzymes.each enzyme participates in a step to make tryptophan
Repressor Proteinrepressor protein (trpR)
transcribed as a different gene from trp operonbinds to operator when tryptophan is presentprevents RNA polymerase from transcribing genes
when bound to operator
When do you want the repressor to bind / not bind to the trp operon?
When tryptophan needs to be made:enzymes are required to make tryptophanrepressor is NOT bound to operatortranscription can proceed
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.
Genes of operon
Inactiverepressor
Protein
Operator
Polypeptides that make upenzymes for tryptophan synthesis
Promoter
Regulatorygene
RNA polymerase
Promoter
trp operon
5’
3’mRNA 5’
trpDtrpE trpC trpB trpAtrpRDNA
mRNA
E D C B A
When cell has enough tryptophan:tryptophan binds to repressorrepressor can now bind operator to prevent
transcriptionDNA
mRNA
Protein
Tryptophan(corepressor)
Active repressor
No RNA made
Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein.
(b)
RNA Polymerase
Effector MoleculeSince tryptophan is the molecule that determines when
the operon is turned on or off, it is known as an effector molecule.
corepressor – effector molecule that binds repressor protein to cause it to bind to the operator
(a) Tryptophan absent, repressor inactive, operon on.
Genes of operon
Inactiverepressor
Protein
Operator
Polypeptides that make upenzymes for tryptophan synthesis
Promoter Promoter
trp operon
5’
3’5’
trpDtrpE trpC trpB trpAtrpRDNA
mRNAE D C B A
DNA
mRNA
Protein
Tryptophan(corepressor)
Active repressor
No RNA made
Tryptophan present, repressor active, operon off.
(b)
http://www.youtube.com/watch?v=8aAYtMa3GFU
Classwork/HomeworkSection 5.5 Questions pg. 258 #1-6