Questions to Ponder How do your cells know what kind of cell they are How do your cells know when to make a particular protein When to stop making it How does the environment affect your cells ID: 776556
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Slide1
Gene Expression
AP Biology
Slide2Questions to Ponder…..
How do your cells “know” what kind of cell they are?How do your cells “know” when to make a particular protein? When to stop making it?How does the environment affect your cells?ANSWER: Gene Expression
Slide3Liver Cells
Cartilage Cells
Red Blood Cells
Stem Cells
What makes cells from the same individual look different?
DNA sequence in each cell is the same, but different cell types have different “GENE EXPRESSION PATTERNS”
Slide4Slide adapted from Genetic Science Learning Center, University of Utah
2013
When a gene is “on” and its protein or RNA product is being made, scientists say that the gene is being EXPRESSED.The on and off states of all of a cell’s genes is known as a GENE EXPRESSION PROFILE.Each cell type has a unique gene expression profile.
InsulinDNA?Protein?Muscle CellXPancreatic Cell
Slide5Gene Expression in Bacteria
Bacteria are single-celled organisms who are surrounded on all sides by their environment. They must be able to regulate expression of their genes in response to environmental changes.
Slide6Bacteria Respond by Regulating Transcription
Bacteria cells that can conserve resources and energy have a selective advantage over cells that cannot do so.Natural selection has favored bacteria that express only the genes they need.
Slide7E. Coli Regulation of Tryptophan
An individual E. coli cell living in the erratic environment of the human colon, is dependent for its nutrients on the whimsical eating habits of its host—you!If the environment is lacking in the amino acid tryptophan, which the bacterium needs to survive, it responds by activating a metabolic pathway that makes tryptophan from another compound. If tryptophan becomes available, it shuts down this pathway.
Slide8Regulation of a Metabolic Pathway
In the pathway for tryptophan synthesis, an
abundance
of tryptophan can both inhibit the activity of the first enzyme (a rapid response) OR repress expression of the genes encoding the enzymes in the pathway (a longer response).
This is an example of
feedback inhibition
. It allows for a cell to adapt to short-term fluctuation in the supply of a substance it needs.
Slide9Fig. 18-2
Regulation
of geneexpression
trpE
gene
trpD gene
trpC gene
trpB gene
trpA gene
(b) Regulation of enzyme production
(a) Regulation of enzyme activity
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
Precursor
Feedbackinhibition
Slide10Gene Expression Controls Which Enzymes are Made and When
In many cases, this occurs in the process of transcription.Many genes may be switched on or off by changes in the metabolic status of the cell.One example was discovered in 1961 by Francois Jacob and Jacques Monod at the Pasteur Institute in Paris. This method is called the Operon Model.
Slide11Operons: The Basic Concept
A cluster of functionally related genes
that can
be under coordinated control
by a single on-off “switch
”.
The regulatory “switch” is a segment of DNA called an
operator
usually positioned within the
promoter.
An
operon
is the entire stretch of DNA that includes the operator, the promoter, and the genes that they
control.
Slide12The operon can be switched off by a protein
repressor
The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase
The repressor is the product of a separate
regulatory gene
Slide13The repressor can be in an active or inactive form, depending on the presence of other
molecules.
A
corepressor
is a molecule that cooperates with a repressor protein to switch an operon
off.
Slide14Bacteria can synthesize tryptophan by utilizing the
trp
operon.
By default,
the
trp
operon is
on
and the genes for tryptophan synthesis are transcribed
When tryptophan is present, it binds to the
trp
repressor protein, which turns the operon off
The repressor is active only in the presence of its
co-repressor
tryptophan; thus the
trp
operon is turned off (repressed) if tryptophan levels are high
Slide15Fig. 18-3a
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
DNA
mRNA 5
Protein
Inactiverepressor
RNApolymerase
Regulatorygene
Promoter
Promoter
trp operon
Genes of operon
Operator
Stop codon
Start codon
mRNA
trpA
5
3
trpR
trpE
trpD
trpC
trpB
A
B
C
D
E
Slide16Fig. 18-3b-1
(b) Tryptophan present, repressor active, operon off
Tryptophan
(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
Slide17Fig. 18-3b-2
(b) Tryptophan present, repressor active, operon off
Tryptophan
(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
Slide18Video for Gene Expression
http://education-portal.com/academy/lesson/regulation-of-gene-expression-transcriptional-repression-and-induction.html
Slide19Different Types of Operons
A repressible operon is one that is usually ON—binding a repressor to the operator turns off transcription. (The
trp
operon is like this)
An inducible operon is one that is usually OFF—a molecule called an inducer inactivates the repressor and starts transcription. (The
lac
operon is this type)
Slide20The lac Operon
The
lac
operon is an inducible operon (usually off) and contains genes that code for enzymes that break down the sugar lactose (found in dairy products)
By itself, the
lac
repressor is active and therefore shuts the
lac
operon off most of the time.
A molecule called an inducer inactivates this repressor which turns the
lac
operon on.
Slide21Fig. 18-4
(b) Lactose present, repressor inactive, operon on
(a) Lactose absent, repressor active, operon off
mRNA
Protein
DNA
DNA
mRNA 5
Protein
Active
repressor
RNApolymerase
Regulatorygene
Promoter
Operator
mRNA
5
3
Inactiverepressor
Allolactose(inducer)
5
3
NoRNAmade
RNApolymerase
Permease
Transacetylase
lac operon
-Galactosidase
lacY
lacZ
lacA
lacI
lacI
lacZ
Slide22Fig. 18-4a
(a) Lactose absent, repressor active, operon off
DNA
Protein
Active
repressor
RNApolymerase
Regulatorygene
Promoter
Operator
mRNA
5
3
NoRNAmade
lacI
lacZ
Slide23Fig. 18-4b
(b) Lactose present, repressor inactive, operon on
mRNA
Protein
DNA
mRNA 5
Inactive
repressor
Allolactose(inducer)
5
3
RNApolymerase
Permease
Transacetylase
lac operon
-Galactosidase
lacY
lacZ
lacA
lacI
Slide24Inducible Enzymes
Inducible enzymes (such as those found in the
lac
operon) are usually catabolic enzymes, which means they break things apart.
Their synthesis is usually induced by some kind of signal.
In the
lac
operon, the signal is the presence of the lactose sugar molecule.
Slide25Repressible Enzymes
Repressible enzymes (such as those in the
trp
operon) usually function in anabolic pathways which build things or put things together.
Since these are almost always ON, they are repressed (shut down) when there are high levels of the
end-product present.