Control of Gene Activity

Control of Gene Activity

Chapter 18: Regulation of Gene ExpressionChapter 18: Regulation of Gene Expression

Gene RegulationGene Regulation

Prokaryotes and eukaryotes alter gene expression in response to their changing environment

In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types

RNA molecules play many roles in regulating gene expression in eukaryotes

Prokaryotes and eukaryotes alter gene expression in response to their changing environment

In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types

RNA molecules play many roles in regulating gene expression in eukaryotes

Prokaryotic regulationProkaryotic regulationGene expression in bacteria is

controlled by the operon modelAn operon is a cluster of

functionally related genes can be under coordinated control by a single on-off “switch”Bacteria do not require

same enzymes all the timeThey produce just enzymes

needed at the momentThe regulatory “switch” is a

segment of DNA called an operator (positioned within the promoter)

Gene expression in bacteria is controlled by the operon model

An operon is a cluster of functionally related genes can be under coordinated control by a single on-off “switch”Bacteria do not require

same enzymes all the timeThey produce just enzymes

needed at the momentThe regulatory “switch” is a

segment of DNA called an operator (positioned within the promoter)

Prokaryotic regulationProkaryotic regulation The operon can be switched off

by a protein repressorThe repressor prevents gene

transcription by binding to the operator and blocking RNA polymerase

The repressor is the product of a separate regulatory gene

The 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

The operon can be switched off by a protein repressorThe repressor prevents gene

transcription by binding to the operator and blocking RNA polymerase

The repressor is the product of a separate regulatory gene

The 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

Operon ComponentsOperon Components

The operon includes the following: 1) Regulatory gene

Located outside the operonCodes for a repressor protein molecule

2) PromoterSequence of DNA where RNA polymerase

attaches 3) Operator

A short sequence of DNA where repressor binds, preventing RNA polymerase from binding

4) Structural genes Code for enzymes of a metabolic pathwayTranscribed as a unit

The operon includes the following: 1) Regulatory gene

Located outside the operonCodes for a repressor protein molecule

2) PromoterSequence of DNA where RNA polymerase

attaches 3) Operator

A short sequence of DNA where repressor binds, preventing RNA polymerase from binding

4) Structural genes Code for enzymes of a metabolic pathwayTranscribed as a unit

E. coli & Tryptophan E. coli & Tryptophan

E. coli is a bacteria that lives in your colon

It has a metabolic pathway that allows for the synthesis of the amino acid tryptophan (Trp)

This pathway starts with a precursor molecule and proceeds through five enzyme catalyzed steps before reaching the final product: tryptophan

It is important that E. coli be able to control the rate of Trp synthesis because the amount of Trp available from the environment varies considerably

E. coli is a bacteria that lives in your colon

It has a metabolic pathway that allows for the synthesis of the amino acid tryptophan (Trp)

This pathway starts with a precursor molecule and proceeds through five enzyme catalyzed steps before reaching the final product: tryptophan

It is important that E. coli be able to control the rate of Trp synthesis because the amount of Trp available from the environment varies considerably

E. coli & Tryptophan E. coli & Tryptophan

If you eat a meal with little or no Trp, the E. coli in your gut must compensate by making more

If you eat a meal rich in Trp, E. coli doesn't want to waste valuable resources or energy to produce the amino acid because it is readily available for use

Therefore, E. coli uses the amount of Trp present to regulate the pathway

If levels are not adequate, the rate of Trp synthesis is increased

If levels are adequate, the rate of Trp synthesis is inhibited

If you eat a meal with little or no Trp, the E. coli in your gut must compensate by making more

If you eat a meal rich in Trp, E. coli doesn't want to waste valuable resources or energy to produce the amino acid because it is readily available for use

Therefore, E. coli uses the amount of Trp present to regulate the pathway

If levels are not adequate, the rate of Trp synthesis is increased

If levels are adequate, the rate of Trp synthesis is inhibited

trp Operontrp Operon The Trp operon has three

components: Five Structural Genes

These genes contain the genetic code for the five enzymes in the Trp synthesis pathway

One Promoter DNA segment where RNA polymerase binds

and starts transcription

One Operator DNA segment found between the promoter

and structural genes It determines if transcription will take place

The Trp operon has three components:

Five Structural Genes These genes contain the genetic code for the

five enzymes in the Trp synthesis pathway

One Promoter DNA segment where RNA polymerase binds

and starts transcription

One Operator DNA segment found between the promoter

and structural genes It determines if transcription will take place

trp Operontrp Operon

trp operontrp operonWhen nothing is bonded to the operator, the operon is "on"

RNA polymerase binds to the promoter and transcription is initiated

The 5 structural genes are transcribed to one mRNA strand

The mRNA will then be translated into the 5 enzymes that control the Trp synthesis pathway

When nothing is bonded to the operator, the operon is "on"RNA polymerase binds to the promoter and

transcription is initiatedThe 5 structural genes are transcribed to one mRNA

strandThe mRNA will then be translated into the 5 enzymes

that control the Trp synthesis pathway

trp operontrp operon

The operon is turned "off" by a specific protein called the repressor

The repressor is inactive in this form and can not bind properly to the operator

To become active and bind properly, a corepressor must associate with the repressorThe corepressor for this operon is

TryptophanThis makes sense because E. coli does not

want to synthesize Trp if it is available from the environment

The operon is turned "off" by a specific protein called the repressor

The repressor is inactive in this form and can not bind properly to the operator

To become active and bind properly, a corepressor must associate with the repressorThe corepressor for this operon is

TryptophanThis makes sense because E. coli does not

want to synthesize Trp if it is available from the environment

trp operontrp operon

trp operontrp operon

An active repressor binds to the operator blocking the attachment of RNA polymerase to the promoter

Without RNA polymerase, transcription and translation of the structural genes can't occur and the enzymes needed for Tryptophan synthesis are not made

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 corepressor tryptophan; thus the trp operon is repressed if tryptophan levels are high

An active repressor binds to the operator blocking the attachment of RNA polymerase to the promoter

Without RNA polymerase, transcription and translation of the structural genes can't occur and the enzymes needed for Tryptophan synthesis are not made

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 corepressor tryptophan; thus the trp operon is repressed if tryptophan levels are high

Repressible vs Inducible

Repressible vs Inducible

The trp operon is a repressible operonThis type is one that is usually onBinding of a repressor to the operator shuts off

transcription The end product, Trp, decreases or stops the

transcription of the enzymes necessary for its production

The opposite is called an inducible operonThis type is one that is usually offA molecule called an inducer inactivates the

repressor and turns on transcriptionAn example of an inducible system is lac (lactose)

operon

The trp operon is a repressible operonThis type is one that is usually onBinding of a repressor to the operator shuts off

transcription The end product, Trp, decreases or stops the

transcription of the enzymes necessary for its production

The opposite is called an inducible operonThis type is one that is usually offA molecule called an inducer inactivates the

repressor and turns on transcriptionAn example of an inducible system is lac (lactose)

operon

lac operonlac operon

The Lac operon has 3 components:Three Structural Genes

Contain the genetic code for the 3 enzymes in the lactose catabolic pathway

One PromoterDNA segment where RNA polymerase binds

and starts transcriptionOne Operator

DNA segment found between the promoter and structural genes

It determines if transcription will take placeIf the operator in turned "on", transcription will

occur

The Lac operon has 3 components:Three Structural Genes

Contain the genetic code for the 3 enzymes in the lactose catabolic pathway

One PromoterDNA segment where RNA polymerase binds

and starts transcriptionOne Operator

DNA segment found between the promoter and structural genes

It determines if transcription will take placeIf the operator in turned "on", transcription will

occur

lac operonlac operon The lac operon is an

inducible operon whose genes code for enzymes used in the hydrolysis and metabolism of lactose

By itself, the lac repressor is active and switches the lac operon offThe active repressor

binds to the operator This blocks RNA

polymerase from transcribing the genes

Basically-- the lac operon is in the “off ” position and needs to be turned “on”

The lac operon is an inducible operon whose genes code for enzymes used in the hydrolysis and metabolism of lactose

By itself, the lac repressor is active and switches the lac operon offThe active repressor

binds to the operator This blocks RNA

polymerase from transcribing the genes

Basically-- the lac operon is in the “off ” position and needs to be turned “on”

lac operon OFFlac operon OFF

lac operonlac operon

A molecule called an inducer inactivates the repressor to turn the lac operon on

What inducer is used in the lac operon? Lactose This makes sense because the cell only needs

to make enzymes to catabolize lactose if lactose is present

When lactose enters the cell it binds to the repressor and changes its shape so that it can't bind to the operator

RNA polymerase can now start transcription of the 3 structural genes that will control lactose catabolism

A molecule called an inducer inactivates the repressor to turn the lac operon on

What inducer is used in the lac operon? Lactose This makes sense because the cell only needs

to make enzymes to catabolize lactose if lactose is present

When lactose enters the cell it binds to the repressor and changes its shape so that it can't bind to the operator

RNA polymerase can now start transcription of the 3 structural genes that will control lactose catabolism

lac operon ONlac operon ON

Eukaryotic RegulationEukaryotic Regulation

Eukaryotes lack a universal regulatory mechanism to control expression of genes coding proteins

In multicellular organisms gene expression is essential for cell specialization

DNA in eukaryotes is packaged as chromatin within a nucleus

Eukaryotes lack a universal regulatory mechanism to control expression of genes coding proteins

In multicellular organisms gene expression is essential for cell specialization

DNA in eukaryotes is packaged as chromatin within a nucleus

Chromatin in Eukaryotes

Chromatin in Eukaryotes Most chromatin is loosely

packed in the nucleus during interphase and condenses prior to mitosis

Loosely packed chromatin is called euchromatinThe genes within this

area are easily accessed, thus easily transcribed

During interphase a few regions of chromatin are highly condensed into heterochromatinGenes within this area

are difficult to access, thus they are usually not transcribed

Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis

Loosely packed chromatin is called euchromatinThe genes within this

area are easily accessed, thus easily transcribed

During interphase a few regions of chromatin are highly condensed into heterochromatinGenes within this area

are difficult to access, thus they are usually not transcribed

Eukaryotic regulationEukaryotic regulation

There are four primary levels of control of gene activity:1. Transcriptional Control2. Posttranscriptional Control3. Translational Control4. Posttranslational Control

There are four primary levels of control of gene activity:1. Transcriptional Control2. Posttranscriptional Control3. Translational Control4. Posttranslational Control

Transcriptional ControlTranscriptional Control

Takes place in nucleus, the site of transcription

Determines which structural genes are transcribed and the rate of transcription

Includes organization of chromatin

Includes the action of regulatory proteins that may activate or inhibit transcription (such as transcription factors)

Takes place in nucleus, the site of transcription

Determines which structural genes are transcribed and the rate of transcription

Includes organization of chromatin

Includes the action of regulatory proteins that may activate or inhibit transcription (such as transcription factors)

Regulatory ProteinsRegulatory ProteinsGeneral transcription factors are essential for the

transcription of all protein-coding genesSome transcription factors function as activators

An activator is a protein that binds to an enhancer and stimulates transcription of a gene

Enhancers are DNA sequences that may be far away from a gene or even located in an intron

Some transcription factors function as repressorsA repressor is a protein that prevents the

expression of a particular geneSome activators and repressors act indirectly by

influencing chromatin structure to promote or silence transcription

General transcription factors are essential for the transcription of all protein-coding genes

Some transcription factors function as activatorsAn activator is a protein that binds to an enhancer

and stimulates transcription of a geneEnhancers are DNA sequences that may be far

away from a gene or even located in an intronSome transcription factors function as repressors

A repressor is a protein that prevents the expression of a particular gene

Some activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription

Posttranscriptional controlPosttranscriptional controlTakes place in the nucleus or cytoplasm Involves differential processing of pre-mRNA

Differential excision of introns and splicing of mRNA can vary type of mRNA

In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns

Involves regulation of transport of mature mRNAThe life span of mRNA molecules in the cytoplasm

is a key to determining protein synthesisThe mRNA life span is determined in part by

sequences in the leader and trailer regions (cap and tail)

Takes place in the nucleus or cytoplasm Involves differential processing of pre-mRNA

Differential excision of introns and splicing of mRNA can vary type of mRNA

In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns

Involves regulation of transport of mature mRNAThe life span of mRNA molecules in the cytoplasm

is a key to determining protein synthesisThe mRNA life span is determined in part by

sequences in the leader and trailer regions (cap and tail)

Translational controlTranslational controlTakes place in the cytoplasm,

the site of translationLife expectancy of mRNA

molecules can vary, as well as their ability to bind ribosomes

Some mRNA's may need additional changes before they are translated

The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA

Takes place in the cytoplasm, the site of translation

Life expectancy of mRNA molecules can vary, as well as their ability to bind ribosomes

Some mRNA's may need additional changes before they are translated

The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA

Posttranslational control

Posttranslational control

Takes place in the cytoplasm after protein synthesisMay involve activation or degradation of the protein After translation, various types of protein

processing, including cleavage and the addition of functional groups, are subject to control

Proteasomes are giant protein complexes that bind protein molecules and degrade them

Eukaryotic Gene Control Animation

Takes place in the cytoplasm after protein synthesisMay involve activation or degradation of the protein After translation, various types of protein

processing, including cleavage and the addition of functional groups, are subject to control

Proteasomes are giant protein complexes that bind protein molecules and degrade them

Eukaryotic Gene Control Animation

Review QuestionsReview Questions1. Differentiate between prokaryotic and eukaryotic gene

regulation.2. Explain the use of an operon as a prokaryotic form of gene

regulation.3. Name and describe the four main parts of an operon.4. Define the following terms: operator, repressor, inducer,

regulatory gene, and corepressor.5. Describe the functioning of the trp operon as a repressible

operon and state its overall significance to E. coli.6. Differentiate between repressible and inducible operons.7. Describe the functioning of the lac operon as an inducible

operon and state its overall significance to E. coli.8. Differentiate between euchromatin and heterochromatin in

eukaryotes.9. Name and describe the important traits of the 4 primary

levels of control of gene activity in eukaryotes.10. Differentiate between activators, enhancers, and

repressors.11. Describe alternative RNA splicing and its significance to

gene control.12. Define proteasome.