3.B.1 Gene Regulation

Gene regulation results in differential gene expression, leading to cell

specialization.

Draw 8 boxes on your paper

Gene regulation accounts for some of the phenotypic differences between

organisms with similar genes.

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Gene regulation in bacteria• Control of gene expression enables

individual bacteria to adjust their metabolism to environmental change

• Cells vary amount of specific enzymes by regulating gene transcription– turn genes on or turn genes off• ex. if you have enough tryptophan in your cell then

you don’t need to make enzymes used to build tryptophan–waste of energy– turn off genes which codes for enzymes

Box #1

What is gene regulation?

Gene expression can involve:

Regulatory Sequences Within Genes

Regulatory Genes

Small Regulatory RNAs (sRNA)

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So how can genes be turned off?• First step in protein production?– transcription– stop RNA polymerase!

• Repressor protein– binds to DNA near promoter region blocking RNA

polymerase• binds to operator site on DNA• blocks transcription

Box # 2

How do you regulate genes?

Regulatory sequences are stretches of DNA that interact with regulatory proteins to

control transcription.

Operons are a type of regulatory sequence consisting of clusters of genes under the

control of a single regulatory region.

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Genes grouped together • Operon – genes grouped together with related functions

• ex. enzymes in a synthesis pathway– promoter = RNA polymerase binding site

• single promoter controls transcription of all genes in operon• transcribed as 1 unit & a single mRNA is made

– operator = DNA binding site of regulator protein

Operators are segments of DNA that a regulator molecule can bind to.

Promoters are nucleotide sequences that allow the genes of an operon to be

transcribed. RNA polymerase binds to the promoter region to begin transcription.

Repressors are small regulatory proteins that halt transcription. They bind to the

operator region of an operon and prevent RNA polymerase from binding.

Box # 3

Describe an operon (include the following terms in description,

promoter, repressor and operator.)

Both positive and negative control mechanisms regulate gene expression

in bacteria and viruses.

Positive

Control

•Stimulate Transcription

Negative

Control

•Inactivate Transcription

The expression of specific genes can be inhibited by the presence of a repressor. A repressor binds to the operator site of an operon, preventing RNA polymerase from

binding and therefore preventing transcription of the operon (negative

control).

Inducers are small proteins that stimulate transcription. They bind to repressors, inactivating them so that

RNA polymerase can bind to the operator and begin transcription.

Positive

Control

•Stimulate Transcription

Negative

Control

•Inactivate Transcription

Inducer

Repressor

Box # 4

Compare and contrast inducers and repressors

Terminators are nucleotide sequences that mark the end of a gene or operon.

Enhancers are short regions of DNA that can be bound with proteins to

enhance transcription.

Box # 5

How do enhancers aid transcription?

A regulatory gene is a sequence of DNA encoding a regulatory protein (such as a

repressor) or RNA.

Certain genes are continuously expressed; that is, they are always turned “on,”

regardless of environmental conditions.

Example of Prokaryotic Gene Regulation:

The Trp Operon

The trp operon consists of a group of genes that code for the enzymes

necessary to synthesize tryptophan, an amino acid.

The trp operon is an example of negative feedback. When there is too much

tryptophan, tryptophan itself acts as a corepressor, which activates the repressor

that shuts down this operon.

The trp operon is an example of a repressible operon; it is usually “on” but can

be turned “off” when there is too much tryptophan.

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operatorpromoter

Repressor protein model

DNATATA

RNApolymerase

repressor

repressor repressor protein

Operon: operator, promoter & genes they controlserve as a model for gene regulation

gene1 gene2 gene3 gene4RNApolymerase

Repressor protein turns off gene by blocking RNA polymerase binding site.

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operatorpromoter

Repressible operon: tryptophan

DNATATA

RNApolymerase

repressor

tryptophan

repressor repressor protein

repressortryptophan – repressor proteincomplex

Synthesis pathway modelWhen excess tryptophan is present, binds to tryp repressor protein & triggers repressor to bind to DNA– blocks (represses) transcription

gene1 gene2 gene3 gene4RNApolymerase

conformational change in repressor protein!

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Tryptophan operonWhat happens when tryptophan is present?Don’t need to make tryptophan-building enzymes

Tryptophan binds allosterically to regulatory protein

Box # 6

Draw the trp operon and how its regulated by a

repressor

Example of Prokaryotic Gene Regulation:

The Lac Operon

The lac operon consists of a group of genes in e. coli that allow the bacteria to metabolize lactose when lactose is

present in the gut of its host.

When there is no lactose present, e. coli does not need to produce the enzymes

to break down lactose, instead using glucose as its primary nutrient.

In the absence of lactose, the lac repressor protein is made, which binds to the

operator and halts the binding of RNA polymerase.

In the presence of lactose, an inducer binds to the repressor, altering its shape so that it is no longer able to bind to the operator. RNA

polymerase can now bind and begin transcribing the operon.

The lac operon is an example of an inducible operon; it is usually “off” but can be turned “on” in the presence of lactose.

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operatorpromoter

Inducible operon: lactose

DNATATARNApolymerase

repressor repressor protein

repressorlactose – repressor proteincomplex

lactose

repressor gene1 gene2 gene3 gene4

Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA– induces transcription

RNApolymerase

conformational change in repressor protein!

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Lactose operonWhat happens when lactose is present?Need to make lactose-digesting enzymes

Lactose binds allosterically to regulatory protein

Box # 7

Describe the lac operon and the inducer

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Operon summary• Repressible operon – usually functions in anabolic pathways

• synthesizing end products

– when end product is present in excess,cell allocates resources to other uses

• Inducible operon – usually functions in catabolic pathways,

• digesting nutrients to simpler molecules

– produce enzymes only when nutrient is available• cell avoids making proteins that have nothing to do, cell

allocates resources to other uses

In eukaryotes, gene expression is complex and control involves regulatory

genes, regulatory elements and transcription factors that act in concert.

Transcription factors are proteins that bind to specific DNA sequences and/or other regulatory proteins. They work

alone or in complex.

Some of these transcription factors are activators (increase expression), while

others are repressors (decrease expression).

The combination of transcription factors binding to the regulatory regions at any one time determines how much, if any, of the gene product will be produced.

Box # 8

What are transcription factors?

How do they involved in eukaryotic gene

expression?

Learning Objectives:LO 3.18 The student is able to describe the connection between the regulation of gene expression and observed differences between different kinds of organisms. [See SP 7.1] LO 3.19 The student is able to describe the connection between the regulation of gene expression and observed differences between individuals in a population.[See SP 7.1] LO 3.20 The student is able to explain how the regulation of gene expression is essential for the processes and structures that support efficient cell function. [See SP 6.2] LO 3.21 The student can use representations to describe how gene regulation influences cell products and function. [See SP 1.4]