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Chapter 19: Control of Gene Expression: It’s How You Play Your Cards That Counts Summary

Gene regulation

  • This is a process through which cells identify the genes to utilize and genes not to use.
  • Cells can change the genes they need depending on signals or the environment.
  • The information in genes is first transcribed into RNA then translated into proteins.
  • Proteins that are essential for the survival of the cells are called housekeeping proteins.
  • Cells have two genes according to gene expression:
    • Constitutive genes: this is the genes that are expressed all the time. They contain protein blueprints for housekeeping functions of the cell.
    • Regulated genes: These are genes that are turned off and on when needed by the cell. They contain protein blueprints needed in some situations or not.

Expression of genes in bacteria

  • The following are steps for bacterial gene expression:
    • An environmental signal is received by cell.
    • The signal inactivates or activates DNA binding proteins.
    • The DNA binding proteins release or bind to the DNA regulatory sequences.
    • The DNA binding proteins switch transcription on or off.
Bacterial genes organization
  • Multiple genes are organized in bacteria under one promoter control.
  • An operon is a set of genes and the promoter.
Elements of a bacterial operon
  • The promoter symbolizes the start point of the operon.
  • Structural genes, protein genes within the operon.
  • The operator which is composed of DNA regulatory sequence found between the structural genes and the promoter.

The lac operon

  • The genes coding for enzymes that digest lactose is found in the lactose operon.
  • The lac operon contains three components:
    • The lac promoter is an RNA polymerase binding site.
    • The lac operator is the DNA binding protein binding site.
    • Three structural genes which code for two proteins needed to digest lactose and one blueprint for a protective enzyme. This includes the following:
      • The lac Z is the beta-galactosidase blueprint.
      • The lac Y which is a-galactosidase permease blueprint for lactose transport into the cell.
      • The lac A, a blueprint gene for transacetylase, allows certain sugars to be excreted from the cell when they are in excess.

Lac operon Repression

  • The lac repressor protein controls lac operon transcription.
  • When active, the lac repressor binds to the operator and blocks transcription.
  • The lac repressor has two binding sites:
    • The DNA binding site
    • An allosteric site
  • The allosteric site controls the activity of the lac repressor protein.
  • The repressor is active when the allosteric site is empty.

Inducing the lac operon

  • The ac operon is induced by lactose.
  • When lactose is available, the lac operon is turned on so that a lactose digesting enzyme can be produced by E. coli.
  • When lactose is present, allolactose attaches to the lac repressor protein which makes the protein inactive.
  • The lac repressor releases the operator which allows RNA polymerase to bind to the promoter.
  • The lac operon then undergoes transcription and produces the enzymes required in lactose breakdown.

Repression of the lac operon by catabolite repression

  • Lactose is not the only sugar that regulates the lac operon.
  • Catabolite activator protein (CAP) is an activator protein that turns on gene transcription.
  • Activation of the CAP turns on the transcription of catabolic operons.
  • CAP is inactive when glucose is available hence the E. coli does not make other enzymes because it is utilizing glucose.
  • CAP is active when glucose is depleted. This turns on catabolic operons hence E. coli makes food through other sources other than glucose.

CAP binding sites

  • A DNA binding site which binds to a DNA sequence called the CAP binding site. The CAP binding site is a DNA sequence next to the catabolic operons promoters.
  • An allosteric site that binds on cAMP. CAP is active when bound by cAMP.
  • The number of cAMP changes with glucose levels in the cell.
    • cAMP levels are low when glucose levels are high.
    • High cAMP levels occur when glucose levels are low.

Gene expression in eukaryotes

  • Gene expression regulation in bacteria and eukaryotes have some similarities.
    • Gene regulation at the transcription level occurs in both eukaryotes bacteria.
    • DNA binding proteins regulate gene activity by binding to regulatory DNA sequences. In bacteria, the regulatory proteins include CAP and repressor proteins. In eukaryotes, several regulatory proteins combine bind to regulatory DNA sequences to control transcription.

How DNA is packaged into the chromosome

  • The molecules of DNA are wound proteins with positive charges called histones that associate together in 8 protein groups. The beads o which DNA is wound are called nucleosomes.
  • The nucleosomes strings and DNA are twisted until 30-nanometer fibers are formed.
  • Continued DNA twisting and twisting of protein fibers condense the DNA more tightly.

Controlling transcription

  • Regulatory proteins control transcription in eukaryotes.
  • The regulatory proteins bind to regulatory DNA sequences near to the promoters.
  • The regulatory sequences that regulate gene expression in eukaryotes include the following:
    • Promoter proximal elements: These are regulatory sequences that are found near to the promoters. Transcription is turned on when regulatory proteins bind to proximal elements on the promoter.
    • Enhancers: These are regulatory sequences located far from genes they regulate. Transcription is turned on when regulatory proteins attach to enhancers.
    • Silencers: These are regulatory sequences found far from the genes they regulate. Transcription is blocked when regulatory proteins attach to silencers.

Transcription factors that control transcription in eukaryotes

  • General transcription factors: these factors are required to fr any gene in all types of cells. They form part of the transcription initiation complex, a protein group that attracts RNA polymerase enzyme and allows it to bind to its promoters.
  • Regulatory transcription factors: these are factors that bind to regulatory sequences of certain genes.
  • Coactivators also play a role in transcription initiation. The coactivators bind to regulatory and general transcription factors to form an initiation complex.

How coactivators and transcription factors work together to initiate transcription

  • Transcription factors attach to enhancers to transcribe the gene.
  • Transcription factors collect and bind general transcription factors and coactivators together.
  • RNA polymerase attaches to the above-formed complex to complete the formation of the protein initiation complex.
  • Transcription starts.

Mechanisms for controlling processing and stability of mRNA

  • Regulation of mRNA processing is performed by proteins that interact with spliceosomes.
  • These proteins affect mRNA splicing. During alternative splicing, and mRNA can be spliced in different ways.
  • This produces different gene products from the former pre-mRNA.
  • mRNA stability is controlled by RNA interference.
  • microRNA associate with complementary sequences on molecules of mRNA marking them for cell destruction.

Controlling translation

  • Regulating translation can also control the expression of genes.
  • The following mechanisms can be used to block mRNA translation in cells:
    • Regulatory proteins can attach to mRNA, blocking the translation of the mRNA.
    • Modification of the poly-A tail or 5’ cap on an mRNA to block translation.
    • The population of proteins in the ribosome can slow or stop translation.

Revision

Exam 3 – Control of Gene Expression
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Enhancers are
A) proteins located adjacent to promoters
B) distant sites where regulatory proteins bind
C) expediters of RNA polymerase capture
D) proteins that bind with repressors, deactivating them
E) a bacterial form of promoters
B) distant sites where regulatory proteins bind
When tryptophan is present in the medium, the transcription of tryptophan producing genes in E. coli is stopped by a repressor binding to the
A) trp repressor
B) trp operon
C) trp promoter
D) trp operator
E) trp polymerase
D) trp operator
1/10

Created by
Anamaria_Astudillo
Key concepts:
Rna Polymerase Binds To The
The Environment
Structural Gene
Terms in this set (10)

Original
Enhancers are
A) proteins located adjacent to promoters
B) distant sites where regulatory proteins bind
C) expediters of RNA polymerase capture
D) proteins that bind with repressors, deactivating them
E) a bacterial form of promoters
B) distant sites where regulatory proteins bind

When tryptophan is present in the medium, the transcription of tryptophan producing genes in E. coli is stopped by a repressor binding to the
A) trp repressor
B) trp operon
C) trp promoter
D) trp operator
E) trp polymerase
D) trp operator

When tryptophan is abundantly present in the environment of E. coli, the tryptophan binds to the
A) trp operon
B) trp promoter
C) trp operator
D) trp repressor
E) trp polymerase
D) trp repressor

In the function of the lac operon in E. coli, the lac genes are transcribed in the presence of lactose because
A) RNA polymerase binds to the operator
B) the repressor cannot bind to the promoter
C) an isomer of lactose binds to the repressor
D) CAP does not bind to the operator
E) of the absence of cAMP
C) an isomer of lactose binds to the repressor

The role of methylation of DNA is
A) up-regulating DNA transcription.
B) down-regulating DNA transcription.
C) prevention of mutation
D) irrelevant to gene transcription
B) down-regulating DNA transcription

E. coli is able to use foods other than glucose in the absence of available glucose, because falling levels of glucose cause an increase of
A) cAMP
B) CAP
C) lactase
D) glu operons
E) tRNA
A) cAMPX

In the absence of glucose, E. coli can import lactose to change into glucose and galactose because CAP binds to the
A) cAMP
B) DNA
C) lac operon
D) operator
E) repressor
B) DNA

Which is not part of the lac operon?
A) repressor
B) activator protein
C) operator
D) promotor
E) structural gene
B) activator protein

In an operon the location of the regulatory region occurs __ the structural genes.
A) after
B) within
C) before
C) before

Proteins that block the passage of RNA polymerase are called:
A) operons
B) activators
C) repressors
D) enhancers
E) promoters
C) repressors

Terms in this set (21)

Original
What are the 3 types of gene regulation in bacterial cells? At what location in the “central dogma” schematic do they act on?
transcriptional between DNA and mRNA, translational between mRNA and protein, and post translational between protein and activated protein.

Of the 3 types of regulation, which is the most energy efficient? (ie. spends the least amount of energy)
transcriptional because it stops the cell at the earliest stage.

Of the 3 types of regulation, which is the most efficient in terms of speed?
post translational because it is like an on/off switch and the protein is already made.

Complete the statement. Gene expression regulation allows organisms to respond to _ _ _
Changes in environment

E. coli can use a variety of sugars to make ATP. Which is most preferred and why?
Glucose–> starting compound for glycolysis

When would an E.coli switch to using Lactose instead of Glucose?
If Glucose is absent

What is an inducer?
something that triggers transcription of a certain gene

What is the inducer in E.Coli’s synthesis of the enzyme that breaks down Lactose?
Lactose itself

What is negative control/regulation?
when a regulatory protein called a repressor binds to DNA and shuts down transcription

What is positive control?
when a regulatory protein called an activator binds to DNA and triggers transcription

Are activators and repressors sequences of DNA or proteins?
proteins

What is a constitutive mutant?
A mutant cell that produces a certain product at all times (no regulation/response to environment)

What is an operon?
a unit of DNA containing a cluster of genes that are regulated by the same single regulatory factor.

What does the operon contain aside from the cluster of genes?
promoter, operator, and genes.

In an operon, do the different genes have different promoters?
NO, the gene cluster acts as one big gene with one single promoter

How does Lactose “turn on” the transcription of the enzyme to break lactose down? What does it bind to and what happens when it does?
binds to the repressor, which is attached to the DNA. Repressor then changes shape and falls off the DNA.

What part of the DNA is the repressor attached to?
the operator ,

What is an operator? (DNA, protein, etc)
sequence of DNA

What is inducer exclusion and how does Glucose act as an inducer excluder?
Inducer exclusion= when an inducer is prevented from activating a gene
Glucose prevents transport of Lactose to the repressor of the lac operon.

What is Global Gene Regulation?
the coordinated regulation of many genes

What is a regulon and how is it an example of global gene regulation?
A regulon is a group of different genes or operons that contain the same regulatory sequences and are controlled by the same type of regulatory protein…regulates many genes/operons at one time.

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