Genetics: Regulation of Transcription and Translation

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Questions and Answers

Describe how regulating mRNA half-life can control gene expression.

By altering how long an mRNA molecule persists in the cell, the amount of protein produced from it can be controlled; a longer half-life leads to more protein, while a shorter one reduces protein production.

Explain why the operon organization of genes is predominantly found in prokaryotes and not eukaryotes.

Prokaryotes often need to coordinate expression of functionally related genes; operons allow for simultaneous transcription of these genes from a single promoter, which is less commonly needed or feasible in eukaryotes due to their more complex gene regulation and cell structure.

Differentiate between a positive and a negative control system in the context of gene regulation.

In a positive control system, a regulatory protein (activator) must bind to the DNA to initiate transcription, whereas in a negative control system, a regulatory protein (repressor) must be prevented from binding to the DNA to allow transcription.

What distinguishes a constitutive gene from an inducible gene?

<p>A constitutive gene is always expressed, while an inducible gene is only expressed in response to specific environmental conditions or signals.</p> Signup and view all the answers

Describe how induction acts in a positive control system.

<p>In a positive control system with induction, an inducer molecule binds to an activator protein, causing a conformational change that allows the activator protein to bind to DNA and promote transcription; without the inducer, the activator protein cannot effectively bind and transcription is minimal.</p> Signup and view all the answers

Explain the role of the operator in negative control of gene expression.

<p>The operator is a DNA sequence located downstream of the promoter where a repressor protein binds; this binding physically blocks RNA polymerase from transcribing the gene, thus preventing gene expression.</p> Signup and view all the answers

Describe how the presence of lactose affects the expression of the lac operon.

<p>Lactose, when present, is converted to allolactose, which binds to the lac repressor, causing it to detach from the operator. This allows RNA polymerase to bind to the promoter and begin transcription of the lac operon genes.</p> Signup and view all the answers

Describe one way that bacteria use two-component regulatory systems to sense and respond to their environment.

<p>A sensor kinase detects an environmental signal and phosphorylates a response regulator. The phosphorylated response regulator then binds to DNA and alters gene expression appropriate to the signal.</p> Signup and view all the answers

What is the final result on protein levels if a gene is turned on?

<p>Turning on a gene ultimately leads to higher levels of the protein.</p> Signup and view all the answers

What are the four ways a cell can change how much protein is made from the same gene?

<p>A cell can regulate: <strong>1.</strong> transcription, <strong>2.</strong> mRNA half-life, <strong>3.</strong> translation, and <strong>4.</strong> degradation of the protein.</p> Signup and view all the answers

Explain what an operon is.

<p>An operon is a cluster of genes transcribed as a single mRNA molecule from a single control region.</p> Signup and view all the answers

Explain the function of an activator protein.

<p>An activator protein promotes transcription when bound to DNA.</p> Signup and view all the answers

Explain the function of a repressor protein.

<p>A repressor protein prevents transcription when bound to DNA.</p> Signup and view all the answers

How does a repressor protein prevent mRNA synthesis?

<p>Binding of the repressor protein downstream of the promoter prevents mRNA synthesis.</p> Signup and view all the answers

How does a activator protein allow mRNA synthesis?

<p>The activator protein binds upstream of the promoter. This helps stabilize RNA polymerase binding to the promoter.</p> Signup and view all the answers

Why would a cell want to regulate genes in response to environmental signals?

<p>A cell regulates genes in response to enviromental signals for efficiency. A small molecule turns on a gene (induction) or turns off a gene (repression).</p> Signup and view all the answers

How does having high levels of amino acids affect gene expression?

<p>When there are high levels of the amino acid arginine, the expression is turned off, blocking transcription.</p> Signup and view all the answers

How does a inducer work in positive control?

<p>The presence of a inducer binds to the activator. The inducer changes the shape of the activator so it will bind. Then RNA polymerase binds to the promoter and initiates transcription.</p> Signup and view all the answers

Name one way in which the bacterium responds to the environmental presence of lactose and glucose.

<p>If lactose is present, the bacterium wants to break it down and get energy. If lactose is not present, it is wasteful to transcribe and translate the genes the cell would use.</p> Signup and view all the answers

What happens when there is glucose and lactose present?

<p>When glucose and lactose are present mRNA polymerase cannot bind and structural genes are not transcribed.</p> Signup and view all the answers

Explain the relationship between cAMP, cAMP receptor protein (CRP), and catabolite activator protein (CAP).

<p>When glucose is low, cells produce cyclic AMP (cAMP), which binds to cAMP recptor protein (CRP), also known as catabolite activator protein (CAP).</p> Signup and view all the answers

Explain how quorum sensing is used as a two-component regulatory system.

<p>In quorum sensing, as the population density increases, so does the concentration of autoinducers. Once a threshold concentration is reached, the autoinducers bind to a receptor, triggering altered gene expression and coordinating group behaviors.</p> Signup and view all the answers

How does the absence of a signal lead to gene OFF in an induction system?

<p>In an induction system, the gene is 'off' unless a specific signal molecule is present. The absence of this signal means there’s no trigger to activate gene expression, thus keeping the gene turned off.</p> Signup and view all the answers

Describe one specific example of an inducible gene.

<p>Genes for breaking down exotic carbon sources are typically inducible, only being expressed when that specific carbon source is available in the environment.</p> Signup and view all the answers

Explain how low levels of amino acids within a cell can lead to the expression of genes involved in amino acid synthesis.

<p>When amino acid levels are low, the repressor protein is inactive and can’t bind to the operator. This allows RNA polymerase to bind to the promoter and initiate transcription of the genes needed for amino acid synthesis.</p> Signup and view all the answers

Describe what happens when a sensor kinase detects an environmental signal.

<p>Upon detecting an environmental signal, a sensor kinase typically phosphorylates itself and then transfers the phosphate group to a response regulator protein, activating it.</p> Signup and view all the answers

How does high cell density affect gene expression in quorum sensing?

<p>As cell density increases, the concentration of autoinducers rises. When this concentration reaches a threshold, it triggers a coordinated change in gene expression across the population, often related to virulence or biofilm formation.</p> Signup and view all the answers

Explain why the lac operon is described as both under negative and positive control.

<p>The <em>lac</em> operon is under negative control because a repressor protein blocks transcription in the absence of lactose. It's under positive control because CAP enhances transcription when glucose is scarce.</p> Signup and view all the answers

How does the binding of allolactose to the lac repressor influence transcription?

<p>Allolactose binds to the <em>lac</em> repressor, causing it to change shape and detach from the operator. This allows RNA polymerase to bind to the promoter and initiate transcription of the <em>lac</em> operon genes.</p> Signup and view all the answers

Under what conditions will the lac operon be most actively transcribed?

<p>The <em>lac</em> operon will be most actively transcribed when lactose is present (to inactivate the repressor) and glucose is absent (allowing CAP to enhance transcription).</p> Signup and view all the answers

Describe the role of homoserine lactone (HSL) in quorum sensing.

<p>HSL is a common autoinducer molecule in quorum sensing. It diffuses out of bacterial cells and, at high concentrations, re-enters the cells and binds to a receptor protein, triggering changes in gene expression.</p> Signup and view all the answers

What is the function of cassette switching in bacteria, and why is it beneficial?

<p>Cassette switching involves moving different gene segments into an expression site, allowing bacteria to alter surface structures or other traits. This helps bacteria evade the host immune system or adapt to changing environments.</p> Signup and view all the answers

What cellular component does the sensor kinase phosphorylate?

<p>The sensor kinase phosphorylates the response regulator.</p> Signup and view all the answers

What is the function of cassette switching?

<p>Cassette switching allows bacteria to alter their gene expression. Recombination is used to swap silent (cryptic) genes, inserting it into an expressed gene location.</p> Signup and view all the answers

Flashcards

Gene regulation

The process of influencing gene expression, either by increasing or decreasing the production of a gene product.

Operon

A unit of linked genes which is thought to regulate other genes responsible for protein synthesis.

Promoter

DNA sequence where RNA polymerase attaches and initiates transcription of a gene.

Control protein

A protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence.

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Negative control

Blocks transcription.

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Operator

A DNA segment to which a repressor binds, thereby blocking the transcription of adjacent genes.

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Repressor protein

Protein that binds to the operator in prokaryotes or to silencers in eukaryotes to prevent RNA polymerase from transcribing the gene.

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Positive control

Stimulates transcription.

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Activator protein

Protein that increases gene transcription.

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Inducer

A molecule that initiates gene expression.

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Induction

The process of stimulating the synthesis of a specific substance

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Inducible

Gene capable of being expressed only when induced.

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Co-repressor

Substance that inhibits the expression of genes.

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Repression

The action or process of suppressing a gene.

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Repressible

Gene whose expression decreases in response to a signal.

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Lac operon

The set of genes required for the transport and metabolism of lactose in E. coli.

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Lac repressor protein

A protein that binds to the operator site of the lac operon and inhibits transcription.

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Negative control with induction

Transcription of the lac operon is activated by the presence of lactose.

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Catabolite activator protein (CAP)

A protein that binds to DNA and increases transcription of the lac operon.

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Cyclic AMP (cAMP)

A derivative of adenosine monophosphate (AMP) used for intracellular signal transduction in many different organisms.

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Positive control with induction

Lac operon transcription enhanced by CAP when glucose is scarce.

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Two component regulatory system

Mechanism used by bacteria to sense their environment and regulate gene expression accordingly, often involving a sensor kinase and a response regulator.

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Sensor kinase

Membrane protein that detects an environmental signal and activates itself by phosphorylation.

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Response regulator

A protein that, once phosphorylated by a sensor kinase, regulates gene expression.

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Quorum sensing

Regulation based on population density.

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Homoserine lactone (HSL)

Small signaling molecule used in quorum sensing.

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Cassette switching

Switch from one gene expression to another.

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Silent (cryptic) gene

A gene that is not expressed although present in the genome.

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Expressed gene

A gene that is transcribed and translated into a functional protein.

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Constitutive gene

A gene that is transcribed continuously regardless of environmental conditions.

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Study Notes

Lecture 16 – Genetics: Regulation of Transcription and Translation

  • Genes can be "turned off" or "turned on," indicating whether they are actively producing proteins
  • Cells can vary protein amounts from the same gene through multiple strategies
  • The operon concept is relevant to prokaryotes
  • Positive control systems activate gene expression
  • Negative control systems repress gene expression
  • Constitutive genes are continuously expressed and not regulated
  • Inducible genes are activated under certain conditions
  • Repressible genes are deactivated under certain conditions

Space Bugs and Gene Expression

  • Salmonella typhimurium flown in space altered the expression of 167 genes
  • These bacteria were almost three times as likely to kill infected mice than standard samples
  • Low fluid shear is a novel environmental signal that reprograms Salmonella by altering gene expression
  • This induces formerly unrecognized stress and virulence responses; bacteria adapt genetic machinery to survive in new environments

Levels of Regulation

  • Gene regulation can occur at multiple levels, including transcription, mRNA half-life, translation, and protein degradation
  • Regulation of transcription is specifically considered

Operons

  • Prokaryotic genes are arranged in operons, which allow multiple genes to be transcribed as a single mRNA from one control region
  • Each gene in the operon has its own Shine-Dalgarno sequence

Gene Regulation by Proteins

  • Gene expression can be turned "on" or "off" using protein regulators
  • Repressor proteins bind downstream of the promoter, blocking mRNA synthesis
  • This action is known as negative control
  • Activator proteins bind upstream of the promoter, enhancing mRNA synthesis
  • This action is positive control

Transcriptional Activators Defined

  • Transcriptional activators are proteins that bind to specific DNA sequences "upstream" of a gene and increase transcription

Environmental Signals

  • Gene expression responds to environmental signals
  • Small molecules turn genes ON (Induction)
  • Signal absence means gene is OFF
  • Small molecules turn genes OFF (Repression)
  • Absence of signal means gene is ON

Gene Transcription

  • Constitutive genes have mRNA synthesized constantly
  • Inducible genes can be "turned on" by certain conditions
  • Repressible genes, mRNA is usually produced, but can be "turned off" by certain conditions

Negative Control Mechanisms

  • Regulation involves both protein and environmental aspects
  • In negative control, a repressor can block transcription, or a corepressor can enable the repressor to bind

Negative Control

  • Low arginine levels mean ribosomes do not bind, so no enzyme created
  • High arginine levels in the cell result in arginine being a corepressor
  • Arginine binds allosterically to the arg repressor
  • Bound complex then binds DNA to prevent transcription

Induction

  • Transcription on is blocked by a repressor, but induced reaction prevents transcription

Positive Control and Induction

  • In positive control, an inducer binds to the activator, enabling its binding to the activator-binding site and allowing RNA polymerase to initiate transcription

The Lac Operon Overview

  • The lac operon enables bacteria to respond to lactose and glucose:
  • For utilization via breakdown for energy, if present
  • if not, transcription is wasteful
  • If glucose is also present, it is prioritized

Lac Operon Specifics

  • The lac operon: negative control/induction example
  • If +Glucose and -Lactose, transcriptional repressor (lacl) binds to the lac operator
  • If -Glucose and +Lactose presence, the enzyme produced by the lacA gene converts lactose allolactose, inducer
  • Lacl (repressor) binds to the inducer and changes its structure, and leaves the lac operator
  • If +Glucose and +Lactose presence, simply removing the lac repressor is not enough to allow for transcription of the operon
  • The lac operon is is controlled by repressor AND activator proteins
  • If -Glucose and +Lactose, low glucose leads to cyclic AMP (cAMP) production
  • cAMP binds to cAMP receptor protein (CRP), also known as catabolite activator protein (CAP)

Transcription Activation

  • Transcription is not activated with low cAMP, the CAP cannot bind and the repressor is bound to operator
  • Transcription is not activated when lactose is present, low cAMP and CAP cannot bind, and the inducer prevents repressor from binding

Transcription Activation pt.2

  • Transcription activated but blocked with no glucose, the repressor is bound
  • Transcription is activated with no glucose or repressor

Regulation Systems

  • Two-component regulatory systems are used
  • (1) sensor kinase senses environment
  • (2) response regulator activator or repressor protein that regulates the cell's response to the environment

Quorum Sensing

  • Quorum sensing regulates bacterial virulence through the production of extracellular polysaccharides, biofilm formation, and iron-binding activities

Cassette Switching

  • Cassette switching is a mechanism used by some bacteria to alter their gene expression
  • Cassette switching involves switching between different gene cassettes to express different traits

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