Molecular Biology: Lac Operon Quiz

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Questions and Answers

What is the default position of an inducible gene?

  • INACTIVE
  • ACTIVE
  • ON
  • OFF (correct)

Which of the following enzymes is encoded by the lacZ gene?

  • Glucosidase
  • Trans-acetylase
  • Beta-galactosidase (correct)
  • Permease

What is the role of cyclic AMP (cAMP) in the regulation of the lac operon?

  • Inhibitor of lactose metabolism
  • Substrate for lactose digestion
  • Cellular alarm signal (correct)
  • Energy source for gene transcription

Which of the following statements about the lac operon is FALSE?

<p>It is activated when glucose is present. (D)</p> Signup and view all the answers

What type of operon is the lac operon classified as?

<p>Inducible operon (A)</p> Signup and view all the answers

Which enzymes are included in the lac operon for lactose metabolism?

<p>β-galactosidase, Permease, Trans-acetylase (B)</p> Signup and view all the answers

How does the lac operon prevent energy wastage?

<p>By turning off transcription when glucose is available (A)</p> Signup and view all the answers

What is described as a polycistronic mRNA in the context of the lac operon?

<p>A single transcript containing multiple gene-coding sequences (B)</p> Signup and view all the answers

What is the primary effect of a nonsense mutation on protein production?

<p>It causes the protein to be truncated or incomplete. (B)</p> Signup and view all the answers

What distinguishes transitions from transversions in point mutations?

<p>Transitions involve substitutions between purines or pyrimidines. (C)</p> Signup and view all the answers

What fundamental change occurs as a result of a frameshift mutation?

<p>It alters the entire reading frame of the genetic code. (D)</p> Signup and view all the answers

Which of the following correctly defines a back mutation?

<p>A second mutation that reverses the effect of an initial mutation. (C)</p> Signup and view all the answers

How does a single base pair deletion affect the reading frame?

<p>It shifts the reading frame for all subsequent codons. (D)</p> Signup and view all the answers

What could potentially happen if two base pairs are inserted into a gene?

<p>It results in a frameshift mutation. (D)</p> Signup and view all the answers

What is the consequence of a mismatch where A pairs with C?

<p>It can result in a mispairing mutation. (D)</p> Signup and view all the answers

What is the impact of a microdeletion in DNA sequence?

<p>It changes the primary sequence of the encoded polypeptide significantly. (C)</p> Signup and view all the answers

What is the primary purpose of the Ames test?

<p>To test the mutagenicity of potentially hazardous chemicals (C)</p> Signup and view all the answers

Which type of bacteria is primarily used in the Ames test?

<p>Histidine auxotrophic strains of Salmonella (C)</p> Signup and view all the answers

What is meant by the term 'reversion' in the context of the Ames test?

<p>The return of bacteria to their wild-type state (B)</p> Signup and view all the answers

What additional component is necessary when conducting the Ames test to activate certain chemicals?

<p>Rat liver extract (A)</p> Signup and view all the answers

How is the degree of mutagenicity assessed in the Ames test?

<p>By measuring the number of revertants observed (D)</p> Signup and view all the answers

In the context of the Ames test, what is a histidine auxotroph?

<p>A mutant strain unable to produce histidine (B)</p> Signup and view all the answers

What observation is expected if a chemical is identified as mutagenic during the Ames test?

<p>Increased rate of reversion in his- bacteria (C)</p> Signup and view all the answers

Why is the Ames test more focused on back mutations than forward mutations?

<p>Revertants can be selected more easily than auxotrophs (B)</p> Signup and view all the answers

Which process involves the uptake of free DNA by a recipient cell?

<p>Transformation (C)</p> Signup and view all the answers

What is required for DNA transfer during conjugation?

<p>Cell-to-cell contact and a conjugative plasmid (B)</p> Signup and view all the answers

What fate does transferred DNA face when integrated into a new cell?

<p>It may recombine with the host chromosome (A), It can only replicate if it has an origin of replication (B)</p> Signup and view all the answers

Which of the following bacterial species is known to undergo natural transformation?

<p>Bacillus spp. (C)</p> Signup and view all the answers

How does a bacterium become competent for transformation?

<p>Chemical treatment and temperature changes (C)</p> Signup and view all the answers

Which statement about restriction enzymes is true?

<p>They degrade transferred DNA within the recipient cell. (C)</p> Signup and view all the answers

Which of the following describes transduction in prokaryotes?

<p>DNA transfer mediated by a virus (A)</p> Signup and view all the answers

What typically dictates a bacterium's competence for transformation?

<p>Genetic determination and regulation (D)</p> Signup and view all the answers

What is the primary reason cells regulate gene expression?

<p>To save energy by only producing necessary proteins (A)</p> Signup and view all the answers

Which type of genes are produced at a constant rate and are crucial for major life processes?

<p>Constitutive (house-keeping) genes (D)</p> Signup and view all the answers

At which level does most gene regulation occur in bacterial cells?

<p>Transcription (A)</p> Signup and view all the answers

What defines an operon?

<p>A group of genes under the control of a single operator (B)</p> Signup and view all the answers

In the context of gene regulation, what is negative control?

<p>Prevention of transcription through a repressor (A)</p> Signup and view all the answers

Which of the following is an example of a structural gene?

<p>Gene coding for a specific protein (C)</p> Signup and view all the answers

What is meant by enzyme induction in terms of gene regulation?

<p>Activation of enzymes in response to specific signals (B)</p> Signup and view all the answers

Which statement about regulatory genes is correct?

<p>They regulate the synthesis of proteins coded by structural genes. (C)</p> Signup and view all the answers

What accumulates in the cell when glucose is no longer available?

<p>cAMP (D)</p> Signup and view all the answers

What is the role of cAMP in the functioning of the lac operon?

<p>It binds to the allosteric site of CAP. (D)</p> Signup and view all the answers

Which type of mutation occurs without external intervention?

<p>Spontaneous mutation (B)</p> Signup and view all the answers

What is a point mutation?

<p>An alteration of a single base pair in DNA. (D)</p> Signup and view all the answers

Which of the following describes a silent mutation?

<p>It does not change the amino acid sequence. (D)</p> Signup and view all the answers

What are induced mutations primarily caused by?

<p>Environmental agents. (C)</p> Signup and view all the answers

What can a missense mutation result in?

<p>A different amino acid at a specific site. (A)</p> Signup and view all the answers

How can oxygen radicals contribute to mutations?

<p>By chemically modifying bases in DNA. (A)</p> Signup and view all the answers

Flashcards

Constitutive Genes

Genes that are constantly expressed at a fixed rate, regardless of environmental conditions.

Inducible Genes

Genes that are only expressed when needed, responding to specific environmental cues.

Structural Genes

Genes that code for proteins that perform specific functions in the cell.

Regulatory Genes

Genes that regulate the expression of other genes, controlling when and how much protein is produced.

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Operon

A cluster of genes that are transcribed together as a single unit, controlled by a single regulatory element.

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Operator

A DNA sequence that a repressor protein binds to, blocking transcription of the genes in the operon.

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Negative Control

A mechanism that regulates gene expression by preventing transcription. A repressor protein binds to the operator, blocking RNA polymerase.

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Positive Control

A mechanism that regulates gene expression by stimulating transcription. An activator protein binds to the operator, enhancing RNA polymerase binding.

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Activator (positive control)

A regulatory protein that binds to DNA and activates transcription.

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Repressible operon

The default state of a repressible operon is ON, meaning gene expression is occurring unless a repressor protein binds to the operator and blocks transcription.

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Inducible operon

The default state of an inducible operon is OFF, meaning gene expression is not occurring unless an inducer molecule binds to the repressor protein, causing it to detach from the operator and allowing transcription to proceed.

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Lac operon

A group of genes that regulate the transport and metabolism of lactose in E. coli.

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Polycistronic mRNA

A single mRNA transcript that includes multiple coding sequences for different proteins.

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β-galactosidase

An enzyme that breaks down lactose into glucose and galactose.

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Permease

A protein that helps transport lactose across the cell membrane.

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cAMP (cyclic AMP)

A molecule that serves as a cellular alarm signal, indicating low glucose levels.

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Mutation

A change in the nucleotide sequence of DNA that can alter the amino acid sequence of a protein and potentially the phenotype of a cell.

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Induced Mutations

Mutations that occur due to external factors like radiation or chemicals.

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Spontaneous Mutations

Mutations that happen spontaneously without external intervention.

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Point Mutation

A point mutation is a change in a single base pair within the DNA sequence.

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Silent Mutation

A type of point mutation where the altered codon still codes for the same amino acid, leading to no change in the protein's function.

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Missense Mutation

A type of point mutation where the altered codon codes for a different amino acid, potentially causing a change in the protein's function.

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Nonsense Mutation

A type of point mutation where the altered codon signals the end of protein synthesis, resulting in a shortened protein.

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Base Substitution

A point mutation where a single base pair is replaced with a different base pair, resulting in a change in the amino acid sequence of the protein.

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Transition

A base substitution where a purine base (A or G) is replaced with another purine, or a pyrimidine base (C or T) is replaced with another pyrimidine.

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Transversion

A base substitution where a purine base (A or G) is replaced with a pyrimidine base (C or T), or vice versa.

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Frameshift Mutation

A mutation that occurs due to the insertion or deletion of one or more base pairs in the DNA sequence, shifting the reading frame of the genetic code.

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Reversion/Back Mutation

A mutation that reverts a previous mutation, restoring the original DNA sequence.

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DNA Repair

The process of correcting mistakes or errors in the DNA sequence, ensuring the integrity of the genetic information.

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Mispairing (Mismatching)

A mutation that occurs when a nucleotide base mispairs with a non-complementary base during DNA replication.

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What is the Ames test?

A test that uses bacteria to determine if a chemical is a mutagen, a substance that can damage DNA and cause mutations.

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What is an auxotroph?

A bacterial strain that cannot synthesize a specific nutrient, making it dependent on an external source of that nutrient.

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What is a reversion mutation?

A mutation that reverses the effect of a previous mutation, restoring the original phenotype.

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What are "his- cells"?

A bacterial strain that lacks the ability to synthesize histidine, requiring it to be provided in the growth medium.

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What are "his+ cells"?

A bacterial strain that has regained the ability to synthesize histidine.

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What is chemical activation in the Ames test?

A process where a chemical is converted by enzymes into a more reactive form that can damage DNA.

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What is the reversion rate?

The rate at which a substance causes mutations in a sample of his- cells.

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Why is rat liver extract used in the Ames test?

Rat liver extract contains various enzymes that can activate chemicals into their mutagenic forms.

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Transformation

The process of transferring genetic information from one prokaryotic cell to another through free DNA.

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Competence

The ability of a cell to take up and incorporate free DNA from its environment.

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Transduction

Transfer of DNA between prokaryotic cells mediated by a bacteriophage (virus that infects bacteria).

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Conjugation

The direct transfer of DNA from one bacterium to another, requiring physical contact between the cells.

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DNA Degradation by Restriction Enzymes

DNA that enters a cell through horizontal gene transfer may get broken down by cellular enzymes.

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Autonomous Replication of Transferred DNA

DNA that enters a cell through horizontal gene transfer can replicate independently if it has its own origin of replication.

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Recombination with the Host Chromosome

DNA that enters a cell through horizontal gene transfer can integrate into the host chromosome, changing the host genome.

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Vertical Gene Transfer

The transfer of genetic material from parent to offspring; the most common way genetic information is passed on in organisms.

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Study Notes

General Microbiology and Immunology: Bacterial Genetics

  • Lecture 2 covers bacterial genetics, focusing on gene regulation, mutations, and genetic element transfer.

Lecture Outline

  • Regulation of gene expression: Examines how cells conserve energy by controlling the production of proteins only when needed.
    • Distinguishes between constitutive (housekeeping) and inducible genes.
    • Discusses the roles of structural and regulatory genes.
    • Uses the lac operon as a key example of gene regulation in prokaryotes.
  • Mutation: Explores changes in DNA sequences, impacting protein production and phenotypes.
    • Categorizes mutations as spontaneous or induced, mentioning various environmental and human-caused agents contributing to induced mutations.
    • Explains how mutations can be caused by oxygen radicals altering DNA bases.
    • Describes point mutations (substitutions) and their effects: silent, missense, and nonsense mutations and their impact on the resulting protein.
    • Details base pair substitutions, including transitions and transversions, to further classify types of point mutations, and how these mutations occur.
    • Frameshift mutations are described as a change in the reading frame due to insertions or deletions of single base pairs. These potentially serious changes have far-reaching consequences in how the polypeptide sequence is produced.
    • Reversion or back mutations can reverse the effects of initial mutations.
  • Transfer of genetic elements: Examines the mechanisms of genetic exchange between prokaryotic cells.
    • Details transformation as free DNA uptake by recipient cells.
    • Highlights transduction, where DNA transfer occurs by viruses.
    • Explains conjugation, a transfer mechanism requiring cell-to-cell contact and a donor plasmid.

Ames Test

  • The Ames test is a bacterial mutagenesis assay.
    • It aids in assessing possible mutagenic and carcinogenic potential of chemicals.
    • Utilizes auxotrophic bacterial strains that do not produce specific nutrients.
    • It's used to detect if a chemical causes an increased rate of back mutations (reversions) in these strains.

Positive Regulation of Lac Operon

  • The lac operon's function relies on both glucose levels and lactose presence.
  • Cyclic AMP (cAMP) levels, derived from ATP and serving as a cellular signal, impact operon activity based on glucose availability.
  • When glucose is low, cAMP builds up, enabling a positive regulatory protein (CAP) to bind to the promoter and enhance RNA polymerase's binding, thus facilitating lac operon transcription.

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