RNA Synthesis and Polymerase
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Questions and Answers

What is generated from an operon?

  • Monocistronic transcript
  • mRNA recognized by sigma factors
  • Single polypeptide
  • Polycistronic transcript (correct)

    • Which sigma factor is most commonly associated with recognizing promoter regions in bacteria?

  • Sigma 32
  • Sigma 70 (correct)
  • Sigma 54
  • Sigma 24

    • How do regulatory proteins control gene expression?

  • Through post-transcriptional modifications
  • Through the binding of small molecules (correct)
  • By modifying histones
  • By degrading mRNA

    • What is the role of CAP in gene regulation when bound to cAMP?

    <p>Enhances efficiency of RNA polymerase entry, binding, and initiation</p> Signup and view all the answers

    Which inhibitor of RNA synthesis works by binding to the beta-subunit of RNA polymerase?

    <p>Rifamycin</p> Signup and view all the answers

    How do bacteria respond to the presence of lactose in the medium with respect to beta-galactosidase production?

    <p>Beta-galactosidase is produced at high levels</p> Signup and view all the answers

    Which of the following statements is true about the lifespans of synthesized RNA?

    <p>mRNA is unstable and has a half-life of 2 minutes.</p> Signup and view all the answers

    Why is the error frequency higher in RNA synthesis compared to DNA replication?

    <p>RNA polymerase lacks 3’-5’ exonuclease proof-reading activity.</p> Signup and view all the answers

    What is the difference between a CORE ENZYME and a HOLOENZYME?

    <p>HOLOENZYME includes α_2, β, β’, and σ subunits.</p> Signup and view all the answers

    Which sequence is located upstream of the transcription start site in prokaryotes?

    <p>Pribnow box</p> Signup and view all the answers

    What is the role of the σ factor in bacterial transcription?

    <p>To allow the core RNA polymerase to specifically recognize promoters.</p> Signup and view all the answers

    How does termination of transcription occur at rho-independent sites?

    <p>A stable GC-rich hairpin followed by 6 uracils causes RNA polymerase to dissociate.</p> Signup and view all the answers

    What event occurs after the RNA polymerase releases the σ subunit?

    <p>RNA-DNA duplex is broken, and polymerase continues elongation.</p> Signup and view all the answers

    Which component is necessary for the opening of the DNA strands around the start site?

    <p>Open complex</p> Signup and view all the answers

    What is the main reason why RNA polymerase is asymmetric?

    <p>It has two alpha subunits and one of the other subunits</p> Signup and view all the answers

    What is the function of the rho protein in transcription termination?

    <p>To unwind the RNA-DNA complex</p> Signup and view all the answers

    What is the purpose of the -10 and -35 boxes in prokaryotic promoters?

    <p>To provide a binding site for the sigma subunit</p> Signup and view all the answers

    What is the consequence of the lack of 3'-5' exonuclease proof-reading activity in RNA polymerase?

    <p>Higher error frequency in RNA synthesis</p> Signup and view all the answers

    What is the role of the sigma subunit in bacterial transcription?

    <p>To bind to the promoter region</p> Signup and view all the answers

    What is the characteristic of the hairpin structure at rho-independent termination sites?

    <p>It is rich in uracil residues</p> Signup and view all the answers

    What is the effect of a strong promoter on gene expression?

    <p>It increases the efficiency of transcription</p> Signup and view all the answers

    What is the consequence of the closed complex formation between RNA polymerase and the promoter?

    <p>The strands of DNA are opened</p> Signup and view all the answers

    What characterizes the mRNA produced from an operon?

    <p>Polycistronic transcript</p> Signup and view all the answers

    Which sigma factor is specifically associated with heat shock response in bacteria?

    <p>Sigma 32</p> Signup and view all the answers

    What happens to LAC repressor when lactose is bound?

    <p>It ceases to function as a repressor</p> Signup and view all the answers

    How does actinomycin D inhibit RNA synthesis?

    <p>Binding to double-stranded DNA between GC pairs</p> Signup and view all the answers

    What effect does cAMP binding have on the CAP protein in bacterial gene regulation?

    <p>It enhances RNA polymerase binding and initiation</p> Signup and view all the answers

    Which environmental condition leads to high levels of beta-galactosidase in bacteria?

    <p>Low levels of glucose and high levels of lactose</p> Signup and view all the answers

    Study Notes

    RNA Synthesis

    • A single RNA polymerase synthesizes all types of RNA: rRNA, tRNA, and mRNA
    • rRNA and tRNA are stable once synthesized, whereas mRNA is unstable with a half-life of 2 minutes
    • RNA synthesis requires a DNA template and riboNTPs

    Error Frequency and Proofreading

    • Error frequency in RNA synthesis is 1 per 10^4 nucleotides, higher than DNA replication
    • This is because RNA polymerase lacks 3'-5' exonuclease proofreading activity and there are no other correction mechanisms

    RNA Polymerase Structure

    • Core enzyme contains α2, β, and β' subunits
    • Holoenzyme contains α2, β, β', and σ subunits
    • σ subunits are less abundant than core subunits
    • RNA polymerase is asymmetric, having two alpha subunits and only one of the other subunits

    Promoters and Transcription Initiation

    • Promoters are transcription start sites recognized by a core RNA polymerase associated with a σ factor
    • Consensus sequence maximizes homology, including the -10/Pribnow box and -35 box conserved upstream of the start site
    • Promoter is asymmetric, matching the asymmetric RNA polymerase structure
    • Holoenzyme binds to the promoter without opening DNA strands (closed complex)
    • Strands of DNA are opened around the start site (open complex), and selection of NTPs occurs

    Transcription Elongation and Termination

    • RNA-DNA duplex is broken, and polymerase dissociates
    • Rho-independent sites terminate transcription due to a GC-rich hairpin followed by 6 uracils
    • Rho-dependent sites have a hairpin but no U tract, preceded by a cytosine-rich region
    • Rho (ATP-dependent helicase) binds to the cytosine-rich RNA, unwinding the RNA-DNA complex

    Regulation of Gene Expression

    • Regulation is partly due to promoter efficiency and regulatory proteins controlling access to RNA polymerase
    • Promoters have different efficiencies due to sequence differences
    • Operon: a set of genes controlled by one promoter, producing a polycistronic transcript
    • Sigma factors recognize different promoter regions, regulating gene expression
    • Regulatory proteins: repressors block RNA synthesis, while positive regulators enhance RNA polymerase activity

    Examples of Gene Regulation

    • Lac repressor and CAP regulator: repressor blocks RNA synthesis when bound to DNA, while positive regulator enhances RNA polymerase activity
    • Bacteria change protein expression in response to environmental conditions, such as the induction of β-galactosidase in the presence of lactose

    Inhibitors of RNA Synthesis

    • Actinomycin D inhibits RNA synthesis by binding to dsDNA between GC pairs
    • Rifamycin, an antibiotic, binds to the β-subunit of RNA polymerase, blocking bacterial synthesis and used to treat tuberculosis

    RNA Synthesis

    • A single RNA polymerase synthesizes all types of RNA: rRNA, tRNA, and mRNA
    • rRNA and tRNA are stable once synthesized, whereas mRNA is unstable with a half-life of 2 minutes
    • RNA synthesis requires a DNA template and riboNTPs

    Error Frequency and Proofreading

    • Error frequency in RNA synthesis is 1 per 10^4 nucleotides, higher than DNA replication
    • This is because RNA polymerase lacks 3'-5' exonuclease proofreading activity and there are no other correction mechanisms

    RNA Polymerase Structure

    • Core enzyme contains α2, β, and β' subunits
    • Holoenzyme contains α2, β, β', and σ subunits
    • σ subunits are less abundant than core subunits
    • RNA polymerase is asymmetric, having two alpha subunits and only one of the other subunits

    Promoters and Transcription Initiation

    • Promoters are transcription start sites recognized by a core RNA polymerase associated with a σ factor
    • Consensus sequence maximizes homology, including the -10/Pribnow box and -35 box conserved upstream of the start site
    • Promoter is asymmetric, matching the asymmetric RNA polymerase structure
    • Holoenzyme binds to the promoter without opening DNA strands (closed complex)
    • Strands of DNA are opened around the start site (open complex), and selection of NTPs occurs

    Transcription Elongation and Termination

    • RNA-DNA duplex is broken, and polymerase dissociates
    • Rho-independent sites terminate transcription due to a GC-rich hairpin followed by 6 uracils
    • Rho-dependent sites have a hairpin but no U tract, preceded by a cytosine-rich region
    • Rho (ATP-dependent helicase) binds to the cytosine-rich RNA, unwinding the RNA-DNA complex

    Regulation of Gene Expression

    • Regulation is partly due to promoter efficiency and regulatory proteins controlling access to RNA polymerase
    • Promoters have different efficiencies due to sequence differences
    • Operon: a set of genes controlled by one promoter, producing a polycistronic transcript
    • Sigma factors recognize different promoter regions, regulating gene expression
    • Regulatory proteins: repressors block RNA synthesis, while positive regulators enhance RNA polymerase activity

    Examples of Gene Regulation

    • Lac repressor and CAP regulator: repressor blocks RNA synthesis when bound to DNA, while positive regulator enhances RNA polymerase activity
    • Bacteria change protein expression in response to environmental conditions, such as the induction of β-galactosidase in the presence of lactose

    Inhibitors of RNA Synthesis

    • Actinomycin D inhibits RNA synthesis by binding to dsDNA between GC pairs
    • Rifamycin, an antibiotic, binds to the β-subunit of RNA polymerase, blocking bacterial synthesis and used to treat tuberculosis

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    Description

    This quiz covers the basics of RNA synthesis, including the role of RNA polymerase, the stability of different types of RNA, and the error frequency of RNA synthesis.

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