Biochem Lecture 14a
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Questions and Answers

What happens when glucose levels are low in relation to cAMP levels and the lac genes?

  • CRP is bound to the DNA
  • CRP-cAMP forms and binds to the DNA (correct)
  • No transcription occurs because CRP is not bound
  • Activation of transcription of the lac genes at high levels

    • In the absence of lactose, why does transcription not occur in the lac operon?

  • Low cAMP levels prevent CRP binding
  • CRP is bound to the DNA
  • The absence of allolactose (correct)
  • No specific reason, transcription should occur

    • How does the binding of allolactose to the repressor affect transcription in the presence of lactose?

  • Facilitates binding of CRP to DNA
  • Enhances CRP-cAMP formation
  • Increases cAMP levels
  • Prevents the repressor from binding to DNA (correct)

    • What is the main characteristic of an operon in bacteria?

    <p>A cluster of genes sharing a promoter and regulatory sequences</p> Signup and view all the answers

    Which enzyme in the lac operon is responsible for the cleavage of lactose into glucose and galactose?

    <p>β-galactosidase (lacZ)</p> Signup and view all the answers

    Which of the following proteins regulate transcription in an operon by either activating or repressing it?

    <p>Proteins binding to regulatory sequences</p> Signup and view all the answers

    In the lac operon, what is the function of lactose permease (galactoside permease; lacY)?

    <p>Transports lactose into the cell</p> Signup and view all the answers

    Which form of regulation is primarily involved in controlling the lac operon?

    <p>Negative regulation via a repressor</p> Signup and view all the answers

    What is the main function of CRP-cAMP in the lac operon system?

    <p>Stimulating transcription</p> Signup and view all the answers

    Under what condition does CRP-cAMP have its stimulatory effect on lac operon transcription?

    <p>Low glucose and high cAMP</p> Signup and view all the answers

    What is the role of allolactose in the lac operon system?

    <p>Causing repressor dissociation from operator</p> Signup and view all the answers

    In the absence of lactose and with high glucose levels, what is the impact on lac operon transcription?

    <p>No transcription due to repressor binding</p> Signup and view all the answers

    What are the two requirements for the strongest induction of the lac operon?

    <p>[Glucose] must be low and cAMP must increase</p> Signup and view all the answers

    What is a common characteristic of enhancers in bacteria?

    <p>Enhancers are often adjacent to weak promoters.</p> Signup and view all the answers

    In positive regulation, what is the role of activators like CRP in the lac operon?

    <p>Activators stimulate transcription in the presence of a signal.</p> Signup and view all the answers

    What is the outcome of repressor dissociation from DNA in negative regulation?

    <p>Transcription is stimulated.</p> Signup and view all the answers

    How does effector binding alter the activity of activators or repressors?

    <p>Effector binding may increase or decrease activator or repressor affinity for DNA elements.</p> Signup and view all the answers

    What occurs when a signal causes a repressor to dissociate from DNA?

    <p>Transcription is stimulated.</p> Signup and view all the answers

    How does the Lac repressor help prevent RNA polymerase from binding to the promoter?

    <p>By binding primarily to the O1 operator adjacent to the promoter</p> Signup and view all the answers

    What role does glucose play in the regulation of lactose-digesting genes in the Lac operon?

    <p>Turns off lactose genes when present</p> Signup and view all the answers

    What is the primary function of the Lac repressor in the lac operon?

    <p>To prevent RNA polymerase from binding to the promoter</p> Signup and view all the answers

    Which molecule mediates catabolite repression in the Lac operon?

    <p>cAMP and cAMP Receptor Protein (CRP)</p> Signup and view all the answers

    What effect does binding of the Lac repressor to secondary operators (O2 or O3) have on transcription?

    <p>Reduces transcription but still occurs at a low rate</p> Signup and view all the answers

    What is the outcome when CRP-cAMP is bound to DNA in the lac operon system?

    <p>Transcription is activated</p> Signup and view all the answers

    Which molecule prevents the repressor from binding to DNA in the lac operon system?

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

    What are the major groove characteristics that facilitate protein binding to DNA?

    <p>Exposes hydrogen bonding groups for alpha-helix binding</p> Signup and view all the answers

    How do Gln/Asn amino acids contribute to DNA-binding specificity?

    <p>Form specific hydrogen bonds with Thymine-Adenine base pairs</p> Signup and view all the answers

    Which condition leads to the absence of CRP binding in the lac operon system?

    <p>[cAMP] is low and glucose levels are high</p> Signup and view all the answers

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