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Genetics: Gene Regulation and the lac Operon

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  • What is the role of allosteric effectors in gene regulation?
    Allosteric effectors bind regulatory proteins, causing conformational changes that affect DNA binding and regulate transcription.
  • How does an inducer affect a repressor protein?
    An inducer binds the repressor's allosteric domain, preventing it from binding DNA and allowing transcription.
  • What happens when a corepressor binds a repressor protein?
    The corepressor-repressor complex binds DNA, blocking transcription by negative regulation.
  • What is coupled transcription and translation in bacteria?
    In bacteria, translation begins on mRNA while it is still being transcribed by RNA polymerase.
  • What are the three structural genes of the lac operon and their functions?
    lacZ encodes β-galactosidase, lacY encodes permease, and lacA encodes transacetylase.
  • What is the function of the lacI gene?
    lacI encodes the lac repressor protein, which binds the operator to inhibit transcription; it is constitutively expressed.
  • How does the lac repressor regulate the lac operon?
    The lac repressor binds the operator sequence to block transcription; binding of allolactose inducer causes it to release DNA.
  • What is catabolite repression in the lac operon?
    Catabolite repression is positive regulation where CAP-cAMP complex enhances RNA polymerase binding to the lac promoter when glucose is low.
  • How do glucose and lactose levels affect lac operon expression?
    High glucose lowers cAMP, reducing CAP binding; lactose presence inactivates the repressor, allowing transcription if CAP is bound.
  • What happens to lac operon transcription when glucose is high and lactose is low?
    cAMP is low, CAP is unbound, and lac repressor binds operator, so transcription is off.
  • What happens to lac operon transcription when glucose is low and lactose is high?
    cAMP is high, CAP binds promoter, lac repressor is inactivated by allolactose, so transcription is on.
  • What is the role of the CAP-cAMP complex in lac operon regulation?
    CAP-cAMP binds the promoter region, increasing RNA polymerase affinity and activating transcription.
  • What is the structure of the lac operon promoter region?
    It includes the CAP binding site, -35 and -10 sequences, operator, and Shine-Dalgarno sequence for translation initiation.
  • What is the significance of symmetrical DNA binding sites in gene regulation?
    Symmetrical binding sites allow dimeric regulatory proteins to bind DNA with high specificity and stability.
  • How does the lac repressor protein bind DNA?
    It uses helix-turn-helix motifs to bind operator DNA sequences and block transcription.
  • What is the metabolic role of β-galactosidase in lactose metabolism?
    β-galactosidase cleaves lactose into glucose and galactose for bacterial metabolism.
  • What is the function of permease in the lac operon?
    Permease facilitates lactose transport into the bacterial cell.
  • What is the role of allolactose in lac operon regulation?
    Allolactose acts as an inducer by binding the lac repressor and preventing it from binding the operator.
  • How does the tryptophan operon differ from the lac operon in regulation?
    The tryptophan operon is repressible and uses a corepressor (tryptophan) to activate the repressor and block transcription.
  • What triggers transcription in the tryptophan operon?
    Transcription occurs when tryptophan is absent, inactivating the repressor and allowing gene expression.