Control of Transcription III: Lac Operon

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  1. a.      Trp operon is repressible because __.     
    b.__operon is usually off but
    can be stimulated when specific small molecule interacts with regulatory
    protein (ex: lac operon)
    • its transcription is usually on but can be inhibited when specific small molecule binds allosterically to regulatory protein
    • Inducible
  2. a.      Lactose:                                                              i.      Lactose metabolism: hydrolysis into __ and __ by __
    1.      Only a few molecules of this enzymes are present in E. coli cell growing in absence of lactose
    a.      If added, number of __increases a thousandfold
    • glucose and galactose
    • beta-galacosidase
    • beta-galactosidase
  3.                                                               i.      __ is part of lac operon, which includes two other genes coding for enzymes that function in lactose utilization
    1.      Entire transcription unit under command of __
    a.      The regulatory gene, lacI, located outside the operon, codes for an Allosteric repressor protein that can do what?
    • Beta-galactosidase
    • one main operator and promoter
    • switch off the lac operon by binding to operator
  4.                                                               i.      Difference between lac and trp
    • 1.      Trp is inactive by itself and requires tryptophan as a corepressor in order to bind to operator
    • 2.      Lac repressor is active by itself, binding to operator and switching the lac operon off
  5. a.      Inducer __the repressor                                                                                                                                      i.      For lac operon, it is __
    ii.      In absence of lactose and hence allolactose, the lac repressor is in its __, and genes of lac operon are __
    • inactivates
    • allolactose                                                                                                                                   
    • active configuration
    • silenced
  6. 1.      If lactose added, allolactose __
    a.      Without bound repressor, the lac operon is transcribed into mRNA for the lactose-utilizing enzymes
    binds to the lac repressor and alters its conformation, nullifying the repressor’s ability to attach the operator
  7. 1.      the enzymes of the lactose pathway= __enzymes because their synthesis is __by a chemical signal
    - the enzymes for tryptophan synthesis are said to be __
    • inducible
    • induced
    • repressible
  8. 1.      repressible enzymes generally function in __pathways, which synthesize essential end products from raw materials
    a.      by suspending production of an end product when it is already present in excess, the cell can __
    • anabolic
    • allocate its organic precursors and energy for other uses
  9. 1.      in contrast, inducible enzymes usually function in __pathways, which break down a nutrient to simpler molecules
    a.      by producing the appropriate enzymes only when the nutrient is available, __
    • catabolic
    • the cell avoids wasting energy and precursors making proteins that are not needed
  10. a.      regulation of both the trp and lac operons involves __control of genes, because the operons are switched off by the active form of the repressor protein                                                              i.      for lac, it is __because allolactose induces enzyme synthesis not by acting directly on the genome, but by freeing the lac operon from the negative effect of the repressor
    • negative
    • positive
  11. when does positive only occur?
    a.      positive only occurs when regulatory protein interacts directly with genome to switch transcription on
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Control of Transcription III: Lac Operon

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