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5.5 - CONTROL MECHANISMS housekeeping genes are always needed, so are constantly transcribed and translated but not all proteins are required at all times  cells control transcription and translation of genes

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5.5 - CONTROL MECHANISMS

  • housekeeping genes are always needed, so are constantly transcribed and translated but not all proteins are required at all times
  •  cells control transcription and translation of genes
  • gene regulation turns genes on and off by using transcription factors that bind to DNA and help RNA polymerase bind

Four Levels of Gene Expression Control

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The lac Operon (see Fig. 2, P. 256)

  • β-galactosidase metabolizes lactose in intestinal E. coli, but it is not needed all the time, especially when milk intake decreases as mammals mature  operons are used by prokaryotes as a simple negative control mechanism which blocks the transcription and translation of a gene if a substance is not present
  •  operons are clusters of genes under the control of a promoter (where RNA polymerse binds) and an operator (DNA sequence to which a repressor protein binds) that collectively codes for the enzymes and proteins needed to metabolize a substance
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the lac operon codes for the proteins that metabolize lactose (to glucose and galactose)

when lactose is not present, Lacl protein (a repressor protein encoded by the regulator gene) binds to the operator such that it overlaps the promoter region and blocks RNA polymerase from transcribing and translating the β-galactosidase genes

when lactose is present it acts as an effector binding to the Lacl protein, which changes it’s shape and does not allow for the lactose-Lacl complex to remain bound to the operater region of the lac operon (removing the roadblock for β-galactosidase transcription and translation)

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when tryptophan is not present, the lack of a corepressor does not allow the trp repressor to remain bound to the trp operon, removing the roadblock to tryptophan transcription and translation

  •  when tryptophan is present, it acts as an effector (corepressor), binding to the trp repressor protein, changing it’s shape and allowing the tryptophan-trp repressor complex to bind to the trp operon, blocking the transcription and translation of the genes to produce tryptophan

The trp Operon (see Fig. 3, P. 257)

E. coli can manufacture the amino acid tryptophan if they are away from a readily available source (i.e. a mammal’s food), but need to able to know when not to produce it

 the trp operon is repressed when high levels of tryptophan are present