Lac operon

• operons

  • in prokaryotes- control of gene expression requires binding of transcription factors to operons

  structure:

  -cluster of structural genes transcribed together

  -control elements, promoter and operator region

  -some have regulatory genes, code for activators or repressors
• promoter

  • a DNA sequence RNA polymerase bind to

  operator

  • where transcription factors bind
• the lac operon(bacteria)

  • controls the production of lactase and 2 other structural proteins
  • lactase breaks down substrate lactose to be used as an energy source in bacterial cells
  • helps bacteria avoid wasting energy
• when lactose absent

  • regulatory gene transcribed and translated to produce LAC repressor protein
  • LAC repressor protein binds to operator region
  • RNA polymerase thus unable to bind to promoter region
  • transcription of structural genes does not take place and no lactase enzxyme is synthesised
• when lactose present

  • uptake of lactose by bacterium
  • lactose binds to repressor protein, distorting shape, repressor protein cannot bind to operator region
  • RNA polymerase then can bind to promoter and transcription takes place
  • MRNA for all structural genes translated, lactase produced and lactose broke down and used for energy by bacterium