Lactose and lac operon

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  • Created by: Steff06
  • Created on: 03-05-16 09:48
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  • Presence/absence of lactose
    • Absence of lactose:
      • 1. Regulator gene is transcribed and translated and repressor protein is synthesised. It has 2 binding sites - One binds to lactose and one binds to operator region.
      • 2. Repressor protein binds to operator region. It also blocks part of the promoter region where RNA polymerase normally attaches.
      • 3. RNA polymerase cannot bind to the promoter region, so the structural genes cannot be transcribed into mRNA.
      • 4. Without mRNA, these genes cannot be translated and the enzymes B-galactosidase and lactose permease cannot be synthesised.
    • Presence of lactose:
      • 1. Lactose molecules bind to the other site on the repressor protein. This causes molecules of repressor protein to change shape so its other binding site cannot bind to the operator region. Repressor protein breaks away from operator region.
      • 2. This leaves the promoter region unblocked so RNA polymerase can now bind to it and initiate transcription of mRNA for genes Z and Y.
      • 3. The operator-repressor-inducer system acts as a molecular switch. It allows transcription and translation of the structural genes Z and Y into B-galactosidase and lactose permease.
      • 4. E.coli bacteria can then use the lactose permease enzyme to take up lactose from the medium into their cells.
        • They can then convert lactose to glucose and galactose using B-galactosidase. Sugars can then be used for respiration, gaining energy from lactose.


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