Why is it that structural genes of the lac operon can not be transcribed without the presence of lactose?

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Why is it that structural genes of the lac operon can not be transcribed without the presence of lactose?

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This is not a cop-out but if you watch the superb animation in the link I guarantee all will be clear. http://www.biostudio.com/d_%20Lac%20Oper… The genes on the lac operon are for the metabolism of lactose – so if there is no lactose there is no need for the genes to be working -this is due to a repressor molecule binding to the operator region of the gene and preventing RNA Polymerase from reading the DNA and producing the mRNA needed to make the proteins required for lactose metabolism. In the prescence of lactose the repressor molecule binds to lactose and in doing so no longer binds to the gene and so the RNA Polymerase can now transcribe the genes and the proteins will be made to metabolise the lactose.