2.1.5 OCR A2 Biology Lac Operon

Notes and a diagram on the presence/absence of lactose

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Emily Summers
In the absence of Lactose:
The regulator gene is expressed so the repressor protein is synthesised
The repressor protein has two binding sites, one to lactose and one to the operator region
The repressor protein binds to the operator region, covering part of the promoter region where RNA
polymerase would attach to
RNA polymerase can't bind to the promoter region so structural genes cannot be transcribed to
The genes cannot be translated and beta galactosidase and lactose permease cannot be
In the presence of Lactose:
Inducer (lactose) molecules bind to the other site on the repressor protein so the molecules change
The repressor protein's other binding site can't bind to the operator region and it dissociates from it
The promoter region is clear for RNA polymerase to bind to and initiate transcription of mRNA for
genes Y and Z
The operatorrepressorinducer system allows transcription and translation of structural genes Y
and Z into the lac enzymes Lactose permease and beta galactosidase

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Emily Summers
Now E coli bacteria is able to use lactose permease to take up lactose from the medium into their
They can then convert lactose to glucose and galactose via beta galactosidase which can be used to
respire…read more


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