Help :( The CAP activator protein (aka CRP) and the Lac repressor both control t
ID: 80618 • Letter: H
Question
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The CAP activator protein (aka CRP) and the Lac repressor both control the Lac operon. You create cells that are mutant in the gene coding for the Lac repressor so that these cells lack the Lac repressor under all conditions. For these mutant cells, state whether the Lac operon will be switched on or off in the following situations and explain why.
a. In the presence of glucose and lactose
b. In the presence of glucose and the absence of lactose
c. In the absence of glucose and the absence of lactose
d. In the absence of glucose and the presence of lactose
Explanation / Answer
Condition Lac operon On or OFF Explaination In the presence of glucose and lactose OFF The attachment of RNA polymerase to the promoter site in the lac operon requires CAP activator protein bound to cyclic AMP.The presence of glucose lowers the intracellular concentration of cyclic AMP by inactivating the enzyme adenylyl cyclase responsible for the formation cAMP.Due to the diminished levels of cAMP the formation of CAP-cAMP is low. Hence, RNA polymerase is not able to bind to the promoter and hence lac operon is switched off. In the presence of glucose and the absence of lactose OFF Explaination same as above. In the absence of glucose and the absence of lactose ON In the absence of glucose there is enough of the CAP-cAMP available and hence RNA polymerase will be able to bind the promoter and transcription will proceed.Eventhough, lactose is not present, but since the cells are mutated and do not have have repressor , there is no need of lactose to inactivate the repressor. In the absence of glucose and the presence of lactose ON In the absence of glucose there is enough of the CAP-cAMP available and hence RNA polymerase will be able to bind the promoter and transcription will proceed.presence of lactose does not determine the switching ON or OFF of the operon since cells lack repressor
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