The lac operon is turned off and on by repressors.
Repressors
The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the lac operon only when necessary. In the presence of lactose, the lac repressor halts production of the enzymes encoded by the lac operon.
When the lac repressor binds to the O region, RNA polymerase is prevented from beginning the process of transcription. In effect, the binding of the repressor protein turns the operon "off" by preventing the transcription of its genes. (Prentice Hall Biology Book .....Chapter 12 page 310)
Lactose is absent
The lac operon is most active when glucose levels are low and lactose is present.
Lactose is present, otherwise the lac operon is not needed and is shut off.
Repressors
explain the regulation of gene expression in lac operon.
The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing the enzymes encoded by the lac operon only when necessary. In the presence of lactose, the lac repressor halts production of the enzymes encoded by the lac operon.
When the lac repressor binds to the O region, RNA polymerase is prevented from beginning the process of transcription. In effect, the binding of the repressor protein turns the operon "off" by preventing the transcription of its genes. (Prentice Hall Biology Book .....Chapter 12 page 310)
Lactose is absent
The lac operon is most active when glucose levels are low and lactose is present.
Lactose is present, otherwise the lac operon is not needed and is shut off.
its an operon required for the transport and metabolism of lactose.
The lac operon is most active when glucose levels are low and lactose is present.
A lac repressor turns off the lac genes by binding to the operator
The lac operon is found in enteric bacteria, predominantly Escherichia coli, therefor it is found in prokaryotes.
My answer is 3, I am pretty sure I am right, but I would double check to make sure. I am in ninth grade taking biology at PineTree.