Question

A laclac repressor turns off the genes by

Select one:
a. binding to the operator.
b. binding to the promoter.
c. DNA polymerase.
d. binding to the lac genes.

Answer 1

The lac repressor turns off the lac operon genes by binding to the operator, which prevents RNA polymerase from starting transcription. This system is regulated by the presence or absence of lactose, which when present, binds to the repressor causing its dissociation from the operator.

Step-by-step explanation:

In the lac operon system of E. coli, regulation of gene expression is achieved through the interaction between the lac repressor protein and the DNA sequence known as the operator. When lactose is absent, the lac repressor binds to the operator sequence. This binding inhibits the RNA polymerase from attaching to the promoter, thereby preventing the transcription of the lac genes which are responsible for the metabolism of lactose. In contrast, when lactose is present, it binds to the repressor and changes its shape, leading to its dissociation from the operator. This removal allows RNA polymerase to bind to the promoter and initiate transcription of the lac genes, which results in the production of enzymes to digest lactose.

Expressing genes only when needed is energetically efficient, as synthesizing proteins requires both energy and materials. This efficiency allows the cell to conserve resources and only produce proteins when they are necessary for the cell's functions.