Question

question 23 of 25
in e. coli bacteria, the lac operon contains genes that code for enzymes that break down lactose. which statement describes when the genes in the lac operon are expressed?
a. the genes are turned off whether or not there is lactose in the bacterias environment.
b. the genes are turned off when lactose is unavailable in the bacterias environment.
c. the genes are turned off when the bacteria make lactose inside their cells.
d. the genes are turned off when there are high levels of lactose in the bacterias environment.

Explanation:

The lac operon is an inducible operon. When lactose is unavailable, the repressor protein binds to the operator region of the lac operon, preventing RNA polymerase from transcribing the genes (so the genes are "turned off"). When lactose is present (as allolactose, its isomer), it binds to the repressor, changing the repressor's shape so it can no longer bind to the operator. Then RNA polymerase can transcribe the genes (genes are "turned on") to produce enzymes for lactose breakdown. So the genes are off when lactose is unavailable (matches option B).

• Option A is wrong because genes aren't always off (they turn on when lactose is present).
• Option C is wrong as bacteria don't "make" lactose in this context (they break down lactose from the environment).
• Option D is wrong because high lactose levels would lead to gene expression (not being turned off) as lactose (allolactose) would inactivate the repressor.

Answer:

B. The genes are turned off when lactose is unavailable in the bacteria's environment.