Question

When is the lac operon most active?

A.
when glucose levels are high and lactose is present
B.
when glucose levels are low and lactose is absent
C.
when glucose levels are low and lactose is present
D.
when glucose levels are high and lactose is absent

Answer 1

Answer:

CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon can only be transcribed at high levels when glucose is absent. This strategy ensures that bacteria only turn on the lac operon and start using lactose after they have used up all of the preferred energy source (glucose).

Answer 2

Answer:

c. when glucose levels are low and lactose is present.