If there is no lactose present, how does bacteria respond? The gene for beta-galactosidase turns on. The gene for beta-galactosidase turns off. Lactose is produced. Beta-galactosidase production increases.

The gene for beta-galactosidase turns off

Explanation:

Beta-galactosidase is an enzyme which cleaves lactose molecule into glucose and galactose.

The lactose-metabolizing enzymes are encoded by the lac operon. The lac operon contains three genes:

lacZ encodes beta-galactosidase lacY encodes lactose permease lacA encodes thiogalactoside transacetylase

In an absence of lactose, Lac repressor binds to lac operator. And as a result of this binding, transcription of lac operon will be prevented. Hence all the above mentioned genes of the lac operon will be repressed or will be turned off.

The gene for beta-galactosidase turns off.

Explanation:

The gene that codifies the beta-galactosidase enzyme is part of the lac operon, which also contains two other genes that produce enzymes involved in the metabolization of lactose.

Between glucose and lactose, the bacteria will preferentially use glucose as an energy source. On the other hand, lactose is a dimer, and thus a series of enzymes are needed to process lactose before its use as an energy source.

If there is no lactose present, the genes contained inside this operon are turned off (the operon is repressed).