Question

The lac operon is expressed when ________.

Answer 1

Answer:

The correct answer is b. glucose is low and lactose is present

Explanation:

Lac operons are found in bacteria and it contains one regulator gene, one promotor sequence, one operator and three structural genes. When lactose is not present in the environment regulatory gene synthesizes repressor proteins that bind to the operator and stops the expression of the lac operon.

When lactose or allolactose is present then it binds to repressor proteins and prevents the protein to bind to the operator region which results in the expression of the lac operon. So to express lac operon lactose should be present.

In presence high amount of glucose lac operon is not expressed because glucose is the easy source of getting energy than lactose so for expression of lac operon glucose should be low and lactose should be present.

Answer 2

Answer:

Option B,  glucose is low and lactose is present

Explanation:]

Lac operon transcription is dependent on two regulatory proteins

• The lac repressor which acts as a  lactose sensor.

• The catabolite activator protein (CAP) which acts as a glucose sensor.

In the presence of glucose, the adenylate cyclase is inhibited due to which cAMP is not produced. But once the glucose level becomes low or negligible, cAMP is produced as adenylate cyclase is not inhibited any more. cAMP produces a complex by combining to catabolite activator protein (CAP), thereby allowing transcription of the lac operon.  

In the absence of lactose, lac repressor binds to operator due to which transcription stops and in case where lactose is present, lac repressor does not binds to operator and hence allows lac transcirption

Hence, option B is correct