Question

What would be true in the following scenario?

a. An E. coli colony (A) is exposed to glucose and lactose at the same time
b. A different E. coli colony (B) is exposed to glucose but no lactose
c. A third E. coli colony (C) is exposed to lactose but no glucose
1. Colony A has lac operon on due to lack of negative control and presence of positive control
2. Colony A has lac operon semi-on due to lack of negative control but also lack of positive control
3. Colony B has lac operon semi-on due to lack of negative control but also lack of positive control
4. Colony B has lac operon on due to lack of negative control and presence of positive control
5. Colony C has lac operon semi-on due to lack of negative control but also lack of positive control
6. Colony A has lac operon off due to negative control only
7. Colony C has lac operon on due to lack of negative control and presence of positive control
8. Colony C has lac operon off due to negative control only Colony B has lac operon off due to negative control only

Answer 1

In the proposed scenarios, scenario A would likely result in semi-on expression of the lac operon (statement 2), scenario B would have the lac operon off due to abundant glucose (last unnumbered statement), and scenario C would have the lac operon fully on as lactose is present without glucose (statement 7).

Step-by-step explanation:

The regulation of the lac operon in E. coli is an excellent scenario to understand the mechanisms of gene expression in response to the presence of sugars like glucose and lactose. The presence of glucose generally represses the lac operon through a mechanism where the repressor protein prevents transcription. However, in the absence of glucose and the presence of lactose, the lac operon is turned on because lactose (specifically allolactose, a derivative of lactose) binds to the repressor and prevents it from attaching to the operator, thus allowing transcription.

In scenario A, E. coli colony (A) is exposed to both glucose and lactose. Since glucose is present, the lac operon would be repressed, making statement 1 incorrect and leaning more toward statement 6 being correct. However, as lactose is also present, some inducer may bind to the repressor, making it a semi-on scenario rather than fully on or off (statement 2).

For scenario B, where E. coli colony (B) is exposed only to glucose, the lac operon would be off due to the repressive conditions created by the glucose, which means statement 3 is incorrect and statement 4 is not applicable; the correct statement is that the lac operon is off due to negative control only (the last unnumbered statement).

Lastly, in scenario C, E. coli colony (C) is exposed only to lactose, meaning that the lac operon would be fully on since lactose binds to the repressor and inhibits it, allowing for the transcription of the lac genes necessary to metabolize lactose (statement 7), and statement 8 would be incorrect.