Which of the conditions would result in the greatest amount of transcription of the lac operon? $$\begin{array} { l c c }
& \text { [glucose } ] & \text { [lactose } ] \
\text { I. } & \text { high } & \text { high } \
\text { II. } & \text { low } & \text { low } \
\text { III. } & \text { high } & \text { low } \
\text { IV. } & \text { low } & \text { high }
\end{array}$$
A) I
B) II
C) III
D) IV

Question

Which of the conditions would result in the greatest amount of transcription of the lac operon? $$\begin{array} { l c c } 
& \text { [glucose } ] & \text { [lactose } ] \
 \text { I. } & \text { high } & \text { high } \
\text { II. } & \text { low } & \text { low } \
\text { III. } & \text { high } & \text { low } \
\text { IV. } & \text { low } & \text { high }
\end{array}$$
A) I
B) II
C) III
D) IV

Quizplus · Accepted Answer

Transcription of the lac operon is regulated by the presence of glucose and lactose. When glucose is present, it inhibits the production of cAMP, which is needed for the binding of CAP to the promoter region of the lac operon. In the absence of cAMP-CAP binding, there is minimal transcription of the lac operon. When lactose is present, it binds to the repressor protein, causing it to release from the operator region, thus allowing RNA polymerase to bind and allowing transcription to occur. 
In conditions III & IV, there is a low level of glucose which means that cAMP levels will be high. As cAMP levels are high, more CAP will bind to the promoter region, leading to increased transcription. However, in condition III there is a low level of lactose preventing the release of the repressor protein affecting the transcription of the operon whereas in condition IV lactose levels are high which will cause the release of the repressor protein leading to increased transcription. Thus, condition IV results in the greatest amount of transcription of the lac operon.

Which of the Conditions Would Result in the Greatest Amount