Consider the signaling pathway depicted below, which involves two different receptors. Assume that a protein is active only when it is activated but not inhibited by its upstream signaling molecules. Activation and inhibition are indicated by normal and blunt arrows, respectively. Under which of the following conditions is target-gene expression induced?
A) Only in the presence of both signals (1 and 2)
B) In the presence of either or both signals (1 or 2)
C) In the presence of either signal but not both of them
D) Only in the absence of both signals
E) Only in the presence of signal 2 but not signal 1

Question

Consider the signaling pathway depicted below, which involves two different receptors. Assume that a protein is active only when it is activated but not inhibited by its upstream signaling molecules. Activation and inhibition are indicated by normal and blunt arrows, respectively. Under which of the following conditions is target-gene expression induced? 
A) Only in the presence of both signals (1 and 2) 
B) In the presence of either or both signals (1 or 2) 
C) In the presence of either signal but not both of them 
D) Only in the absence of both signals 
E) Only in the presence of signal 2 but not signal 1

Quizplus · Accepted Answer

In this imaginary example, each of the two parallel pathways depicted can stimulate gene expression through the same transcription regulator; however, when both signals are present, each pathway inhibits the other one, leading to no response.

Consider the Signaling Pathway Depicted Below, Which Involves Two Different