G proteins are a family of receptor proteins that are involved in transmitting signals from outside a cell to inside a cell. When a signaling molecule binds to a G protein, the G protein is activated. The G protein then activates an enzyme that produces a second messenger called cAMP. Which of the following describes a critical role of cAMP during the transduction stage of a G protein signal transduction pathway?
a. cAMP carries the signal to the nucleus of the cell and results in new sequences of nucleotides being added to the cell’s DNA.
b. cAMP binds the extracellular signal molecule and carries it to the intracellular target specified by the signal.
c. cAMP modifies a specific monomer so that it can be added to an elongating structural macromolecule.
d. cAMP results in the activation of an enzyme that amplifies the signal by acting on many substrate molecules.