What role do protein kinases and phosphatases play in cell signaling pathways?

• A. Protein kinases degrade proteins, while phosphatases synthesize them.
• B. Protein kinases and phosphatases both act as second messengers.
• C. Protein kinases bind to DNA, while phosphatases bind to RNA.
• D. Protein kinases add phosphates to proteins, while phosphatases remove them, modulating protein activity.

Answer: (D)