Which of the following is an example of a secondary messenger?

A secondary messenger is a molecule that supports the transmission of signals within cells, typically following the activation of a primary messenger such as a hormone or neurotransmitter. These secondary messengers play a crucial role in amplifying the strength of the signal as it moves through the cell and interacts with various pathways and processes.

Calcium is a well-known secondary messenger because it can rapidly increase intracellular concentrations, triggering a variety of cellular responses. It is involved in numerous signaling pathways, including muscle contraction, neurotransmitter release, and various enzymatic activities. When the primary messenger binds to a receptor, it can lead to the release of calcium ions from the endoplasmic reticulum or influx from the extracellular space, thus propagating the signal further.

In contrast, DNA and RNA are nucleic acids involved in the storage and transfer of genetic information, and while they are critical for cellular function, they do not act as secondary messengers in signaling pathways. Proteins, although they can serve various roles in signaling and cellular processes, are not classified as secondary messengers either. Their function often entails acting as enzymes or structural components rather than as signaling intermediates responding to primary messengers. Therefore, calcium stands out as the clear example of a secondary messenger in this context.