"Secondary messenger" is ":

Options

A. Cyclic A.M.P.
B. ATP
C. ADP
D. DNA

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks to identify which of the given options is a "secondary messenger." Secondary messengers are molecules that relay signals received at receptors on the cell surface to target molecules inside the cell, thus amplifying the signal.

2. **Evaluating the Options**:
- **Option 1: Cyclic AMP (cAMP)**: This is known as cyclic adenosine monophosphate. It is a well-known secondary messenger involved in various biological processes, including signal transduction pathways.
- **Option 2: ATP (Adenosine Triphosphate)**: This is the primary energy carrier in cells, not a secondary messenger. It plays roles in energy transfer and storage.
- **Option 3: ADP (Adenosine Diphosphate)**: This is a product of ATP dephosphorylation and is involved in energy metabolism, but it is not classified as a secondary messenger.
- **Option 4: DNA (Deoxyribonucleic Acid)**: This is the genetic material of the cell and does not function as a secondary messenger.

3. **Identifying the Correct Answer**: After evaluating all the options, it is clear that the only correct answer is **Cyclic AMP (cAMP)**, as it is the only molecule listed that functions as a secondary messenger.

4. **Conclusion**: Therefore, the correct answer to the question "Secondary messenger is:" is **Cyclic AMP (cAMP)**.

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