Give the structure of cyclic AMP and discuss its role as a second messenger by giving suitable examples. (UPSC 2017, 15 Marks, )

साइक्लिक AMP की संरचना दें और उपयुक्त उदाहरण देकर इसे एक द्वितीयक संदेशवाहक के रूप में इसकी भूमिका पर चर्चा करें। (Give the structure of cyclic AMP and discuss its role as a second messenger by giving suitable examples.)