MYSTERY PLOT POINTS

What are the turning points in a mystery? When we plot romances, we have certain emotional plot points, like first kiss and big dark moment. What about the traditional whodunit mystery? For example, is it necessary to have a dead body in chapter one? Does the crime always have to be a murder? How many suspects is too many? How can the sagging middle be avoided?

Each author will have a different answer, and they're all right. I've read mysteries where no one gets killed for the first hundred...