Part I1. A signaling pathway has four essential parts: (1) the initial signal, (2) the receptor that binds the signals, (3) the signaling molecule or molecules that transmit the message, and (4) the effector or effectors that result in a short-term or long-term cellular change. 2. Long term changes are generally the result of changes in DNA transcription. For example, a protein could be made to begin cellular replication by activating the cell cycle. 3. The addition of phosphate groups to amino acids can alter the behavior of their proteins by changing their enzymatic activity, changing their association with other proteins, or changing their localization in the cell. Each phosphorylation site can change the protein kinase activity. 4. Each phosphorylation site can change the kinase’s activity. In a protein kinase cascade, a receptor activates a signaling molecule that in turn activates a protein kinase that activates another protein kinase.