I have a 97 jeep grand cherokee that will not start I have gas but no spark ?

I have changed the plugs, wires, distributor cap, rotor and the ignition coil and I still have no spark. The engine turns over and I have a full tank of gas. So what else could be the problem. Like I said it's a 97 jeep grand cherokee 6 cyl 4.0 please help