Need help with Making my tron bike turn 90 degrees

Iv been working on a tron game and i'm having trouble getting my bike to move 90 degrees when i hit the d or a keys. Also how do I make it so that you can hold down the button and it makes it turn constantly i want to have to press the button then lift my finger off then have to press again instead of being able to hold it down. Ill attach my script. I don't want it to turn smoothly i want it to turn 90 degrees at a time like the old tron games.

 function Update ()
     { 
         if(Input.GetButton("LEFT"))
     {
             transform.Rotate(Vector3.up * Time.deltaTime * -90);
     }
         if(Input.GetButton("RIGHT"))
     {
             transform.Rotate(Vector3.up * Time.deltaTime * 90);
     } 
     }