When the jury is selected the court may direct the selection of one or more additional jurors, in its discretion, to be known as alternate jurors. Each party shall be entitled to one peremptory challenge for each alternate juror to be selected. When several defendants are on trial together, each defendant shall be entitled to one challenge in addition to the challenge provided above, with discretion in the trial judge to afford the prosecuting authority such additional challenges as circumstances warrant. If at any time before submission of the case to the jury a juror is found unable to perform his or her duties the court shall order the juror discharged, and the clerk shall draw the name of an alternate who shall take the jurors place on the jury.