Whenever, in the opinion of the court the trial is likely to be a protracted one, the court may direct that not more than four jurors, in addition to the regular jury, be called and impanelled to sit as alternate jurors. Alternate jurors in the order in which they are called shall replace jurors who, prior to the time the jury retires to consider its verdict, become unable or disqualified to perform their duties. Alternate jurors shall be drawn in the same manner, shall have the same qualifications, shall be subject to the same examination and challenges, shall take the same oath and shall have the same functions, powers, facilities and privileges as the regular jurors. An alternate juror who does not replace a regular juror shall be discharged after the jury retires to consider its verdict. Each side is entitled to one peremptory challenge in addition to those otherwise allowed by law if one or two alternate jurors are to be impanelled, and two peremptory challenges if three or four alternate jurors are to be impanelled. The additional peremptory challenges may be used against an alternate juror only, and the other peremptory challenges allowed by this section may not be used against an alternate juror.