In all trials, the court may call and impanel alternate jurors to replace jurors who are disqualified or who the court may determine are unable to perform their duties prior to deliberation unless the alternate juror is excused by the court. Alternate jurors shall be summoned 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, and privileges as regular jurors. An alternate juror who does not replace a regular juror shall be discharged at the time the jury retires to consider its verdict, unless otherwise provided by law, by agreement of the parties, or by order of the court. The seating of an alternate juror entitles each party to an additional preemptory challenge, which may be exercised as to any prospective juror. (Code 1979, § 25-89)