Mo. Rev. Stat. § 494.485: Alternate jurors.

If in any case to be tried before a jury it appears to the court to be appropriate, the court may direct that a number of jurors in addition to the regular jury be called and impaneled 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 or are found to be unable or disqualified to perform their duties. Alternate jurors shall be selected 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 principal jurors. Alternate jurors who do not replace principal jurors 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 for each two alternate jurors to be impaneled. The additional peremptory challenge may be used against an alternate juror only, and the other peremptory challenges allowed by law shall not be used against the alternates.