What must happen before a cell can begin mitosis?

Answer - The chromosomes has to be crucially duplicated and it happens actively during the resting phase or interphase.

Reasoning - During the resting phase or interphase you call it, Is important to have the same exact DNA or Chromosomes to be duplicated. If the chromosomes did not duplicate during the interphase then the split of the cell would be unbalance and uneven and likely to cause defects.