Mitosis consists of specific phases that occur in a specific order. The picture to the left is a group of plant cells taken from a sample. When the nucleus is present, we say the cell is in interphase and not preparing to divide. If chromosomes are visible as dark lines, we say the cell is in mitosis and preparing to divide.

Scientists look at microscope slides like the one to the left in order to determine how many cells are currently in each phase of mitosis. This allows us to determine how many cells are at rest and how many are dividing.