The interphase, or growth, period of the cell cycle (indicated by "I" in the figure at right) alternates with mitosis ("M" in the figure). It's the time when the cell isn't undergoing division. So it isn't part of mitosis (even though it ends in "-phase" like the names of the various steps of mitosis).
When this stage of the cycle begins, the chromosomes have not yet replicated, but by the beginning of prophase replication is complete, so that each chromosome is composed of two sister chromatids. Replication occurs during the synthesis, or S phase ("S" in the diagram at right).
S phase is preceded by G₁ phase, which in many cells is a time when cell growth occurs. From G₁, a cell may exit the cell cycle and go into a long-term stable state known as G₀ where the cell functions but does not divide.
At the beginning of the third stage of interphase, G₂ phase, replication is complete. During G₂ the cell prepares for mitosis as it undergoes rapid growth.
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