Interphase

Before a cell can enter cell division, it needs to prepare itself by replicating its genetic information and all of the organelles. All of the preparations are done during the interphase. Interphase proceeds in three stages, G₁, S, and G₂. Cell division operates in a cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokenesis.

[Diagram showing the cell cycle. The size of the arrows show the relative length of each stage of the cell cycle. Notice mitosis (M) is quite short.]

G₁ Phase

After mitosis is complete the new daughter cell begins to accelerate its biochemical processes which were slowed down by mitosis. The length of the G₁ phase creates the difference between fast dividing cells and slowly dividing cells. The G₁ phase can be slowed by reducing the nutrients available in a system - thus the cell will take longer to build up the resources necessary for cell division. If there is a severe depletion in nutrients the cells can virtually stop growing. It is interesting to note that cells that aren't growing are always stopped in the G₁ phase, being mitotically arrested. This suggests that once the cell enters the S phase, it is committed to cell division, regardless of the external cell conditions.

[Animal cell interphase. The DNA has been replicated. Also, notice the increased cell size as the cytoplasm has been enlarged.]

S Phase

The S phase begins with the replication of the cellular DNA. This is described in further detail in DNA replication. When the cellular DNA has been duplicated, leaving the cell with twice as many chromosomes (each chromosome is made up of two identical chromatids), the cell moves onto the G₂ phase.

G₂ Phase

During this phase proteins, such as kinase (which catalyzes protein phosphorylation), which are necessary for cell division are synthesized at this time. The chromosome begins to condense and the proteins necessary for construction of the mitotic spindle also are synthesized. When the chromosomes become visible the cell enters the first stage of mitosis, prophase.

Continue on to Mitosis 1: Prophase

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