The Cell CycleIn the life of a cell, mitosis is only one phase in a cycle. All the phases that describe the activities of the cell are part of the cell cycle (see Figure 12.1). This cycle consists of:
- G1 (gap) phase, the phase prior to DNA replication in which cellular activities occur.
- S phase (synthesis), when the DNA replicates.
- G2, a second gap, which occurs after S.
- M phase (mitosis), when the cell divides.
G1, S and G2 are collectively known as interphase. Some cells, such as neurons, function but do not cycle and do not divide. They arrest at a stage in the cell cycle known as G0.
The Cell Cycle
The time it takes for a cell to go through the cell cycle is characteristic of the cell type. Check points within the cycle ensure that the cell is ready to continue on to the next phase. The cell will not be allowed to continue if certain events are not completed. For example, a check point in G2 ensures that the cell is physically big enough to divide in M phase. If the cell has not grown to a critical size, the cell will be arrested in G2 until it is ready to proceed. In many types of cancer, these check points are disabled, and cells divide in an uncontrolled fashion.