Cell Cycle
The cell cycle is composed of interphase and mitotic (M) phase (Figure 1). Most of the time, the cells are in interphase, which is composed of three different subphases: the G1 phase (first gap), the S phase (synthesis), and the G2 phase (second gap). The mitotic phase, which includes both mitosis and cytokinesis, is the shortest part of the cell cycle.
The goal of interphase is to grow by producing proteins and cytoplasmic organelle such as the endoplasmic reticulum. DNA is replicated during the S phase only. During M phase, the replicated chromosomes are segregated into individual nuclei (mitosis), and the cell splits into two (cytokinesis).
Figure 1. Phases of the cell cycle: the key steps a cell must go through in order to divide.
Progress through the cell cycle is strictly regulated by proteins called cyclins and CDKs. There are also checkpoints in the cell cycle. Only when all the necessary processes for each phase have been completed, does the cell progress past the checkpoint into the next phase. Briefly, the G1 checkpoint is passed when the cell received signals to grow, the G2 checkpoint when the DNA is accurately replicated and the M checkpoint when all chromosomes are attached to the microtubules of the mitotic spindle.