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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).

The cell cycle is represented by a circular graph divided into 4 parts. First part is called the G1 phase, takes approximately 35% of the circle, and in that phase cell growth takes place. Second part is called the S phase, it also takes around 35% of the circle, and in that phase the DNA synthesis takes place. Third part is called the G2 phase, takes 25% of the circle, and in that phase also cell growth takes place. Fourth part is called mitosis, it takes the remaining 15% of the circle. Mitosis phase is divided in half, first half is called mitotic phase, second is called cytokinesis, and their result is formation of 2 daughter cells. The red arrows go around the circle in clockwise manner, from M to G1 phase, then to S phase, and then to G2 phase, to reach the M phase. They show the direction of the cycle.

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.