During the G1 or first gap phase, the cell grows and performs normal metabolic roles, including producing proteins and organelles. During the S phase, which stands for synthesis of DNA, the cell’s chromosomes are copied. Lastly, in the G2 phase, the cell continues to grow and prepares for cell division. This phase of the cell cycle is regulated by stop and go signals called checkpoints, which are located between G1 and S, G2 and M, and M and G1. These make sure that the cell cycle can proceed and is being performed correctly.
The hallmark of this phase is that the DNA appears as loosely packed chromatin and the centrioles are together. Centrioles play an important role in animal cell division. They are composed of microtubules which become spindle fibers that guide the movement of chromosomes in mitosis. Mitosis is the division of the nucleus, which accounts for only 10 percent of the cell cycle. Mitosis can be broken down into five sub phases: prophase, prometaphase, metaphase, anaphase, and telophase; cytokinesis takes place last. In prophase, the chromatin condenses to chromosomes and the two sister chromatid are joined at the centromere.
As the centrioles begin to separate, the mitotic spindle, made of microtubules, begins to form and the nucleolus disappears. In prometaphase, the nuclear membrane dissolves, which allows the spindle to interact with the chromosomes, although the spindle isn’t fully formed yet. The sister chromatids of the chromosomes develop proteins within the centromeres called kinetochores. These allow the chromosomes to attach to the microtubules of the spindle, which are called kinetochore microtubules. Nonkinetochore microtubules aren’t connected to chromosomes, but they interact with other microtubules rom the other pole of the cell. By metaphase, the centrioles are at the opposite poles of the cell and the spindle is fully formed. The chromosomes, attached to the spindle, move to the middle of the cell and line up at the imaginary metaphase plate. Anaphase begins when the centromeres of the chromosomes split and the chromatid is now a chromosome. The chromosomes begin to move toward the opposite poles of the cell and the poles move farther apart. In telophase, two daughter nuclei form at the poles of the cell, the nucleolus and nuclear membrane reappears, and the chromosomes unravel.
Cytokinesis, the splitting of the cytoplasm, occurs simultaneously as telophase. Two daughter cells appear by the creation of a cleavage furrow, which pinches the cell in two. After mitosis, the cell can grow and mature to divide again like epithelial, blood, and stem cells, or grow and mature to never divide again like brain, nerve, and muscle cells. These cells go to the G0 phase, a non-dividing resting period. Liver cells can be called back from the G0 phase to divide again, which makes the liver the only organ to regenerate. This demonstrates the importance of the cell cycle.