The cell cycle is divided into interphase, when cellular components duplicate, and cell division (mitosis and cytokinesis), when the cell splits in two, distributing its contents into two cells. Interphase is divided into two gap phases (Gi and G2), when specific molecules and structures duplicate, and a synthesis phase (S), when the genetic material replicates.
The series of changes that a cell undergoes, from the time it forms until it divides, is called the cell cycle (fig. 3.35). Superficially, this cycle seems rather simple—a newly formed cell grows for a time, and then divides in half to form two new cells, which in turn may grow and divide. Yet the specific events of the cycle are quite complex. For ease of study, the cell cycle can be considered to consist of distinct stages, which include interphase, mitosis, cytoplasmic division, and differentiation.
The actions of several types of proteins form "checkpoints" that control the cell cycle. One particularly important checkpoint determines a cell's fate—that is, whether it will continue in the cell cycle and divide, stay specialized yet alive, or die.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.