Unlike the life of an organism, which can be viewed as a linear progression from birth to death, the life of a cell follows a cyclical pattern. Each cell is produced as a part of its "parent" cell; when the daughter cell divides, it in turn becomes two new cells. In a sense, then, each cell is potentially immortal as long as its progeny can continue to divide. Some cells in the body divide frequently; the epidermis of the skin, for example, is renewed approximately every 2 weeks, and the stomach lining is renewed every 2 or 3 days. Other cells, such as striated muscle cells in the adult, do not divide at all. All cells in the body, of course, live only as long as the person lives (some cells live longer than others, but eventually all cells die when vital functions cease).
The nondividing cell is in a part of its life cycle known as interphase (fig. 3.27), which is subdivided into Gi, S, and G2 phases, as will be described shortly. The chromosomes are in their extended form, and their genes actively direct the synthesis of RNA. Through their direction of RNA synthesis, genes control the metabolism of the cell. The cell may be growing during this time, and this part of interphase is known as the G1 phase (G stands for gap). Although sometimes described as "resting," cells in the G1 phase perform the physiological functions characteristic of the tissue in which they are found. The DNA of resting cells in the G1 phase thus produces mRNA and proteins as previously described.
If a cell is going to divide, it replicates its DNA in a part of interphase known as the S phase (S stands for synthesis). Once DNA has replicated in the S phase, the chromatin condenses in the G2 phase to form short, thick, structures by the end of G2. Though condensed, the chromosomes are not yet in their more familiar, visible form in the ordinary (light) microscope; these will first make their appearance at prophase of mitosis (fig. 3.28).
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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.