Once thought to be a time of rest, interphase is actually a very active period. During interphase, the cell grows and maintains its routine functions as well as its contributions to the internal environment (fig. 3.36).

Figure 3.36

(a) Interphase lasts until a cell begins to undergo mitosis. (b) A micrograph of a cell in interphase (250x micrograph enlarged to 1,000x). Although present, centrioles and chromatin fibers are not clearly visible at this magnification.

If the cell is developmentally programmed to divide, it must amass important biochemicals and duplicate much of its contents so that two cells can form from one. For example, the cell must take on the tremendous task of replicating its genetic material. It must also synthesize or duplicate membranes, ribosomes, lysosomes, peroxisomes and mitochondria.

Interphase is divided into phases based on the sequence of activities. DNA is replicated during S phase (S stands for synthesis), and is bracketed by two G phases, G1 and G2 (G stands for gap or growth). Structures other than DNA are synthesized during the G phases, and cellular growth occurs then too (see fig. 3.35).

Essentials of Human Physiology

Essentials of Human Physiology

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.

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