Everything a cell does ultimately results from the action of its proteins; DNA directs the synthesis of those proteins. Cells, of course, synthesize many other substances as well—glycogen, fat, phospholipids, steroids, pigments, and so on. There are no genes for these cell products, but their synthesis depends on enzymes that are coded for by the genes. For example, even though a cell of the testis has no genes for testosterone, testosterone synthesis is indirectly under genetic control (fig. 4.5). Since testosterone strongly influences such behaviors as aggression and sexual drive (in both sexes), we can see that genes also make a significant contribution to behavior. In this section, we examine how protein synthesis results from the instructions given in the genes.
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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.