Heroes, Rogues, & Lovers: Testosterone and Behavior Read Online Free Page B

storage of energy from food, and release of stored energy in emergencies. Testosterone and estrogen are the major players in the sex hormone family. They start off as cholesterol, one of the building blocks of the body. Cholesterol is converted into testosterone by the action of enzymes, substances that change molecules from one form to another. This conversion takes place in the testes, ovaries, and adrenal glands. Testosterone can then be converted into a more potent form called dihydrotestosterone by the enzyme 5-alpha-reductase, or into estrogen by the enzyme aromatase (so named because its products sometimes have an "aroma" like that of benzine). Testosterone can be converted into estrogen, but estrogen cannot be converted back into

 

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testosterone. It is an oddity of sex hormones that estrogen in males and females comes from testosterone. Both sexes have testosterone and estrogen, although men have more testosterone and women have more estrogen. The same individual can have high levels of both hormones. Stallions have high levels of estrogen and testosterone, as do football players and rattlesnakes. 8
Figure 1.1 shows the molecular structure of testosterone and estrogen. Testosterone molecules have twenty-one carbon and oxygen atoms. Removing one carbon atom changes testosterone into estrogen. The similarity of the two hormones is perhaps a metaphor for the similarity of men and women. Men and women are similar in many waysin their bodies, their minds, their hopes and fears. We should remember this underlying similarity when thinking about the ways in which they differ.

Figure 1.1
Molecular structure of testosterone  (left)  and estrogen  (right) . The circles
represent carbon and oxygen atoms in each molecule. Testosterone has one
extra carbon atom, shaded gray in the figure. The molecules are so similar
that you can merge them into a single three-dimensional image. If you cross
your eyes slightly and look at the tick marks above the figures, they will
gradually merge to form a new, third tick mark in the center. When the
center mark is clearly in focus, move your eyes down to the figure below
it, and a three-dimensional molecule will "pop out" into view.
Hormones operate by moving through the bloodstream from cells that produce them to target cells. When they reach the target cells they fit into receptor molecules, like keys into locks. What happens next depends on what the cells are designed to do. Cells are diverse, ranging from those that make up the brain to those that are parts of muscles, toenails, and eyelashes. In the nucleus of each cell, genetic material in

 

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the form of chromosomesdouble strands of long DNA moleculestells the cell what to do. Genes are sections on the DNA strands, and they contain information needed to build and maintain the body. Each cell has a complete set of genes, but it uses only a few of them. When a cell uses a gene, we say it "expresses" that gene. Gene expression begins when part of the double-stranded DNA unravels into a single strand, called RNA. The RNA moves out of the nucleus and into the main body of the cell, where it attracts small building-block molecules to form stable groups that make up new protein molecules. Creating the new protein is a step toward meeting the need of the moment, whether that need is to digest a meal, add a bit of muscle, attract a lover, or prepare for a fight.
Testosterone has many functions. Among other things, it signals cells to build muscle, make red blood cells, produce sperm, and release neurotransmitters in the brain. A testosterone molecule acts upon its target cell in one of two ways. It binds with a receptor at the membrane surrounding the cell and triggers action there, or it passes through the membrane to bind with a receptor inside the cell and triggers the expression of a gene. Action at the membrane can be completed within