6.5–6.8: Mitosis replaces worn-out old cells with fresh new duplicates.

For most of the cell cycle, chromosomes resemble a plate of spaghetti more than the familiar condensed, replicated “X” shape seen in photos.

Look around your room. Dust is everywhere. What is it? It is primarily dead skin cells. In fact, you (and your friends and family) slough off millions of dead skin cells each day—yet your skin doesn’t disappear. Why not? Because your body replaces the sloughed-off cells. When the cells wear out, your body creates replacements through the process of mitosis.

Mitosis has just one purpose: to enable existing cells to generate new, genetically identical cells. There are two different reasons for this need (FIGURE 6-10).

Figure 6.10: Reasons for mitosis. Mitosis is important in an organism’s growth and the replacement of cells.

1. Growth. During development, organisms get bigger. Growth happens in part through the creation of new cells. In fact, if you want to see cell division in action, one surefire place to find it is at the tip of a plant root: at a growth rate of about half an inch per day, the root is one of the fastest-growing parts of a plant.

2. Replacement. Cells must be replaced when they die. The wear and tear that come from living can physically damage cells. The daily act of shaving, for example, damages thousands of cells on a man’s face. It’s nothing to worry about, though. Microscopic views of human skin reveal several distinct layers, with the outermost layers—the layers under assault during shaving—made up primarily of dead cells. These cells help protect us from infection and also reduce the rate at which the underlying living cells dry out. The living cells that exist just below the layers of dead cells are produced at a high rate by mitosis.

Some other cells that must be replaced actually die on purpose, in a planned process of cell suicide called apoptosis (A-pop-TOE-siss). This seemingly counterproductive strategy is employed in parts of the body where the cells are likely to accumulate significant genetic damage over time and are therefore at high risk of becoming cancer cells (a process described later in this chapter). Cells targeted for apoptosis include many of the cells lining the digestive tract as well as those in the liver, two locations where cells are almost constantly in contact with harmful substances.

Every day, a huge number of cells in an individual must be replaced by mitosis. In humans this number is in the billions. Nearly all the somatic cells of the body—that is, all cells except sperm- and egg-producing cells—undergo mitosis. There are a few notable exceptions, as we’ve already seen. Heart muscle cells and most neurons, in particular, do not seem to divide, or, if they do divide, they do so at very, very slow rates. (We don’t know why this is so.)

The rate at which mitosis occurs in animals varies dramatically for different types of cells. The most rapid cell division takes place in the bone marrow (as red blood cells are produced) and in the cells lining various tissues and organs. The average red blood cell, for example, is in circulation for only about two to four months and then must be replaced (FIGURE 6-11). The cells lining the intestines are replaced about every three weeks. Hair follicles contain some of the most rapidly dividing cells.

Figure 6.11: Short-lived cells. Red blood cells in your body are replaced about every 120 days.