6.6: Overview: mitosis leads to duplicate cells.

For mitosis to begin, the parent cell replicates its DNA, creating a duplicate copy of each chromosome. Once this task is completed, mitosis can take place. Mitosis occurs in just four steps, in which the now-duplicated chromosomes are separated into identical sets in two separate nuclei, after which the cytoplasm and the rest of the cell are divided into two cells that pinch apart (FIGURE 6-12). Where once there was one parent cell, now there are two identical daughter cells (FIGURE 6-13).

Before we explore the process of mitosis in more detail, it is helpful to clarify some of the terminology used to describe the important structures and processes.

Generally, when you look at a cell through a microscope, you won’t see any chromosomes because the cells you’re looking at are in some part of their interphase. During all of the cell cycle except mitosis, chromosomes are uncoiled and spread out in a diffuse way, like a mass of spaghetti. And because they are so stretched out, they are not dense enough to be visible.

Before mitosis begins, two important events occur.

• 1. The chromosomes replicate, becoming two identical linear DNA molecules. The two DNA molecules are held together at a region called the centromere. Throughout mitosis, until the centromere splits, each of the identical DNA molecules is called a chromatid; together, the two are called sister chromatids (FIGURE 6-14).
  
• 2. The sister chromatids begin the process of condensation, in which they coil tightly and become compact—in contrast to the uncondensed and tangled state of the chromosomes prior to replication, during most of interphase.

When the sister chromatids condense, they look like the letter X. This appearance can be confusing. As we saw earlier, chromosomes are not X-shaped. They are linear. The reason the genetic material appears X-shaped in most photos is that the only time it is coiled tightly and is thus thick enough to be seen (and photographed) is after it has condensed in preparation for cell division. But this occurs only after replication.

Because a lot of room is required for the sister chromatids to separate, the membrane around the nucleus is dismantled and disappears early in mitosis. At the same time, a structure called the spindle, composed of proteins—mostly hollow tubes called microtubules—is assembled. The spindle microtubules, part of the cell’s cytoskeleton, can be thought of as a group of parallel threads stretching across the cell between its two ends, or poles. In animal cells, the threads connect at each pole to a structure called the centriole. These threads (known as spindle fibers) attach to the centromeres and pull the sister chromatids to the middle of the cell. During mitosis, they’ll eventually pull the chromatids apart as cell division proceeds.