6.7: The details: mitosis is a four-step process.

Let’s look at the process of mitosis in a bit more detail, keeping in mind that the ultimate result is the production of two cells with identical chromosomes.

Interphase: In Preparation for Mitosis, the Chromosomes Replicate Processes essential to cell division take place even before the mitotic phase of the cell cycle begins. During the DNA synthesis part of interphase, sister chromatids are formed as every chromosome replicates itself. Each pair of sister chromatids is held together at a centromere.

Mitosis The actual process of cell division occurs in four steps (FIGURE 6-15).

1. Prophase: following replication, the sister chromatids condense. Prophase begins when the sister chromatids have condensed sufficiently to be seen with a light microscope. At this point, the spindle forms and the nuclear envelope breaks down.

2. Metaphase: the chromatids congregate at the cell center. After condensing, the pairs of sister chromatids seem to move aimlessly around the cell, but eventually they line up at the cell’s center, pulled by spindle fibers attached to a disk-like group of proteins, called a kinetochore, that develops on the centromere of each pair. At the end of metaphase, all the chromatid pairs are lined up in an orderly fashion, straddling the center in a “single-file” congregation that is called the metaphase plate. The chromatids are at their most condensed during this part of mitosis.

3. Anaphase: the chromatids separate and move in opposite directions. In anaphase, the spindle microtubules attached to the centromeres begin pulling each chromatid in the sister chromatid pairs toward opposite poles of the cell. From each pair of sister chromatids, the centromere splits as one DNA molecule is pulled in one direction and the other, identical DNA molecule is pulled in the opposite direction. At the end of anaphase, one full set of chromosomes is at one end of the cell and another identical full set is at the other end. These chromosome sets will eventually occupy the nucleus of each of the two new daughter cells that result from the cell division.

4. Telophase: new nuclear membranes form around the two complete chromosome sets. With two full, identical sets of chromosomes collected at either end of the cell, the parent cell is prepared to divide into two genetically identical cells. In this last step, called telophase, the chromosomes begin to uncoil and fade from view, nuclear membranes reassemble, and the cell begins to divide into two.

The process of mitosis, the division of a cell’s nucleus into two nuclei, each of which includes an identical set of chromosomes, is generally accompanied by cytokinesis. During cytokinesis, the cell’s cytoplasm is also divided into approximately equal parts, with some of the organelles going to each of the two new cells. When cytokinesis is complete, the two new daughter cells, each with an identical nucleus containing identical genetic material, enter interphase and begin the business of being cells.

Usually, mitosis occurs without errors and only when needed. In rare cases, however, something goes wrong that can cause cell division to proceed unchecked. In such cases, cancer can arise.