Mitosis Phases

There are four phases in mitosis (adjective, mitotic): prophase, metaphase, anaphase, and Telophase.

1. In prophase, three activities occur simultaneously. First, the nucleoli disappear and the chromatin condenses into chromosomes. Second, the nuclear envelope breaks down. Third, the mitotic spindle is assembled. The development of the mitotic spindle begins as the MTOCs move apart to opposite ends (or poles) of the nucleus. As they move apart, microtubules develop from each MTOC, increasing in length by the addition of tubulin units to the microtubule ends away from the MTOC. Microtubules from each MTOC connect to a specialized region in the centromere called a kinetochore. Microtubules tug on the kinetochore, moving the chromosomes back and forth, toward one pole, then the other. In addition to these microtubules, the completed spindle also includes other microtubules from each MTOC that overlap at the center of the spindle and do not attach to the chromosomes.

2. Metaphase begins when the chromosomes are distributed across the metaphase plate, a plane lying between the two poles of the spindle. Metaphase ends when the microtubules, still attached to the kinetochores, pull each chromosome apart into two chromatids. Each chromatid is complete with a centromere and a kinetochore. Once separated from its sister chromatid, each chromatid is called a chromosome. (To count the number of chromosomes at any one time, count the number of centromeres.)

3. Anaphase begins after the chromosomes are separated into chromatids. During anaphase, the microtubules connected to the chromatids (now chromosomes) shorten, effectively pulling the chromosomes to opposite poles. The microtubules shorten as tubulin units are uncoupled at their chromosome ends. Overlapping microtubules originating from opposite MTOCs, but not attached to chromosomes, interact to push the poles farther apart. At the end of anaphase, each pole has a complete set of chromosomes, the same number of chromosomes as the original cell. (Since they consist of only one chromatid, each chromosome contains only a single copy of the DNA molecule.)

4. Telophase concludes the nuclear division. During this phase, a nuclear envelope develops around each pole, forming two nuclei. The chromosomes within each of these nuclei disperse into chromatin, and the nucleoli reappear. Simultaneously, cytokinesis occurs, dividing the cytoplasm into two cells. In animals, microfilaments form a ring inside the plasma membrane between the two newly forming nuclei. As the microfilaments shorten, they act like purse strings to pull the plasma membrane into the center, dividing the cell into two daughter cells. The groove that forms as the purse strings are tightened is called a cleavage furrow.

In plants, vesicles originating from Golgi bodies migrate to the plane between the two newly forming nuclei. The vesicles fuse to form a cell plate, which subsequently becomes the plasma membranes for the two daughter cells. Cell walls develop between the membranes. Once mitosis is completed and interphase begins, the cell begins a period of growth. This growth period is divided into three phases, designated G1, S, and G2 to distinguish special activities that occur. Although you can associate the labels G1 and G2 with growth and S with synthesis, it is important to recognize that growth takes place during all three phases. In addition, S phase marks the time during which the second DNA molecule for each chromosome is synthesized. As a result of this DNA replication, each chromosome that appears at the beginning of the next mitotic division will appear as two sister chromatids. During the G2 period of growth, materials for the next mitotic division are prepared. The time span from one cell division through G1, S, and G2 is called a cell cycle.

A cell that begins mitosis in the diploid state, that is, with two copies of every chromosome, will end mitosis with two copies of every chromosome. However, each of these chromosomes will consist of only one chromatid, or one DNA molecule. During interphase, the second DNA molecule is replicated from the first, so that when the next mitotic division begins, each chromosome will, again, consist of two chromatids.