At the beginning of prophase the cell becomes round and the chromosomes appear in the nucleus as convoluted threadlike structures. Simultaneously, the nuclear membrane disappears and the two centrioles move apart. They migrate toward the poles of the cell and form the socalled central spindle. In metaphase, which follows, the chromosomes become shorter and thicker, the two chromatids become visible and can be distinguished clearly by their size and shape. As the process continues, the chromosomes arrange themselves in the so-called equatorial plane between the two poles.
Schematic representation of cell division (mitosis) (After Hadorn and Wehner)
a Prophase: Chromosomes in the nucleus become visible by coiling and the nuclear spindle apparatus develops the central spindle;
b Early metaphase: the central spindle stretches and the chromosomes migrate toward the equatorial plane;
c Late metaphase: The division of each chromosome into two chromatids is clearly visible and arrangement along the spindle’s equator is complete;
d, e Anaphase: the daughter chromosomes are moving away from each other in the direction of the spindle’s pole;
f Telophase: the chromosomes have uncoiled, a nuclear membrane has formed and the cell body is constricted.
At the end of metaphase the chromosomes have arranged themselves in the equatorial plane in such a way that each of their constrictions (centromere) is oriented toward the central axis. Because the arrangement looks star-shaped when viewed from the two poles, it is called a “monaster” (Greek for single star). As anaphase begins, the chromatids (chromosome halves) of each chromosome separate, forming two starshaped figures called “diasters” (double star). Since each of the halves of a chromosome (daughter chromatids) migrates to one or other of the two opposite poles, all of the genetic material is divided equally between the two daughter cells.
In the ensuing telophase the chromatids, which now form the chromosomes of the two daughter cells, collect near the centrioles, uncoil, and again become invisible. As a newnuclear membrane forms, two new interphase nuclei have been created. There follows a complete division of the cell body, resulting essentially in two equal, independent daughter cells.
On average one mitosis takes about 60 minutes to complete. Anaphase is the shortest phase, lasting about 3 minutes.