Amitosis (Direct Cell Division)

Amitosis, also called direct or incipient cell division, is the simplest of the three patterns of cell division. Unlike mitosis or meiosis, no spindle apparatus is ever formed and the chromatin material never condenses into visible chromosomes; division simply proceeds in two mechanical steps. In karyokinesis (nuclear division), the nucleus first develops a constriction at its centre that gives it a dumbbell shape; this constriction deepens progressively until it splits the nucleus into two separate nuclei. Cytokinesis (cytoplasmic division) follows the same pattern: the plasma membrane develops a matching constriction along the line of the nuclear constriction, which deepens centripetally (inward from the surface) until it finally divides the whole cell into two daughter cells (Figure 7.2). Amitosis is seen in a limited range of cells, including the cartilage cells of mammals, the macronucleus of the ciliate Paramecium, and old, degenerating cells of higher plants. Because there is no spindle to guide chromosomes precisely to each pole, amitosis has real drawbacks: it causes unequal distribution of chromosomes between the two daughter nuclei, whi …