Inoue, the microtubules that comprise the spindle fibers are in a dynamic equilibrium with a pool of microtubule subunits. In effect, the disassembly of the microtubules at the poles frees the chromosomes to move in response to the sliding of the actin and myosin.Īccording to the dynamic equilibrium model proposed in 1967 by S. Movements of the cytoplasmic filaments are not reflected by corresponding movement of the chromosomes until anaphase begins and the microtubules begin to break down at the two poles of the spindle. The role of the microtubules was relegated to that of serving as a structural framework on which the chromosomes are mounted. This prompted the suggestion that it is the sliding of the actin and myosin filaments past one another that accounts for chromosome movement. Forer showed that actin microfilaments are present in the spindle, and later myosin filaments were also shown to be present. For a number of years many investigators have attempted to determine the nature of the mechanism that is responsible for this movement.Īmong the different models that have been proposed are:

This arrangement suggests that the chromosomes are being pulled toward the poles of the spindle. During the telophase, nucleoli reappear, as does a new nuclear envelope enclosing the chromosomes.ĭuring the anaphase of mitosis, the centromere of each chromosome advances toward one of the two poles of the spindle, with the arms of the chromosomes lagging behind (Fig. In the final phase of mitosis, called the telophase, the chromosomes reach the poles of the spindle and begin to undergo decondensation. Cytokinesis is distinct from but frequently synchronized with nuclear division, occurring during the later stages of mitosis. During anaphase, a process called cytokinesis begins and divides the cell into two halves, thereby physically separating the two complements of chromosomes. The onset of anaphase is characterized by the movement of the chromosomes toward opposite poles of the spindle. The centromeres are duplicated so that each chromatid becomes an independent chromosome and is attached to a spindle fiber connected to one of the two poles. Some of the spindle fibers do not form associations with any chromosomes and extend directly from one pole to the other. At this time the centromeres are linked to the spindle fibers.

Diagram of prophase