Movement of Chromosomes during Anaphase (With Diagram)

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During nuclear division or mitosis, there is a progres­sive change in the structure and appearance of the chromosomes.

Although mitosis is a continuous pro­cess (Figs. 20-20 and 20-21), for convenience it is usu­ally divided into four major stages: prophase, meta- phase, anaphase, and telophase.

Prophase, the first stage of mitosis, is characterized by the condensation of the chromosomes, the disap­pearance of nucleoli and the nuclear envelope, and the formation of the microtubules of the spindle. If, prior to prophase, the cell contained a centriole, then a sec­ond centriole is formed; the two centrioles move apart as the spindle forms.

The chromosomes become distin­guishable by light microscopy as a result of their pro­gressive shortening and thickening and eventually arc seen to be composed of two sister chromatids held together at the centromere or kinetochore. The sister chromatids are the products of the replication of chro­mosomal DNA during the interphase of the cell cycle.

Toward the end of prophase (sometimes called pro­metaphase), the spindle extends between two poles positioned diametrically opposite one another in the cell and the chromosomes migrate toward the center of the spindle. In metaphase, the centromeres of each chromosome are aligned midway across the spindle on a plane called the equatorial plate.

At this time the centromeres are linked to the spindle fibers. Some of the spindle fibers do not form associations with any chromosomes and extend directly from one pole to the other. 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.

The onset of anaphase is characterized by the move­ment of the chromosomes toward opposite poles of the spindle. During anaphase, a process called cytokin­esis begins and divides the cell into two halves, thereby physically separating the two complements of chromosomes. Cytokinesis is distinct from but fre­quently synchronized with nuclear division, occurring during the later stages of mitosis. In the final phase of mitosis, called the telophase, the chromosomes reach the poles of the spindle and begin to undergo decon­densation. During the telophase, nucleoli reappear, as does a new nuclear envelope enclosing the chromo­somes.

Movement of Chromosomes during Anaphase:

During 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. 20-21). This arrangement suggests that the chromosomes are being pulled toward the poles of the spindle. For a number of years many investigators have attempted to determine the nature of the mechanism that is responsible for this movement.

Among the different models that have been proposed are:

(1) The microtubule model,

(2) The sliding cytoplasmic filament model, and

(3) The dy­namic equilibrium model.

The microtubule model was first proposed in the late 1960s by J. R. Mcintosh. According to Mcintosh, the microtubules extend inward from the poles of the spindle and overlap at the center of the cell. During anaphase, the microtubules slide past one another in the region of overlap, thereby extending the cell and pushing the poles further apart.

At the same time that the overlapping ends of the microtubules slide past each other, “other microtubules connecting the centro­meres to the poles are disassembled at the poles. As a result the overall length of these microtubules is de­creased and the chromosomes move closer to the poles. Because microtubule sliding is known to be in­volved in other forms of cell movement, such as in the beating of cilia and flagella, this model is entirely plausible. However, it appears not to be the complete story.

In 1974, A. Forer showed that actin microfilaments are present in the spindle, and later myosin filaments were also shown to be present. This prompted the sug­gestion that it is the sliding of the actin and myosin fil­aments past one another that accounts for chromo­some movement.

The role of the microtubules was relegated to that of serving as a structural framework on which the chromosomes are mounted. 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. 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.

According to the dynamic equilibrium model pro­posed in 1967 by S. Inoue, the microtubules that com­prise the spindle fibers are in a dynamic equilibrium with a pool of microtubule subunits. The addition of new subunits to fibers that extend from one pole of the spindle to the other but do not have attached chro­mosomes serves to increase the inter-polar distance, while at the same time the removal of subunits from either the polar ends or the centromere ends of other spindle fibers serves to shorten them and draw the chromosomes closer to the poles.

In addition to bio­chemical evidence supporting this latter notion is the recognition that the movement of chromosomes toward the poles of the spindle during anaphase is a relativity slow process compared to the beating mo­tions of cilia and flagella and the contraction of muscle cells. Indeed, anaphase movement takes place at a rate that is more akin to the rate3 at which filaments grow or shorten by either an influx or loss of subunits.

From the preceding discussion it is apparent that several different models attempting to account for chromosome movement during the anaphase of mito­sis are supported by independent observations. This is not to suggest that the actual mechanism may not in­volve some combination of all the models.