The nucleolus or nucleoli (when they are more than one) begin to
destruct and gradually disappear. From cytoplasmic microtubules the
mitotic (division) spindle is formed, which is a bipolar fibrous structure. The
assembly of microtubules occurs out of the nucleus. In animal cells, where
there are centrioles, they appear like asterisks determining the direction
and plane where the spindle will lie. A bipolar division spindle is formed.
Prometaphase. The prometaphase begins with disruption of the nuclear
envelope (membrane) into small fragments, which morphologically do not
differ from those of the endoplasmic reticulum. The outside formed mitotic
spindle installs itself in the nuclear zone. From the centromeres of prometaphase
chromosomes specific structures called kinetochores are formed. During the
metaphase a special group of kinetochore microtubules are attached
to each kinetochore (Fig. 3–11). These microtubules radiate from the two
chromosome ends. They are opposed to each other and interact with the fibres of
bipolar spindle. By means of kinetochore microtubules the chromosomes start to
move. The mechanism of their motion is unclear. For the present, still unclear
remains the chemical composition of kinetochores.
image
Figure 3–11. Schematic drawing of a metaphase chromosome showing its two sister chromatids attached to kinetochore microtubules (After Alberts et al., 1989).
The number and size of
microtubules associated with kinetochores varies in the different kinds of
organisms. In some fungi the kinetochore is connected with only one pair of
microtubules, and in the cells of man they are from 20 to 40.
Metaphase. At a definite moment all chromosomes are positioned in
such a way, that their centromeres lie at equal distance from the poles on
the same flatness known as metaphase plate. Each chromosome on the
metaphase plate is kept by a pair of kinetochores and fascicles of
kinetochore microtubules, directed to the opposite poles of division spindle.
Anaphase. After completing the metaphase and dividing the
kinetochores in chromosomes, in conformity with intracellular signals each
chromatid begins to move slowly to the corresponding pole. All chromatids
move with equal speed. By their approaching the poles, the kinetochore
fibres are beginning to shorten. At that time mitotic spindle becomes longer
and the distance between the two poles increases. While metaphase is
more long process, anaphase can complete in a few minutes.
Telophase. When the daughter chromatids settle down on the poles,
the kinetochore fibres disappear. A new nuclear envelope is formed around