The next two major events that take place in mitosis are the alignment of
chromosomes at the center of the cell and the subsequent separation of sister
chromatids to opposite mitotic spindle poles. These two events occur in
metaphase and anaphase, respectively. In this section we will review
the events of both of these phases.
Metaphase
Figure %: Metaphase
At the end of prometaphase, the
centrosomes
have aligned at
opposite ends, or poles of the cell and
chromosomes
are being moved
toward the center of the cell. Metaphase is marked by the alignment of
chromosomes at the center of the cell, half way between each of the mitoic
spindle poles. Movement is mediated by the kinetochore
microtubles, which push and pull on the chromosomes to align
them into what is called the metaphase plate. Chromosomes on the metaphase
plate are held there tightly by pushing and pulling forces from the
microtubules.
Microtubule structure allows them to be dynamic molecules. The subunit of
microtubules is called tubulin and it is constantly added and removed from the
ends of microtubules leading to a state of treadmilling. The chromosomes
are held tightly by these forces constantly pushing and pulling on them.
Metaphase can occupy a large portion of the total time of mitosis because
chromosome alignment at the center of the cell on the metaphase plate acts as a
checkpoint for progression into the next phase, anaphase. Cells can arrest
in metaphase for days until the chromosomes are properly aligned and the cell
enters anaphase.
Anaphase
Figure %: Anaphase
Entrance into anaphase is triggered by the inactivation of M phase-promoting
factor that follows mitotic cyclin
degradation {see Mitotic cyclin.
During anaphase, the kinetochore microtubules retract, increasing the seperation
of the sister chromatids as they are moved further toward the opposite spindle
poles.
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Anaphase can be broken into two distinct phases. In the first phase, called
anaphase A, chromosomes move poleward, away from the metaphase plate with
the retraction of the microtubules. This movement occurs at approximately 2
micrometers per minute (the entire length of a cell is between 10 and 30
micrometers). In the second phase, anaphase B, the mitotic poles marked
by the centrosomes themselves separate by the elongation of a specific type of
non-kinetochore microtubule, called a polar microtubule. The extent of the
separation of the poles varies from species to species. The entire duration of
anaphase is relatively short, usually only lasting a few minutes.
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