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.


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.


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. /PARAGRAPH PARAGRAPH 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. /PARAGRAPH