Mitosis is a process of cell division resulting in two genetically
identical daughter cells. The stage between mitoses is called the interphase
which covers the G1, S, and G2 phases of the cell
cycle. During the G2 phase, DNA has been replicated and the
chromosome number is 4n. In the interphase, chromosomes are not visible by
light microscopy because the chromatin diffuses
throughout the nucleus.
Figure 8-B-1. Phases of mitosis.
Mitosis may be divided into the following phases.
- Prophase: The chromatin condenses into
chromosomes. Two centrioles move toward the opposite ends (poles)
of the cell. Microtubules radiate from each centriole, forming a structure
called "spindle". By the end of prophase, the nuclear envelope breaks down into small vesicles.
- Metaphase: The chromosomes align at the equatorial plane of the
cell, as directed by the microtubules attached to the kinetochore
(located at the centromere of each chromosome).
- Anaphase: Sister chromatids separate and move apart.
The separation happens simultaneously for the entire set of
chromosomes. The microtubules attached to the kinetochore also play a
critical role in this process.
- Telophase: The chromosomes begin to uncoil and become less
condensed. The spindle disappears while the nuclear envelope
reappears. The cell elongates and eventually divides into two daughter
cells. The process of cell division is called "cytokinesis."
The morphological changes in early mitosis are mainly due to the phosphorylation of related
proteins catalyzed by MPF. The mitosis is terminated by the activation of DBRP.