Mitosis is a continuous process of cell division, but for didactic reasons, to better understand it, let's divide it into phases: prophase, metaphase, anaphase and telophase.
Some authors often cite a fifth phase - the prometaphase - intermediate between prophase and metaphase. The end of mitosis, with the separation of the cytoplasm, is called cytokinesis.
Prophase - Beginning phase (pro = before)
- Chromosomes begin to become visible due to spiraling.
- The nucleolus begins to disappear.
- A set of fibers (nothing but microtubules) originating from the nuclei is organized around the nucleus. centrosomes, constituting the so-called division spindle (or mitotic spindle).
Although centrioles participate in the division, it is not from them that the spindle fibers originate. In animal cell mitosis, the fibers around each pair of spindles opposite the spindle constitute the aster (from the Greek, aster = star).
- The core absorbs water, swells, and the library becomes disorganized.
- At the end of the prophase, short spindle fibers from the centrosomes attach to the centromeres. Each of the sister chromatids is attached to one of the cell's poles.
Note that the centrosomes are still aligned in the equatorial region of the cell, which makes some authors designate this prometaphase phase.
Formation of a new pair of centrioles begins in phase G1, continues in phase S and in phase G2 duplication is completed. However, the two pairs of centrioles remain together in the same centrosome. As the prophase begins, the centrosome splits into two, and each pair of centrioles begins to move toward opposite poles of the dividing cell.