The final two events of M phase are the re-forming of the nuclear envelope around the separated sister chromatids and the cleavage of the cell. These events occur in telophase and cytokinesis, respectively. In this section we will review the events that comprise these final phases of M phase.
Telophase is technically the final stage of mitosis. Its name derives from the latin word telos which means end. During this phase, the sister chromatids reach opposite poles. The small nuclear vesicles in the cell begin to re-form around the group of chromosomes at each end. As the nuclear envelope re-forms by associating with the chromosomes, two nuclei are created in the one cell. Telophase is also marked by the dissolution of the kinetochore microtubules and the continued elongation of the polar microtubules. As the nuclear envelopes re-form, the chromosomes begin to decondense and become more diffuse.
Cytokinesis is the process in which the cell actually divides into two. With the two nuclei already at opposite poles of the cell, the cell cytoplasm separates, and the cell pinches in the middle, ultimately leading to cleavage. In most cells, the mitotic spindle determines the site where the cell will begin to invaginate and split. The first signs of this puckering are usually visible sometime during anaphase.
Earlier we mentioned that in prophase, the cell's cytoskeleton becomes disassembled. The disassembled cytoskeletal filaments are used in a different way during cytokinesis. Cleavage occurs by the contraction of a thin ring of actin filaments that form the contractile ring. The contractile ring defines the cleavage line for the cell. If the ring is not positioned at the center of the cell, an asymmetrical division takes place. The ring contracts and eventually pinches the cell until it separates into two independent daughter cells. In higher order plants, the cytokinesis process is slightly different because the cytoplasm splits with the formation of the cell wall.