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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
Figure %: Telophase
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
Figure %: Cytokinesis
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.
