Animation 17.5 Mitosis in an Animal Cell
Textbook Reference: The Events of M Phase, p. 670
Mitosis consists of four stages—prophase, metaphase, anaphase, and telophase. At the beginning of prophase, chromatin coils and condenses, resulting in compacted chromosomes that are visible with a light microscope. A chromosome consists of two sister chromatids—products of DNA replication in the previous interphase—that are held together at a site called the centromere.
Structures called centrosomes serve as two poles of the mitotic spindle, which begins to form during late prophase. In higher eukaryotes the end of prophase corresponds to the breakdown of the nuclear envelope.
After prophase, the cell enters prometaphase—a transition period between prophase and metaphase. The microtubules of the mitotic spindle attach to protein structures, called kinetochores, on the centromeres of the chromosomes. The kinetochores of sister chromatids attach to microtubules emanating from opposite poles of the spindle.
The chromosomes shuffle back and forth until they align on the metaphase plate in the center of the spindle. At this stage, the cell has reached metaphase.
The transition from metaphase to anaphase is triggered by breakage of the link between sister chromatids, which then separate and move to opposite poles of the spindle.
Mitosis ends with telophase, during which nuclei re-form and the chromosomes decondense.
Cytokinesis usually begins during late anaphase and is almost complete by the end of telophase, resulting in the formation of two interphase daughter cells.