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Following a cell's successful passage through the G2/M checkpoint, which we observed in the previous section, mitosis occurs. In this animation, we can watch a typical diploid animal cell nucleus as it divides mitotically to form two diploid daughter nuclei. Mitosis can be broken down into four stages: prophase, metaphase, anaphase, and telophase.

Prophase is the first stage of mitosis. During this stage, the chromosomes begin to shorten and thicken. Each chromosome consists of the two sister chromatids that were made during S phase, held together by a centromere. The centrioles migrate towards the poles of the cell, creating a mitotic spindle made up of microtubules. The nuclear membrane and nucleoli disappear, allowing the mitotic spindle to attach to the centromeres.

Next, the mitotic spindle aligns the chromosomes along the equator, or center, of the cell during metaphase.

Sister chromatids separate and start to move away from each other during anaphase. Each sister chromatid is now its own individual chromosome, and each travels toward an opposite pole of the cell, pulled by the spindle fibers. By the end of anaphase, each half of the cell has its own equal and entire set of chromosomes.

Telophase is characterized by the formation of two new daughter nuclei. The chromosomes reach the poles, and the spindle fibers disappear. As telophase progresses, the chromosomes, decondense to form chromatin, the nuclear membranes reform, and the nucleoli reappear. Note that the resulting daughter nuclei, which can be seen in the final frame of this animation, have the same number and kinds of chromosomes as the parent nucleus shown at the beginning of the animation.

In animal cells, telophase is generally followed by cytokinesis. In this process, a cleavage furrow serves to pinch in the cell's membrane to form two distinct daughter cells.





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