M Phase: The Stages of Mitosis

M phase, the mitotic phase of the cell cycle, is the visibly dramatic interval in which the duplicated genetic material assembled during interphase is precisely divided between two daughter nuclei -- a process called karyokinesis -- and is then followed by division of the cytoplasm itself, called cytokinesis, yielding two separate daughter cells. Karyokinesis in a somatic cell most commonly proceeds by mitosis, which is conventionally described as a continuous process artificially broken, for convenience of study, into four successive stages: prophase, metaphase, anaphase and telophase.

Prophase is the first and generally the longest stage of mitosis. The chromatin, which has remained loosely dispersed throughout interphase, now begins to condense -- coiling and supercoiling progressively -- into shorter, thicker, individually distinguishable threads that will become the visible chromosomes; because DNA replication already occurred back in S phase, each chromosome visible at prophase already consists of two identical sister chromatids joined at the centromere. Outside the nucleus, the two pairs of centrioles (in an animal cell) begin moving apart toward opposite ends of the cell, organising between them the network of microtubules that will form the mitotic spindle. Toward the end of prophase, the nucleolus disappears and the nuclear envelope itself begins to break down, so that the spindle fibres gain direct access to the chromosomes.

Metaphase begins once the nuclear envelope has completely disintegrated. The condensed chromosomes, each still consisting of two sister chromatids, are moved by the spindle fibres until every one of them lies precisely along a single imaginary plane at the exact centre of the cell, midway between the two poles -- this plane is called the metaphase plate (or equatorial plate). Spindle fibres from each of the two poles attach to the kinetochore, a protein structure on the centromere of each chromosome, so that every chromosome is held under tension from both poles simultaneously. This precise alignment is essential: it is what allows each of the two chromatids of every chromosome to be sent to a different pole in the next stage, guaranteeing that each daughter cell receives a complete and identical set of chromosomes.

Anaphase is the shortest stage of mitosis but one of the most decisive. It begins the instant the centromere joining each pair of sister chromatids splits, releasing the two chromatids of every chromosome from one another; from this moment on, each separated chromatid is considered a full, independent chromosome in its own right. The spindle fibres attached to each kinetochore then shorten, drawing the two new sets of daughter chromosomes steadily toward the two opposite poles of the cell, while the cell itself typically begins to elongate slightly along the axis of division. …