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Newt lung cell during late anaphase. Chromosomes are in blue, the spindle in green.

Anaphase, from the ancient Greek ανα (up) and φασις (stage), is the stage of mitosis when chromosomes separate in a eukaryotic cell. Each chromatid moves to opposite poles of the cell, the opposite ends of the mitotic spindle, near the microtubule organizing centers.

Anaphase is preceded by metaphase, by the end of which fully condensed sister chromatids are arranged in a straight line down the midline of the cell, defining a structure referred to as the metaphase plate. Spindle fibres, which are microtubules containing γ-tubulin and other Microtubule-associated proteins extend from the poles to the centromeres. The point of contact is a protein complex called the kinetochore, and these fibres are sometimes referred to as kinetochore fibers or k-fibers. Other spindle fibres do not come in contact with the chromosomes but either connect directly with spindle fibres from the opposing pole as overlap microtubules or interpolar microtubules or with the cell cortex as astral microtubules.

Anaphase begins abruptly with the highly-regulated triggering of the metaphase-to-anaphase transition. At this point the Anaphase Promoting Complex (APC) becomes activated. This terminates metaphase (M-phase) activity by cleaving and inactivating the M-phase cyclin required for the function of M-phase cyclin dependent kinases (M-Cdks). It also cleaves securin, a protein that inhibits the protease known as separase. Separase then cleaves cohesin, a protein responsible for holding sister chromatids together. The consequent separation of chromatids marks the cytological onset of anaphase. After separation they are referred to as daughter chromatids.

Within anaphase two distinct processes occur. During early anaphase the chromatids abruptly separate and move towards the spindle poles. This is achieved by shortening of the spindle microtubules, and forces are mainly exerted at the kinetochores. When the chromatids are fully separated late anaphase begins. This involves the polar microtubules elongating and sliding relative to each other to drive the spindle poles further apart.

These two processes were originally distinguished by their different sensitivities to drugs, and mechanically they are distinct processes. Early anaphase involves shortening kinetochore mictrotubules by depolymerization at both ends. During this, motor proteins at the kinetochores pull on the kinetochore microtubules. Late anaphase involves both the elongation of overlap microtubules and the use of two distinct sets of motor proteins: one of these pulls overlap microtubules past each other, the other pulls on astral microtubules that have attached to the cell cortex.

The contributions of early anaphase and late anaphase to anaphase as a whole vary with cell type. In mammalian cells, late anaphase follows shortly after early anaphase and extends the spindle to around twice its metaphase length; in contrast yeast and certain protozoa use late metaphase as the main means of chromosome separation and can extend the spindle to up to 15 times its metaphase length in the process.

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