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A cell during anaphase.

Anaphase, from the ancient Greek ἀνά (up) and φάσις (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. During this stage, anaphase lag could happen.

Anaphase begins abruptly with the regulated triggering of the metaphase-to-anaphase transition and accounts for approximately 1% of the cell cycle's duration. At this point the Anaphase becomes activated. This terminate 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.

During early anaphase (or Anaphase A) 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 (or Anaphase B) begins. This involves the polar microtubules elongating and sliding relative to each other to drive the spindle poles further apart. Anaphase B drives separation of the sister centrosomes to their opposite poles through three forces. Kinesin proteins attached to polar microtubules push the microtubules past one another. A second force involves pulling of the microtubules by cortex-associated cytosolic dynein. The third force for the separation of chromosomes involves lengthening the polar microtubules at the plus end.

These two processes were originally distinguished by their different sensitivities to drugs, and they are mechanically distinct.

  • Early anaphase (anaphase A) involves shortening kinetochore microtubules by depolymerization at the plus end. During this, a sliding collar allows the movement of the chromatids. No motor protein is involved since and ATP-depletion does not inhibit the early anaphase.
  • Late anaphase (anaphase B) 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 protozoans 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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