Anaphase
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Anaphase (Greek: ανά = up and φάσις = stage) is the stage of meiosis or mitosis when chromosomes separate. Each chromatid moves to opposite poles of the cell (opposite ends of the nuclear spindle).
Anaphase is preceded by metaphase, by the end of which fully condensed sister chromatids are arranged in pairs, their centromeres lying along a surface known as the metaphase plate. Spindle fibres (microtubules containing γ-tubulin) extend from the spindle poles to the centromeres. The point of contact is a protein complex called the kinetochore, and these fibres are sometimes referred to as spindle microtubules. Other spindle fibres do not contact the chromosomes but either connect directly with spindle fibres from the opposing pole (overlap microtubules) or with the cell cortex; these are 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 had been inhibiting separase. Separase then cleaves cohesin, the protein responsible for holding sister chromatids together. They immediately separate, marking the cytological onset of anaphase. After separation they are referred to as daughter chromatids.
Within anaphase two distinct processes occur. During anaphase A the chromatids abruptly separate and move towards the spindle poles. This is achieved by shortening of the kinetochore microtubules, and forces are mainly exerted at the kinetochores. When the chromatids are fully separated anaphase B begins. This involves the overlap microtubules elongating and sliding relative to each other to drive the spindle poles further apart.
Anaphases A and B were originally distinguished by their different sensitivities to drugs, and mechanistically they are distinct processes. Anaphase A involves shortening kinetochore mictrotubules by depolymerisation at both ends. During this, motor proteins at the kinetochores pull on the kinetochore microtubules. 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 anaphase A and anaphase B to anaphase vary with cell type. In mammalian cells anaphase B follows shortly after anaphase A and extends the spindle to around twice its metaphase length; in contrast yeast and certain protozoa use anaphase B as the main means of chromosome separation and can extend the spindle to up to 15 times its metaphase length in the process.