prophase

The first stage of mitosis or meiosis, consisting of linear contraction and increase in thickness of the chromosomes (each composed of two chromatids) accompanied by migration of the two daughter centrioles and their asters toward the poles of the cell. In meiosis, p. is complex and can be subdivided into stages: preleptotene, leptotene, zygotene, pachytene, diplotene, and diakinesis. [G. prophasis, from prophaino, to foreshadow]

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pro·phase -.fāz n

1) the initial stage of mitosis and of the mitotic division of meiosis characterized by the condensation of chromosomes consisting of two chromatids, disappearance of the nucleolus and nuclear membrane, and formation of the mitotic spindle

2) the initial stage of the first division of meiosis in which the chromosomes become visible, homologous pairs of chromosomes undergo synapsis and crossing-over, chiasmata appear, chromosomes condense with homologues visible as tetrads, and the nuclear membrane and nucleolus disappear and which is divided into the five consecutive stages leptotene, zygotene, pachytene, diplotene, and diakinesis

pro·pha·sic (')prō-'fā-zik adj

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n.

the first stage of mitosis and of each division of meiosis, in which the chromosomes become visible under the microscope. The first prophase of meiosis ocurs in five stages (see leptotene, zygotene, pachytene, diplotene, diakinesis).

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pro·phase (proґfāz) the first stage in cell reduplication. In mitosis, the chromosomes condense, the nucleolus disassembles, and the centrosome separates into two centrioles surrounded by arrays of microtubules (asters), which migrate toward the poles of the cell. In meiosis, the prophase of the first division is highly specialized and consists of five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis (qq.v.). Prophase of the second meiotic division resembles that in mitotic division. See meiosis and mitosis.