Mitosis (normal cell division) as part of the cell cycle. Mitotic stages (interphase, prophase, metaphase, anaphase, telophase and cytokinesis) in onion root tip cells.
DNA replication occurs at interphase during the so-called S phase (S = synthesis). This stage is followed by the G2 phase (G=gap) during which structures required for division begin to assemble. At interphase the chromosome material is dispersed within the nucleus and the nuclear membrane (envelop) is intact. Mitosis is announced in plant cells by the appearance of a circular pre-prophase band that consists of microtubules (not shown). A flat sheet of cytoplasmic strands (called phragmosome = partition body) is formed in the center of vacuolate cells, which helps the nucleus to migrate to a central position. Mitosis starts with prophase, a process during which the nucleus looses its membrane (nuclear envelop) and chromosomes spiralise. As chromosomes become shorter and thicker they appear first like threads (mitos = thread) and later like distinct rods. The chromosomes consist each of two sister chromatids attached together at their centromere (centromere = central part). At late prophase a kinetochore (kinetochore = motion body) develops on each sides of the centromere. At metaphase (meta = middle) spindle fibers that originate from two poles in the cell and consist of microtubules and actin filaments (not shown here) capture the kinetochores and move the chromosomes to the equatorial (central-mid) plane of the cell. This spindle fiber pulls the chromosomes apart at anaphase and carries each sister chromatid from a pair to an opposite pole at telophase. In plant cells a cell plate (not shown here) grows like a disk in the former phragmosome plane at this stage. In the next stage, at cytokinesis, the cell plate has developed into a new cell wall and plasma membrane that separate the two daughter cells. Cytokinesis is the end of mitosis: the chromosomes decondense en the nucleus is again surrounded by an envelop. After cytokinesis a period of intense biochemical activity follows, the G1 phase, during which each daughter cell duplicates its molecular material and the number of organelles. Then the cycle is closed when a new S-phase begins.
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