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Mitosis and Cytokinesis

  

Mitosis and Cytokinesis

 

The most common type of nuclear cell division is mitosis. Mitosis yields two

genetically identical daughter nuclei. It is followed by cell division (cytokinesis).

The inactive period between active periods of cell division is called

interphase. Nuclear division and cell division are the high points of the cell

cycle. This cycle is segmented into the phases G1, S, G2 and mitosis. Mitosis is

completed in six consecutive steps.

The figure on the right presents different stages of mitosis. The dividing cells

were Indian muntjac fibroblasts in cell culture (Muntiacus muntjac).

a) Interphase. Phase between divisions. The chromatin is dispersed or forms

small aggregates. It is often possible to distinguish between highly condensed

chromatin, called heterochromatin, and less densely packed chromatin,

called euchromatin. One or two nucleoli are routinely present.

b) Prophase. Begin of mitosis. The chromatin condenses to form chromosomal

threads, which become increasingly thicker and shorter (coil stage,

spirem). The nucleolus dissolves and the nuclear membrane falls apart.

The nuclear spindle forms and provides the apparatus for chromosome

movement. The nuclear membrane dissolves in the following prometaphase.

c) Metaphase. The chromosomes are arranged in the equatorial plane of the

spindle, themetaphase plane. It nowbecomes apparent that there are two

identical sister chromatids in each chromosome. Each chromatid has an

area for the attachment to one of the spindle fibers, which extends to both

spindle poles. This is the stage when karyotyping works best.

d-f) Anaphase. The two identical sister chromatids separate and move to the

cell pole. Phase of the double (daughter) stars or diaster. In a later anaphase

stage, a cytokinetic actin ring forms in the position of the equatorial

plane.

g) Telophase. Completion of the nuclear division. The chromatids combine

and form a coil. A newnuclear membrane forms, the cytoplasm divides by

contracting the cytokinetic actin ring (division ring) (cytokinesis). A new

cell wall is synthesized in the place of the cytokinetic actin ring.

h) Interphase.