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  Section: Genetics » Physical Basis of Heredity » Cell Division (Mitosis and Meiosis)
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Physical Basis of Heredity 2.  Cell Division (Mitosis and Meiosis)
Significance of meiosis
Reproductive cycles
Stages of meiosis
Synaptonemal complex
Recombination nodules
Comparison of meiosis and mitosis

Division of one nucleus into two is often called karyokinesis and is followed by cytokinesis, which divides cytoplasm into two cells and can be brought about in two ways.

Cell furrow. In case of animals, outer layers are more flexible due to absence of cell wall. In such cases, a circular constriction appears at equator and it converges on all sides finally separating two daughter cells (Fig. 7.4a).

Cell plate. In plant cells, a more rigid cell plate is usually initiated at centre and is completed towards periphery (Fig. 7.4b). After the cell plate is laid down, primary walls are deposited on either side. The thick secondary cell-walls of cellulose may be laid down later on.

Different stages of mitosis are shown in Fig. 7.3 (a-h). The process takes from 10 minutes to several hours. Metaphase stage of mitosis is often used for determining chromosome numbers in animals as well as in plants. Chromosome numbers of some important animals and plants as determined by mitotic metaphase are given in Table 7.3.
Various stages of mitosis in a somatic cell.
Fig. 7.3. Various stages of mitosis in a somatic cell.

Two methods of cytokinesis : (a) cytokinesis by cell furrow; (b) cytokinesis through formation of cell plate.
Fig. 7.4. Two methods of cytokinesis : (a) cytokinesis by cell furrow; (b) cytokinesis through formation of cell plate.

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