This is the most important and dramatic period of the cell cycle involving a major reorganisation of virtually all components of the cell cycle. And since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division. This has mainly consists of nuclear division in various stages (karyokinesis). This phase is complex and highly regulated and sequence of events are divided into phases. Karyokinesis involves following stages:
Prophase which is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.︎︎︎ Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere. Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus. Cells at the end of prophase, when viewed under the microscope, golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope are not present.
The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, and therefore the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres which are formed by the spindle fibres to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.︎︎︎︎ main event Spindle fibres attach to kinetochores of chromosomes. Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.
At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei. They basically begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. Anaphase stage is characterised by spli`ng of centrosome and separation of chromatids and chromatids move to opposite poles.
At the beginning of telophase which is the final stage of karyokinesis, the chromosomes that have reached their respective poles de-condense and lose their individuality as in they just get dissolved and are not in the shape of chromosomes anymore. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles. Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei. Nucleolus, golgi complex and Endoplasmic Reticulum now appears again.
Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei which is called as karyokinesis, but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed. In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two. Plant cells are enclosed by a relatively inextensible cell wall. And they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells. At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium.
SIGNIFICANCE OF MITOSIS
- Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis.
- Mitosis usually results in the production of diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis.
- Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
- A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
- Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.