Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle. Mitosis is also called equational division.
Prophase
Prophase is the longest stage in mitosis. During prophase the cell nucleus becomes spheroid, and there is an increase in viscosity of cytoplasm. The chromosomes become visible as long thin threads. The chromosomes start to coil up and become shorter and thicker. By the end of prophase some chromosomes may contract up to 1/25 of their length in early prophase. The double-stranded nature of the chromosomes is now visible. Towards the end of prophase, each chromosome can be seen to consist of two chromatids held together by a centromere. With the progress of prophase, the chromosomes, which were essentially distributed linearly during prophase, migrate towards the nuclear membrane, leaving a clear central area. The centrosome, which had undergone duplication during interphase, now begins to move towards opposite poles of the cell. Protein microtubules develop from each centriole, forming spindle fibres. Some of these extend from pole to pole. In plant cells, there are no centrioles and the spindle forms independently. The spindle consists of microtubules that are made of the proteins called tubulins and proteins associated with them. The spindle is a dynamic structure, and undergoes a cycle of dissolution and reformation. The asters that surround the centriole and the spindle together constitute the mitotic apparatus. Prometaphase The nuclear membrane dissolves, marking the beginning of prometaphase. When the nuclear membrane dissolves, there is no differentiation between cytoplasm and nucleoplasm. The chromosomes are attached to the spindles through their centromeres. Such mitosis is called extra-nuclear mitosis or eumitosis. In several protozoans and some animal cells, though, the nuclear membrane does not disappear during cell division.
The mitosis takes place within the nuclear membrane and is called intranuclear mitosis or premitosis. In some protists, the centriole is present within the nucleus. In such cases mitosis is both intranuclear and centric. When the centriole is outside the nucleus, mitosis is extranuclear and centric. When the nuclear membrane dissolves, a fluid area is observed in the centre of the cell. The chromosomes move freely through this area as they proceed towards the equator.
Metaphase
At metaphase, spindle fibres align the chromosomes along the middle of the cell nucleus. This line is referred to as the equatorial plate or metaphasic plate. Occasionally, only the centromere lies on the equatorial plane, while the chromosome arms are directed away from the equator. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.
Anaphase
The chromosomes are arranged on the equatorial plate for a short period only. The centromeres of the chromosomes divide at the same time as anaphase commences, and the two chromatids of each pair separate. They are now called daughter chromosomes. These now behave as if they repel each other. The two sets of chromosomes migrate towards the poles. The shortening of spindle fibres attached to the centromeres brings about the chromosome movement.
Telophase
The two of daughter chromosomes arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The nucleoli reappear at constrictions called nucleolar organizers, in one or more pairs of chromosomes. The chromosomes disperse and are no longer visible under the light microscope. They eventually lose their staining ability. The spindle fibres disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.
Cytokinesis
In animal cells a cleavage furrow appears at the beginning of telophase. This furrow or constriction becomes progressively deeper as the spindle breaks down. Ultimately, the ingrowing constrictions join and separate two daughter cells. This division of cytoplasm is called cytokinesis. When nuclear division takes place without cytoplasmic division it results in the formation of syncytium, which is a condition where large number of nuclei are present in a single cell.
Cytokinesis in plant cells occurs by a process different from that seen in animal cells. The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing. Instead, there is a formation of cell plate between the two daughter nuclei. This grows from the middle towards the periphery, and finally joins the cell wall. The cell plate represents the middle lamella between the walls of two adjacent cells. Amitosis The nuclear division in amitosis occurs by a process other than mitosis. A dumbbell shaped cleavage of the cell nucleus occurs during which chromosomes are not recognisable and spindle is not formed.
Amitosis may or may not be followed by the division of the cell, and nuclei so formed are normally of unequal size. This process occurs in certain protists, ciliates, in specialised animal tissues, and old degenerating cells of higher plants. W. Fleming in 1882 described amitosis.
Significance of Mitosis
1. Equal distribution of chromosomes: The important feature of mitosis is that the chromosomes are distributed equally between the two daughter cells. Every cell involves division of chromosomes with repeated divisions by mitosis from the zygote onwards, maintenance of identical genetic constituents for all the cells of the body is ensured at each division. Thus, the constant number of chromosomes is maintained in all the cells of the body due to mitosis. 2. Surface/Volume ratio: Mitosis restores the surface/volume ratio of the cell. By undergoing division, the cell becomes smaller in size and the surface volume ratio is restored. 3. Nucleoplasmic ratio: An efficient cell has a high nucleocytoplasmic ratio. Increase in size lowers the ratio. It is brought back to efficient level through division. 4. Growth: As multicellular organisms grow, the number of cells making up their tissues increases. The new cells must be identical to the existing ones. Growth by mitosis takes place over the whole body in animals. In plants, growth is confined to certain areas called meristems. 5. Repair of tissues: Damaged cells must be replaced by identical new cells. Your skin cells and the cells lining your gut are constantly dying and being replaced by identical cells. This is achieved by mitosis.
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