The growth and development of every living organism depends on the growth and multiplication of its constituent cells. Negeli in 1846 pointed out that new cells are formed through the division of pre existing cells. Virchow 1885 supported the above theory and investigated the process of cell division.
Three types of cell division have been distinguished:
1. Amitosis or direct cell division
2. Mitosis or indirect cell division
3. Meiosis or reduction division
Mitosis involves the exact replication of parent cell followed by its division into two daughter cells which are identical and contain the same number of chromosomes as found in the parent cell. This nuclear division was first observed by Strassburger (1870) in plant cells and Flemming 1882 in animal cells. Flemming used the term mitosis for this process with reference to the thread like appearance of chromosome early in cell division. Darlinnton has given an illustrated account of behaviour of chromosomes during the period of cell division.
Process
The process of mitosis is characterized by the duplication of chromosomes, their separation into two and then their movement to opposite poles so as to construct two daughter nuclei. It is followed by the constriction of cytoplasm to form two daughter cells. The replication and distribution of chromosomes is known as karyokinesis while the division of cell cytoplasm and separation into two daughter cells is known as cytokinesis.
It means cell can be separated into two cwtegories:
The nuclear division or Karyokinesis
The division of cytoplasm or cytokinesis
The Process of Karyokinesis is divided into Prophase, Metaphase, Anaphase and Telophase.
The Prophase- The nuclear division (mitosis) begins with Prophase. The important events during this cycle are:
: : Nuclear changes
���� The chromatin material of nucleus gradually condenses into distinct chromatin threads by loosing water.
���� The chromatin threads coil like cylindrical spring and in so doing these gradually become shorter and thicker and form the chromosomes.
���� The proteinous matrix gets deposited around the chromosomes, so that these gradually become shorter and thicker to form the chromosomes.
���� Each chromosome is already doubled due to the doubling of DNA contents in interphase.
���� By the end of prophase the two chromatids of each chromosome become more distinct and each chromosome appears to be splitted up lengthwise.
���� The nucleolus and nuclear membrane disappears by the end of prophase.
: : Cytoplasmic Events
���� The centriole divides into two and then one part of the daughter centrioles moves towards the opposite pole.
���� Astral rays radiate out from each daughter centriole.
The Metaphase- The metaphase is marked by appearance of spindle and arrangement of chromosomes on the equator of spindle.
���� The microtubules in the cytoplasm of the cell orient in between the centrioles of the opposite poles and from the spindle. Such a spindle is known as amphistar.
���� The chromosomes from periphery of the nucleus migrate towards equator of the spindle. These orient themselves on the equator in such a fashion that their centromeres lie on the equator and are attached to the chromosomal fibers of the spindle, whereas the arms are oriented towards each other.
���� Each chromosome becomes more compact and short and its two chromatids separate except at the centromere, which has not divided so far.
The Anaphae
���� The centromere of each chromosome divides and allows the separation of two sister chromatids into two daughter chromosomes.
���� The daughter chromosome moves apart and migrates towards opposite poles.
���� The movement of chromosomes is governed by the contraction of spindle fibers, the centromere is pulled first towards the pole of the spindle and the arms of chromosomes are dragged behind.
���� In anaphase, the arms of daughter chromosomes are directed towards the equator and centromeres towards the pole of the equator.
Telophase
���� Chromosomes reach poles of the spindle and form two groups.
���� Chromosomes begin to uncoil and form chromatin net.
���� The nuclear wall and nucleolus reappears.
Cytokinesis is division of cell cytoplasm into two separate cells. The process differs in plant and animal cells.
���� In Animal Cells cytokinesis starts by the appearance of a shallow groove or furrow in the cytoplasm at the equator of spindle. Slowly and slowly it deepens and constrict the cytoplasm and cell into two parts.
���� In Plant Cells cytokinesis is accomplished by the formation of phragmoplast and cell plate at the equator of the dividing cell.
At the end of mitosis two diploid cells are formed.
