What is Mitosis?
Mitosis is a process of cell division in which a single cell divides into two genetically identical daughter cells. The parent cell, in this way, successfully passes on its genetic material to each of the daughter cells. However, before cell division cell’s DNA must be duplicated during interphase. Mitosis is a part of the cell cycle during which a cell divides only once to produce two equal-sized daughter cells. Major reasons for mitosis to take place include the growth of the organisms and repair of wear and tear in them.
Phases of Cell Cycle and Mitosis
Interphase is the first phase of cell cycle and mitosis is divided into four Phases viz. Prophase, Metaphase, Anaphase and Telophase.
Mitosis is a short relatively short phase of cell cycle than the much longer Interphase. Both Interphase and mitosis alternate with each other. During interphase, the cell prepares itself for the process of division as well as performs specific cell functions i.e. takes up nutrients and grows. The DNA of the cell is copied resulting in two identical and full sets of chromosomes. The microtubules extend from the two centrosomes present outside the nucleus. Interphase is further divided into three stages: G1 (Gap 1), S (Synthesis) and G2 (gap 2). G1 accounts for cell growth, S for the synthesis of a duplicate set of chromosomes and G2 for more growth and preparation for mitosis. Cells may also undergo G0 phase during which the cell stops dividing.
It is the first mitotic phase. During prophase, the nuclear membrane starts to dissolve and membranous organelles disperse towards the margins of the cell. The centrosomes start moving to the opposite poles of the cell and the nucleolus disperses. Microtubules spread between the centrosomes and their fibres lengthen. The sister chromatids coil up and become visible under a light microscope.
During this phase of mitosis, the processes which were begun in prophase continue to progress. Nuclear envelope in completely dissolved up and no visible nucleus is found at this stage. Proteins attach to the centromeres forming the kinetochore fibres. Now chromosomes start moving.
During metaphase, spindle fibres align the chromosomes in the centre of the cell thus creating a line called metaphase plate. This organisation of chromosomes helps in the next phase to equally divide the pair of chromosomes in two daughter nuclei. The sister chromatids are still attached to each other with the help of cohesion proteins. The chromosomes are extremely condensed at this stage.
During anaphase the proteins which bind the sister chromatids together degenerate. The sister chromatids separate from each other at the centromere. Now each separated chromatid is called a chromosome. The chromosomes now pull apart to the opposite pole thus making the cell oval shaped now.
Telophase and Cytokinesis
At this stage, a full set of chromosomes is present at each pole. A membrane starts to form around each set of chromosomes and creates two nuclei. The cell then starts to pinch from the centre. The contractile ring cleaves the cell from the middle to create two identical daughter cells during cytokinesis. Microtubules now reorganise themselves to form a new cytoskeleton and the cell cycle continues.