Describe_the_Process_of_Mitosis

Describe the process of mitosis. Mitosis is the most common form of nuclear division, which produces two progeny, or ‘daughter’, nuclei which are genetically identical to each other and to the ‘parent’ nucleus. It is usually followed by cytokinesis (division of the cytoplasm) and forms part of cell division, which in turn forms part of the cell cycle. Cell division involving mitosis is involved in processes such as growth and asexual reproduction etc. Mitosis is generally considered to consist of four main dividing stages – prophase, metaphase, anaphase and telophase, which account for approximately 5-10% of the cell cycle, with the rest spent in the non-dividing stage in between divisions known as interphase. During interphase, energy is stored and the cell grows in preparation for division. The chromosomes in the nucleus have replicated but are still in the form of diffuse chromatin and not visible. In prophase, the chromatin condenses into discrete visible chromosomes. Each replicated chromosome consists of two chromatids joined at a centromere. At either ‘end’ of the nucleus is a centrosome, these having migrated to the poles of the nucleus at the start of prophase. The centrosomes each contain cylinders of microtubules called centrioles. By the end of prophase, these microtubules have extended outward from each centrosome to form spindle fibres that reach across the cell between the two centrosomes. Some of these fibres attach to an area of the centromere on the chromosomes called the kinetochore. Some fibres also branch outward from the centrosome towards the edges of the cell (the nuclear envelope having disintegrated earlier in prophase) – these are known as astral fibres. During the next phase, metaphase, the spindle fibres attached to the kinetochores on the chromosomes ‘pull’ the chromosomes to the centre of the cell where the centrosomes align themselves exactly, forming a metaphasic plate, with the chromatids trailing off randomly. As anaphase begins, the chromosomes now begin to separate, splitting at the centromere and being pulled back towards the centrosomes, centromere first, by the spindle fibres. As this happens the centrosomes are moving further apart as the spindle elongates. In telophase, the chromosomes arrive and gather at the poles of the cell and begin to uncoil and disperse back into the form of chromatin. The spindle fibres disappear, the nucleoli reappear and nuclear envelopes reform around the two groups of daughter chromosomes, leaving two ‘new’ identical daughter nuclei, which, after cytokinesis has occurred, will subsequently re-enter interphase and begin the process again.