Mitosis is known as the somatic cell division as it occurs in the somatic cells for vegetative growth. It is an equational type of cell division, resulting in two daughter cells equal with number of chromosomes and identical genes as that of parent cell.
Mitosis Occurs in Two Phases:
1. Karyokinessis or division of nucleus.
2. Cytokinesis of division of cytoplasm.
Basing on chromosomal events, karyokinesis is distinguished into four stages:
1. Prophase (property-first, phasis-state):
It is of longest duration. At early prophase, chromosomes start condensing longitudinally. At mid-prophase, they become shorter, thicker and distinct from each other with two visible chromatids (formed during s-phase of interphase) joined at a single centromere. During late prophase nuclear membrane and nucleolus disappear, thus very much condensed chromosomes remain scattered all over the cytoplasm.
Spindle formation occurs towards the end of prophase and beginning of metaphase by end to end arrangement of protein microtubules. Such bipolar spindles are extended from both the poles and attach to the centromere of each chromosome and are called tractile fibres. Same continuos fibrs are also formed extending from one pole to other without touching the chromosome.
It is the next stage during which the chromosomes move towards equator of the cell due to spindle action and lie on a single metaphase plane with chromatids directed towards the poles and centormeres arranged on metaphase plate. This is called auto-orientation.
3. Anaphase (ana-up):
It is of shortest duration during which centrimores of each chromosome split. Thus, the sister chromatids are with their own centromere, resilting in two identical chromosomes. These identical chromosomes move to their respective poles due to:
(i) Repulsion between identical centromeres.
(ii) Contraction of tractile fibres.
(iii) Explanation of interzonal fibres developed between separating centromeres.
4. Telephase (telo-end):
It is last stages of karyokinesis during which two identical groups of chromosomes each, the poles elongate longitudinally and coil among themselves. Nucleolus and nuclear membrane reappear to form two identical daughter nuclei.
This occurs by cell plate formation at the equator resulting in two identical daughter cells.
Thus, meiosis is equational and homotypic cell division.
(i) Mitosis maintains the chromosome number and genetic stability in organisms as it is equational and homotypic.
(ii) It provides new cells for repair of wounds and regeneration of organs.
(iii) It helps in asexual reproduction in lower organisms.
(iv) It checks overgrowth of cells.