The following points highlight the four important phases of mitosis. The phases are: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase.
Mitosis: Phase # 1. Prophase:
During prophase, the individual chromosome appears in double threaded structure composed of two chromatids from the beginning. During this phase, the volume of the chromosomes too increases considerably and there is initiation of coiling leading to shortening and thickening of chromatids. As prophase advances there is a tendency of chromosomes to move towards the periphery of the nucleus.
Mitosis: Phase # 2. Metaphase:
The onset of metaphase marks the dissolution of nuclear membrane and nucleolus, formation of spindle, and chromosomes arrange themselves in the centre of cell-equator, the centromere being attached, to the spindle fibres which constitute the spindle.
The arrangement of the centromeres is in the form of a ring which together constitute the metaphase plate. Based on the position of the centromeres, chromosomes appear as V or J or simple rod-like depending on the morphology of chromosome metacentric, acrocentric or telocentric. The chromosomes at metaphase are highly spiralized.
Mitosis: Phase # 3. Anaphase:
The onset of anaphase marks the division of chromosome into two longitudinal halves, and triggers their movements towards the two opposite poles. The movement of the homologous centromeres to the opposite poles is facilitated by shortening of the fibres thus dragging the chromosomes towards the poles. The completion of the movement of chromosomes to the two opposite poles marks the end of anaphase.
Mitosis: Phase # 4. Telophase:
During telophase, the two sets of chromatids at the two poles become organized into two sets of chromosomes, start uncoiling from the coiled state and lose their chromaticity. De-spiralization and de-condensation along with hydration are associated with the formation of nuclear membrane and nucleolus resulting in two daughter nuclei.
The telophase stage is followed later by the formation of cell plate, dividing the cell into two daughter cells, each carrying the same chromosome numbers as the original.
The process of chromosome duplication without cell division is called endomitosis. In this process a cell with successive S phases without entering into divisional phase subjected to endopolyploidy. This resulted in polytene chromosomes as found typically in the salivary gland of Drosophila as well as in tapetum, endosperm and suspensor of many plants.
They arise due to repeated longitudinal splitting’s of chromatids and consequent non-separation of split portions.