Mitotic phase
1. Prophase (Pro=before)
It is the longest mitotic stage and it marks the beginning of cell division. In this stage chromatin networks condense and appear as distinct, long thread0like chromosomes. The term chromosome was first introduced by Waldeyer in 1888. Each chromosome comprises two identical filaments called sister chromatids. The chromatids lie close to each other along their entire length and remain attached only at a site called centromere. Thus the number of chromatids is just double the number of chromosomes. As prophase progresses, the chromosomes (called structures) become thicker and shorter by the process of increased coiling.
Other changes are also observed at this phase. The nucleoli gradually become less conspicuous. In animal cells, centrioles divide into two and move to the opposite poles (sides) of the cell. Twop star-like structures with radiating fibres known as asters are formed around the centriole at each pole. In higher plants, the centrioles and asters are not found.
At the end of prophase, the nucleolus and the nuclear membrane disappear.
2.Metaphase (Meta=between)
During this stage spindle fibres are formed which attach themselves to the centromeres of the chromosomes. Of all the stages, the chromatids become shortest and thickest at this stage due to dehydration and condensation. They acquire a specific shape and size. They are arranged on the equatorial plan. They are arranged in such a way that the centromere of each chromosome lies at the equator, while the arms of the chromosomes extend outwards. This stage is the shortest period of mitomis.
3. Anaphase (Ana=back)
During this stage the centromere of each chromosome divides into two parts so that each chromatid has its own centromere. Both of these may be called daughter chromosomes. Spindle fibres are attached to the centrtomeres of their side. The two sister chromatids now separate from each other and move to their opposite poles. This is achieved because of the contraction of spindle fibres and repulsion between divided sistger centromeres. As the chromatids move, they assume a characteristic shape (V, J, or I) depending upon the position of centromere in the chromosome. After the end of anaphase, each pole receives one set of chromatids which new corresponds to the daughter chromosomes.
4. Telophase (Telo=end)
The stage of telophase starts after the arrival of daughter chromosomes at their respective poles. Each daughter chromosome now becomes uncoiled, thin, long and indistinct. The spindle and the astral rays start disappearing. The nucleolus and nuclear membrane, which had disappeared during late prophase, now reappear. Thus, two nuclel are formed which have the same number and type of chromosomes as that of parent cell. This is due to the fact that each chromosome, during nuclear division gets separated into chromosomes. For example, if a cell having 2 chromosomes undergoes division, it will form two nuclei having 2 chromosomes in each at the end of telophase. The nuclear sap reappears and each nucleus increases in size. In general, the events of prophase are reversed in telophase.