Mitotic phase

Mitosis is a cell multiplication or reproduction process in which a single cell produces two identical cells. Mitosis is a term that refers to the replication and redistribution of chromosomes, which are the structures that carry genetic information. It is the process through which somatic cells divide. Mitosis is also referred to as “equational division” because the daughter cells are genetically identical and have the same number of chromosomes as the parent cells. Mitosis involves major redistribution and rearrangement of all cellular components. Mitosis is generally seen in diploid cells and haploid cells of some plants and insects.

Mitosis

Flemming was the first to use the term “mitosis”. He observed the activities of chromosomes in the nucleus of a cell throughout normal cell division in a systematic way (mitosis). He discovered the order in which changes in the nucleus occur during cell division. He demonstrated that the threads (later referred to as chromosomes) shortened and divided longitudinally into two halves, with each half travelling to opposite poles. 

Stages Of Mitosis

 The Mitotic Phase consists of karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm). The karyokinesis of mitosis includes four stages: Prophase, Metaphase, Anaphase & Telophase. 

Prophase

 It is the longest phase in mitosis. It is divided into early and late prophase.

Early Prophase:

• The tangled chromatin fibres condense to chromosomes.
• The nucleolus is seen attached to the chromosome at the nucleolar organiser.

Late prophase:

• Each chromosome divides into two chromatids; the chromatids are attached at the centromere.
• Condensation of chromosomes continues.
• Chromosomes radiate out astral rays, these are the microtubular fibrils. Spindle fibre is made up of tubulin protein and aster. Aster consists of astral rays, centriole pairs and help in the formation of spindle fibres.
• The disappearance of the nucleolus, nuclear envelope, endoplasmic reticulum, and Golgi complexes can be seen.

Metaphase

• The nuclear envelope completely disintegrates, and the chromosomes can be seen through the cell’s cytoplasm.
• The condensation of chromosomes is completed. At this stage, the chromosomes can be easily studied under the microscope. The presence of two sister chromatids can be seen.
• Chromosomes are present at the equator, and the plane of alignment of the chromosomes is known as the metaphasic plate.
• The spindle fibres present at both the poles are connected to chromatids by their kinetochores in the centromere.

Anaphase

• The shortest phase of mitosis in anaphase.
• Longitudinal division of the centromere of each chromosome leads to the formation of two daughter chromatids.
• At the poles, the spindle fibres contract, and as a result, the chromatids move from the equator to the opposite poles.

Telophase

• Chromosomes form clusters at opposite poles and untangle themselves into chromatin fibres.
• Nuclear envelope assembles around the chromatin fibres. Thus, two daughter nuclei are formed.
• Nucleolus, Golgi complex, and ER reappear.
• The spindle fibres disappear.

Cytokinesis

• The division of the cytoplasm of the parent cell to form two daughter cells. Cytokinesis starts when telophase is in progress.
• Cytokinesis in the animal cell: An animal cell divides with the formation of the cleavage furrow in the plasma membrane. It starts to appear from the side of the plasma membrane and gradually deepens and joins in the centre dividing the cytoplasm into two.
• Cytokinesis In plant cells: Due to the presence of a cell wall in plant cells, the method is different from that in animal cells. Vesicles are formed from Golgi bodies, which helps in cytokinesis. They fuse to form the cell plate. In the end, the cell plate gets converted into middle lamella.
• During cytokinesis, organelles like mitochondria and plastids get distributed between the daughter cells.
• In some organisms, karyokinesis is not followed by cytokinesis. As a result, a multinucleate condition (syncytium) arises. An example is liquid endosperm in coconut.

Significance of Mitosis

• Mitosis helps produce the similar genetic constitutions of daughter cells.
• Mitosis helps somatic cells to attain the same chromosome number as parent cells.
• In multicellular organisms, mitosis helps in the growth of the body. Meristematic tissue in plants continues to grow throughout life with the help of mitosis.
• When a cell divides, the nucleo-cytoplasmic ratio gets disturbed. Mitosis helps in its restoration.
• Mitosis replenishes cells as it helps in cell repairing & replacement. Examples are the cells of the upper layer of the epidermis, stomach lining & blood cells.

Conclusion

Mitosis represents the actual cell division. In the human cell cycle, it lasts for only about an hour. Some cells do not show division, e.g., heart cells. Some cells that do not divide further exit the G1 phase and enter an inactive G0 or quiescent stage. These cells remain metabolically active but do not proliferate. Mitosis is initiated with the prophase, in which the chromosome starts thickening and tangling. The formation of the equatorial plate shows metaphase, then comes anaphase, and in the end, telophase. Finally, cytokinesis separates the parent cell into two daughter cells.