Initially mitosis and karyokinesis was considered to be the same and cytokinesis was a separate process. But later both of karyokinesis and cytokinesis were considered as the two phase of mitosis. In somatic cells of higher organisms mitosis takes place. The daughter cells formed due to mitosis are identical in all respect to the parent cell. The daughter nuclei are not formed directly from the parent nucleus but via the formation of chromosomes hence it is also called Indirect cell division .
Occurrence of Mitosis: -
In the following cases the Mitosis occurs.
i) In the somatic cell of all eukaryotes.
ii) In the meristematic tissues of plants.
iii) In the lower animals during rege3narative growth.
iv) In the damaged plant parts for the purpose of wound healing.
v) In the development and differentiation of embryos.
Factors Including Mitosis: -
i) Greater amount of cytoplasm in comparison to that of nucleus.
ii) The increase in the metabolic process which liberate more energy in the form of ATP molecules.
iii) The increase in the amount of RNA in comparison to DNA.
iv) Synthesis of specific proteins required during mitosis.
v) Availability of adequate substrate.
Mechanism of Mitosis :
Mitosis is the process in which the chromosomes of the parent nucleus are distributed equaly into the two daughter cells. The entire process consists of two major phases:
i) KARYOKINESIS: The division of the nucleus.
ii) CYTOKINESIS: The division of the cytoplasm.
What is Miotic Cycle or Cell Cycle?
The Mitotic Cycle or Cell Cycle is the entire sequence of events happening from the end of one nuclear division to the beginning of the next of the mitosis process of cell division.
Control of Cell Cycle:
The cell cycle is controlled by different proteins in the cytoplasm, those are include:
i) Cyclin D or G1 helping in synthesis of RNA.
ii) Cyclin E and A or, S phase Cyclins helping in the DNA replication.
iii) Mitotic Cyclin in Cyclin B are helping in the divisional stages.
b) Cyclin dependent kinases (Cdks): - Cyclin activated these enzymes and only get activated by binding with specific Cyclin belonging to a particular stage of cell division and add phosphate groups to different protein substrates.
c) Anaphase promoting complex: The group of protein molecules helps in the destruction of cohesive protein compounds and as a result the chromosomes can easily separated during anaphase.
Cell cycle checkpoints: -
The check points include the DNA damage checkpoints and spindle checkpoints operating during the Interphase and Metaphase, which stops cell division and initiate apoptosis.
Time taken for Mitotic cell division: -
in every organism when the current phases of mitosis are going on, maximum time is necessary for the Interphase and little time is necessary for the rest part of the cell division.
What is the importance of Interphase?
Interphase is a stage between two mitotic cycles in an eukaryotic cell, during which various physical and chemical changes for the preparation of cell division takes place. It is the cell division of longest period and called Inter-mitotic phase. The chromosomes appear as long and thin and forms diffused chromatin net. Due to highly hydrated condition it is quite less visible. There have three sub-phases of interface those are: -
i) Post mitotic Gap phase (G1): -this phase occupies 30 to 50% of the cell cycle and being the early stage of Interphase, it may call the past first Growth Phase. In this phase RNA and various regulatory proteins are synthesised. Enzymes required for DNA synthesis like DNA polymerase are synthesised. No synthesis of DNA takes place in this phase of mitosis. Accumulation of tubulin occur which causes elongation of the nucleus.
ii) Synthetic phase (S-phase): – this phase constitutes 35 to 45% of the cell cycle. It is the mid-interphase stage in which all the major metabolic activities of the findings cell take in place. In this phase of mitosis the DNA is synthesised. Volume of the nucleus becomes double due to DNA replication. Histone proteins are also synthesised. This single chromatid of the chromosome gets duplicated to form two identical Chromatids.
iii) Premitotic Gap phase(G2): – 10 to 20% of cell cycle is constitutes by this phase and also referred to as the second growth phase and actually is the gap between S-phase and prophase. In this phase of mitosis in major events include synthesis of specific RNA and protein, and stopping of DNA synthesis.
Major characteristics of interphase: –
i) The replication of DNA.
ii) Nuclear membrane remains intact and distinct.
iii) Due to accumulation of RNA and ribosome protein enlargement of nucleus.
iv) Synthesis of energy rich ATP molecules and microtubules take place.
v) Due to accumulation of cytoplasm the cell increases in size.
vi) A new centrosome is formed in animals cell from the existing one with a pair of centrioles placed at right angles to each other.
vii) In the cells of animal the net membrane biosynthesis increases and it remains stored at the cell surface in the form of blebs and helps the sale is to divide.