Main Stages in Which the Process of Mitosis Takes Place are:
All living organisms start their life from a single cell. However, consist of large number of cells at maturity which are formed from a single cell by repeated divisions e.g. adult man has 1014 cells at maturity.
Growth in higher organisms takes place by increasing number of cells i.e. division, enlargement and differentiation of the cells produced. Cell is the structural and functional unit of life of all organisms.
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The division of chromosomes and cytoplasm of a cell into two daughter cells is known as cell division.
The cell under goes division is termed as parent cell. Cells derived from the division of parent cell are known as daughter cells.
Cell division is the process through which cells reproduce themselves and organism grows. Thus it is a process of duplication or multiplication rather than division.
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The event of cell division consists of:
1. Division of nucleus called mitosis or karyokinesis
2. Division of cytoplasm is called cytokinesis.
The division of the cells which leads to multiplication of cell number and their accumulation resulting in growth and development of individual is known as somatic cell division. There are mainly two types of cell division namely mitosis and meiosis.
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In addition bacterial cells divide by a process referred as fission which is very similar to mitosis in higher organisms and may be regarded as a primitive form of mitosis.
Mitosis:
It is the process in which each of the chromosomes, principle constituents of nucleus undergoes a longitudinal doubling and then separating into two identical groups and forming two daughter cells which resemble each other as well as parent cell quantitatively as well as qualitatively.
The process of mitosis helps in increasing surface area of the cells. It is a complicated process however, for the sake of convenience it is divided into five stages.
1. Resting stage/Inter-phase:
It is the period between two division cycles. Chromosomes are fully extended and uncoiled, so that they do not take up sufficient stain to be visible under light microscope.
Nuclei are surrounded by well defined nuclear envelope and nucleoli are also present. During this stage cell is not dividing state hence called as resting stage?
But inter-phase is not a resting stage since chromosome replication (based on DNA replication) and protein and RNA synthesis takes places during this stage.
These events are essential for the cells to enter mitosis. It is divided into three phases namely; G1: Resting phase (pre DNA synthesis phase), S: DNA synthesis phase and G2: Post DNA synthesis phase.
G1 phase:
It is the period between the beginning of inters phase and the beginning of DNA synthesis (S phase).
The duration of G1 is highly variable. In some cells G1 may be very short or absent. The enzyme and substrates necessary for DNA synthesis during S phase are also synthesized during G1 phase.
S phase:
It is the period of inter phase during which DNA synthesis takes place as a result of which the chromosome replicate.
DNA synthesis during S leads to the production of two sister chromatids from the single chromatid of each of the chromosome in the nucleus.
At the end of S phase each chromosome is composed of two morphologically and genetically identical sister chromatids. RNA and protein synthesis are very low in this state.
G2 phase:
It is the period between termination of DNA synthesis and beginning of prophase of the next division.
The duration of G1 is generally between 1-4 hr but in some cases it may be as long as 12 hrs. There is considerable RNA and protein synthesis.
Some proteins synthesized during this stage are essential for the entry of the cells into mitosis.
2. Prophase:
The chromosomes become more visible, coiled, shortened and thicken distinct structures as prophase progresses.
The chromosomes are longitudinally split up except centromere giving raise to identical (sister) chromatids.
The chromatids are sometimes twisted which is known as relational coiling. The chromosomes take more stain and become dark colored. Nuclear membrane and nucleolus disappear at late prophase.
3. Metaphase:
It is characterized by formation of spindle fibers and disappearance of nuclear membrane and nucleolus.
The chromosomes are arranged on equatorial plane or in the centre of the cell. The spindle fibers are attached to the centromere of chromosomes.
Chromosomes show maximum contraction and well spread and hence it is used for examination of individual chromosome (chromosome morphology) as well as confirming the chromosome number of particular species of plants and animals.
4. Anaphase:
During this stage centromere divides and the sister chromatids daughter chromosomes separate from each other and move towards opposite poles.
Thus each chromosome has produced two daughter chromosomes which form two groups at opposite poles.
5. Telophase:
The two groups of chromosomes reach at opposite poles and reorganize into two new nuclei. The spindle fibers degenerate and nuclear membrane and nucleolus reappear.
Thus giving rise to two daughter nuclei at two poles. The chromosomes uncoil, elongate and lose their stain ability and identity and finally appear as inter phase nucleus.