All cells come from cells; with these words Rudolf virchow captured the crucial importance of cellular reproduction for both are descended from pre existing cells, cellular reproduction is absolutely essential for continue existence of life on arch. Cell reproduces by division process, form daughter cells. Each daughter cell also inherits about half of the parents’ cell cytoplasm, including full complement of organelles. Each round of growth and cell division is called a cell cycle.
It is a natural question what makes the cell divide or why the reproducing cells stop dividing. If we take a unicellular organisms for example Amoeba it is seen that it feeds, grows and attains certain size, afterwards. It divides; on the other hand if amoeba is starved it shrinks and stops dividing. Therefore it appears as if the cell division is a way to keep a constant ration between the amount of cytoplasm and nucleus. It means that the division of cell is regulated by the amount of nucleoplasm. This can be explained by the fact that the nucleus governs the activities of cell and can efficiently control only over certain amount of cytoplasm. Hence the growth and development of every living organism depends on the growth and multiplication of its cell.
In unicellular organisms cell division is means of reproduction and by this process, two or more new individuals arise from the mother cells. In multicellular organisms new individuals develop from single cell, the zygote; it is multiplication of this cell and its descendants that determine the development and growth of the individual.
In general energy cell has two periods in the life cycle. Interphase (non division) and division (which produce two daughter cells). Many cells undergo a continuous alteration between divisions and non-division.
The enents occurring from the completion of one division until the beginning of next division constitute cell cycle. This cycle is repeated at each cell generation but the length of the cycle varies considerably in different types of cells. Division may take place by Mitosis or Meiosis. The cell cycle can be considered as the complex series of phenomena by which cellular material is divided between daughter cells.
Interphase
The non dividing initial phase of the cycle as the interval between two divisions is called interphase. During interphase cell caries out work, grows and prepares itself for next division and duplicates its DNA for it.
Interphase is divided into three sub stages G1 (gap one) S (synthesis) and G2(gap two) phase.
G1 is phase of where no DNA synthesis occurs. In this phase synthesis and organisation of substrate (RNA) and formation of enzymes required to DNA synthesis occur. At a point late in G1, a cell follows one of two paths, either it withdraws form the cycle and enters a resting phase or G0stage or enters into S phase to complete the cycle. Cells that enter G0remain viable and metabolically active but do not divide.
S phase is synthesis stage because during S-phase DNA synthesis takes place and DNA content of nucleus gets doubled. This phase of constant duration in similar cells of species. In G2 sub-stage ribosomes are spindle formation.
Chromosomes become double stranded with two chromatids each of various periods of cell cycle G1 period is variable, it lasts, for 3 to 4 hours. S and G2periods are constant. After completion of interphase cell division (mitosis = M) is a dynamic period of vigorous and continual activity. It is also divided into 4 phases with specific events.
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