One of the most important characteristics of living being is its ability to reproduce. Growth involves increase in the size of organisms and increase in the number of cells. Cell division is the process through which this is achieved. The new cells arise from pre existing cells only through the process of cell division.
In unicellular organisms too cell division is a means of reproduction and population growth. Two or more daughter cells are produced by cell division. Multi-cellular organisms are made up of millions of cells. For example, a new born baby has 2(10 to the power 12) cells. In all multicellular organisms, life starts from a single cell i.e., zygote. Zygote is the product of union of male and female gametes. Zygote divides and re-divides to produce a complex multicellular organization. After each division, there is a gap period in which cells take nutrients from their surroundings and convert them into the living substance, protoplasm. The cells in plants divide throughout the life of the plant. However, in animals, cell division stops after a certain age, but takes place when cell division is required to replenish damaged cells. Growth and development result from this action.
Cells divide by three different methods – amitosis, mitosis and meiosis. In amitosis, spindle fibers are not formed. Mitosis is the type of cell division which takes place in somatic (body) or vegetative cells. It takes place during growth and asexual reproduction of plants and animals. It is also known as somatic (soma means vegetative body) cell division. Meiosis is the type of cell which takes place in reproductive found in gonads of organisms. This process can be observed during gametogenesis i.e., formation of gametes (eggs and sperms).
A cell has broadly two parts: nucleus and cytoplasm. In cell division both parts divide successively. The division of nucleus is called nuclear division or karyokinesis (karyon means nucleus). This is followed by the division of cytoplasm which is known as cytokinesis. The processes of mitosis and meiosis are complicated and are studied in different steps.
Cell cycle Cell cycle is a series of events spanning two cell divisions. Howard and Pelc divided the cell cycle into four intervals: G1, G2, and mitosis. G refers to gap and S refers to synthesis.in the G1 (gap one) period, a cell has to make a critical decision as to whether it should divide or remain as such. If a cell has passed the checkpoint of G1, it enters a phase of DNA duplication. This phase is called S phase. The mass of DNA is doubled upon the completion of S phase. The next phase is G2 phase. In this phase RNA synthesis occurs, which in turn leads to synthesis of proteins needed during mitosis. After the completion of G2 phase, the cell enters the mitotic phase marked by prophase. G1, S, and G2 are included within the interphase.
The term mitosis (mitos means thread) was given by Walther Fleming in 1877. In a typical plant or animal cell, the cell cycle requires about 20 hours for its completion. The actual process of mitosis requires about an hour or so but the rest of this time is needed for interphase (G1, S, G2). The process of mitosis can be divided into four progressive stages: (1) prophase, (2) metaphase, (3) anaphase and (4) telophase. In different cells, the time taken for the completion of each of the above stages differs and depends upon various physical and chemical factors.