A single cell divides many times and forms a multicelled organism. In unicellular bacteria and protozoa divide and increase in number. Mostly cells in a multicellular organism grow and then divide but in nerve and muscle cells of animals and guard cells of plants do not divide. Cell division process is same in all organisms. In cell division, the cell that is dividing is called the parent cell. The parent cell divides into two daughter cells. The process then repeats in what is called the cell cycle.
The cell cycle:
Cell cycle is series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell. Before each daughter cell undergoes division, it must grow to the same size as its mother cell. Cell cycle is divided into two basic phases; Interphase or non-dividing phase (growth phase) and M phase or dividing phase (Mitosis phase).
The interval between two successive cell divisions is termed interphase. It is longest period in the cell cycle. Interphase involves a series of changes that prepares the cell for division. In involves the period of cell growth and cell division in an orderly manner. It is divided into three phases;
G1 phase: It involves growth of cell and preparation of DNA for replication. It is the longest phase.
S phase: It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same. Each chromosome now consists of two sister chromatids.
G2 phase: It involves protein synthesis and further growth of cell, which prepares it for division. Cytoplasmic organelles such as mitochondria, Golgi bodies get duplicated. Centriole also divides into two centrioles contained in a single centrosome.
M-phase or dividing phase:
It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. It is also called equational division. The process of mitosis keeps the chromosome number equal in daughter as well as parental cell. Mitosis takes place in somatic cells.