Life in living organisms starts as a single cell or zygote or spore. In multicellular organisms, the cell divides into two and continues to divide but in unicellular organisms, the cell divides into two separate daughter cells. Cell division is the development of an individual from the fertilized egg to the adult. During this process, a mature cell divides into two daughter cells which have essentially the same composition as the parent cell although they are usually smaller. They take in food materials, grow into the adult size and divide once again. This goes on and on finally resulting in a mature organism that may contain billions of cells.
Some of the most important structures in a cell are chromosomes, which are found in the nucleus. Each chromosome is a long thread-like molecule that is made up of two parallel strands called chromatids which are connected at one point by a structure called centromere. Chromosomes are composed of a chemical substance known as deoxyribose nucleic acid (DNA) and are the carriers of hereditary information in a cell and they also determine the characteristics of each cell and those of the whole organism.
The cells of all members of a species contain an identical number of chromosomes although this number varies from one species to another. The chromosomes in each cell (except gametes) occur in pairs, a condition called the diploid state. For example, in man there are twenty three pairs of chromosomes in each cell. The two chromosomes constituting a pair are called homologous chromosomes which are identical in size and structure. One member of the pair is a progeny of the chromosome that was derived from the sperm, while the other is a progeny of the chromosome that was derived from the egg.
Just before cell division, the number of chromosomes in a cell is doubled. This occurs through a process called replication. In the process, each chromosome makes an exact copy (replica) of itself. The daughter chromosomes then redistribute themselves forming a pair of identical nuclei which are also identical to the nucleus from which they were derived. In addition to the nuclei, new cellular organelles, like mitochondria, ribosomes are also formed. Finally, the whole cell then divides into two, each enclosing one of the daughter nuclei. The result is the formation of the two daughter cells that are identical in composition to one another and to the cell from which they were derived.
This form of cell division in which the daughter nuclei have the same chromosomal constitution as the parent nucleus is called mitotic division or simply mitosis. It is the form of division that occurs during the growth of an individual from a fertilized egg to the adult and also during asexual reproduction. The word mitosis refers to the replication of chromosomes and their subsequent redistribution to form a pair of identical nuclei. The actual division of cytoplasm that results in two daughter cells is called cytokinesis.
A different type of cell division is seen during the formation of gametes (sex cells). In this type, which is called meiotic division or simply, meiosis, the parent cell has a diploid number of chromosomes. However, the daughter cells arising from the division have half the number of chromosomes found in the parent cell, a condition known as haploid state.