Cell is the basic unit of life. Basic unit refers to the structural, functional and reproductive unit. A11 the cells of a living organism together give rise to the characteristic shape and structure of the body, together carry out the sumtotal functions and form the basis of growth in the present generation and perpetuation to the next generation.
Thus the cell is the basis of continuity through generations. This is a basic theory of biology known as cell theory postulated by Schleiden and Schwann.
The cell theory was extended by Rudolph Virchow (1885) by suggesting ‘omnis cellula e cellulae’ which means every cell arises from a pre-existing cell. One cell gives rise to another cell by cell division. Cell division is initiated by the preparation of nucleus or more specifically the chromosomes to divide.
By cell division, there may be production of identical daughter cells like mother cell or production of specilised cells not like the mother cell.
Such divisions are mediated in two different cell lines: somatic cell line producing cells alike the mother cell and reproductive cell line producing specilised cells (gametes, spores) not like the mother cell.
These two types of divisions are called mitosis and meiosis, respectively. The basic processes of these two divisions are somehow identical and always become perceptible by the activity of the chromosomes.
Chromosomes are bearers of hereditary characters and are responsible for transmission of characters from one generation to the next.
In nondividing eukaryotic cells the chromosomal material remains dispersed as long and thin thread like stinctures known as chromatin (coloured threads due to staining). During cell division the chromatin threads shorten and condense to be converted to the individual chromosomes.
Chromosome is composed of DNA, proteins and small amount of RNA. In all types of cell divisions, the DNA of each chromosome replicates to produce an identical DNA. These become surrounded by protein coat to produce identical strands called chromatids attached to each other at the centromere.