All the multicellular organisms grow from an original single cell (zygote) formed by joining of two sex cells which divides repeatedly to form the cells of the adult organism. New cells come only from the pre-existing cells by cell division. Hence, cell division is very important for the continuity of life.
The process where new cells are derived from other cells is called cell divisions. Before the cell divides the nucleus of the cells divides first (nuclear division) and it is followed by the division of the cytoplasm (cytoplasm division). Chromosomes appear clear only at cell division and they become short and thick. Before that the chromosomes are very long and thin. Each chromosome has a specific shape and size so it is easy to identify different chromosomes. Each chromosome exist in pairs (homologous; exactly the same, pairs).
The nucleus and particularly the chromosomes within it play an important role in the continuity of one generation to another. In definition chromosomes are thread like structures found inside the nucleus which carries hereditary information from one generation to another. Chromosomes contain chemical DNA which makes exact copy of itself during nuclear divisions resulting in the doubling of the chromosomes.
Each species has a fixed number of chromosomes. Example human beings have 46 chromosomes, meaning 23 pairs of chromosomes. The number of chromosomes for one species is the same for all of its body (somatic) cells. The number of chromosomes in a sex cell would be half that of the number found in the body cell. Example the human egg or sperm cell will contain 23 chromosomes and in this case they are not in pairs. Diploid and Haploid cells

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