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CELL DIVISION¡¡ Mitosis When a cell is not dividing, there is not much detailed structure to be seen in the nucleus even if it is treated with special dyes called stains. Just before cell division, a number of long, thread - like structure called chromosome. Although they are present in the nucleus all the time, they show up clearly only at cell division because at this time they get shorter and thicker. Each chromosome is seen to be made up of two parallel strands, called chromatids. When the nucleus divides into two, one chromatid from each chromosome goes into each daughter nucleus. The chromatids in each nucleus now become chromosomes and later they will make copies of themselves ready for the next cell division. The process of copying is called replication because each chromosome makes a replica of itself. Human cells contains 46 chromosomes. (Click to view an illustration of human chromosomes)
Mitosis will be taking place in any part of a plant or animal which
is producing new cells for growth or replacement. Bone marrow produces
new blood cells by mitosis; the epidermal cells of the skin are replaced
by mitotic divisions in the Malpighian layer; new epithelial cells lining
the alimentary canal are produced by mitosis; growth of muscle or bone
in animals, and root, leaf, stem or gruit in plants, results from mitotic
cell divisions. Meiosis
In a cell which is going to divide and produce gametes, the diploid
number of chromosomes shorten and thicken as in mitosis. The pairs of
homologous chromosomes lie alongside each other and, when the nucleus
divides for the first time, it is the chromosomes and not the chromatids
which are separated. This results in only half the total number of chromosomes
going to each daughter cell. (Click to view a comparison between meiosis and mitosis) ¡¡ ¡¡
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