Reproduction means a cell does not go on growing indefinitely in size but
at a certain optimum point divides into two daughter cells. Further, certain
cells will undergo division to replace worn-out cells or those destroyed by
disease. This kind of cell division is called mitosis or karyokinesis.
Activity begins in the nucleus. The nuclear membrane
disappears and the chromatin changes character and becomes long filaments
called chromosomes.
The centrosome divides and the two
new centrosomes move away from each other to each end of the nucleus
called the poles.
The chromosomes are then attracted to the poles and
lie near the new centrosomes. The chromatin of which nucleus is formed now
comes to rest and two new nuclei exist. Finally the protoplasm of the cell
constricts and divides and the two new cells are complete.
Each new daughter cell resulting from mitosis contains
forty six chromosomes, so that during mitosis each chromosome must duplicate
itself. The process of chromosomal duplication is one of the least understood
of the cell’s activities.
However, mitosis is not the only kind of cell
division. In the sex organs, the ovary and testis, another kind of cell
division occurs called meiosis. During the formation of the
sex cells, or gametes, the number of chromosomes is halved, so that the spermatozoon
contains only twenty-three chromosomes and the egg-cell or ovum,
twenty three.
When fertilization occurs, that is when
spermatozoon and ovum fuse to form the cell (zygote) which develops
into a new individual the normal chromosomal complement of forty six is
restored. By this means a mixing of the hereditary determinants or genes, from
male and female is achieved.
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