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Human Body
duplication and transmission of characteristics from parent to offspring. Reproduction at the cellular level occurs by a process of cell division in which the original cell splits into two cells. We have dealt with the process of reproduction, in detail, in a later chapter. Here we shall give a brief account of a process of normal cell division called mitosis.
In almost every tissue, cells wear out and must be replaced. Even after overall growth of the body has ceased, growth still occurs at the cell and tissue levels, providing for the replacement of worn out or damaged structures. New cell formation occurs through cell division. Human life itself begins with the division of a single cell, the ovum (female egg) fertilized (by a male sperm) in the mother's womb. This single cell divides into two cells which in turn divide into four. Cell-division (Mitosis)
Mitosis is a continuous process with characteristic sequences of events as one cell becomes two. It begins with the replication of the DNA (deoxyribonucleic acid) of the nucleus. The division continues till there are millions of cells of the human body. Even this wonderful phonomen of multiplication is nothing when compared to the transmission of enormous amount of information stored within the fertilized egg. This tiny fragment of life contains the genes—the messengers of heredity. They store complete blueprints for building complex chemical plants like liver and coded information on colour, texture and size of the body.
Genes are strung into long thin chains called chromosomes. There are 46 (23 pairs) chromosomes in each human cell. The genetic material is a mass, consisting of long thin tangled strands. In the next phase the tangled strands become shortened and thickened into discrete, rod-like structures. Each of these actually consists of two separate strands jointed together by a small body. In the next phase these chromosomes begin to move and align themselves. When the alignment is complete, each double stranded chromosome divides producing two single-stranded daughter chromo
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