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| Summary |
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 Every multicelluar organism begins its life as a single cell called zygote which by cleavage forms numerous identical cells. |
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 As development proceeds, these cells become specialised into components of various tissues and organs. |
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 The process of specialisation called differentiation makes the cells of the same genotype, assume different phenotypes. |
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 All the cells of an organism receive the complete set of genes that are represent in the zygote. However, resulting embryonic cells lose their potential for the expression of all their genes, by process of induction and repression. |
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 During development each gene switch on its turn and some factor may switch off the next gene in the programme. |
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 The masking of given genes is known to be caused by the embryonic cells themselves by a process called induction or organisation. |
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 Induction is known to be brought about by certain chemical substances produced by the embryonic cells, undergoing differentiation. |
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 Any change in the expression of a gene follows an interaction between nucleus, cytoplasm and environment. |
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 Cancer is a differentiation related disease. In cancer, the cells continue to divide due to lack of the property of contact inhibition. |
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 Cancer is caused by activation of genes called oncogenes brought about by viruses and mutagens. |
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 Alteration of oncogenes may involve breaking into fragments, amplification or shifting to a new chromosomal location. |
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 Alteration of the oncogenes leads to the formation of abnormal proteins, change in the gene environment and suspension of the mechanism which controls cell division. |
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