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| Molecular Basis of Differentiation |
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| All the cells of an organism receive the complete set of genes (total information) present in the fertilized egg through mitotic divisions. However, cells of an early embryo lose the potential for the expression of all the genes through the processes of induction and repression, perhaps by mechanisms similar to those explained in gene regulation. Only certain genes remain functional in particular cells. Thus, the cells synthesize some proteins and lose the ability to synthesise others. This enables the cells containing the similar genes to assume different structure and function, which in turn results in differentiation. Thus, only a small proportion of the total genetic information present in any cell is actually used, and the more specialized the cell, the fewer the genes utilised. This is why only a muscle cell synthesises myosin, an erythroid cell forms globin, brain cell produces myelin, and a leaf cell manufactures RuBP corboxylase. Thus, altered gene activity or expression is the molecular basis of differentiation. |
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| Development involves precisely coordinated series of events. Each gene is switched on in its turn, and then something resulting from the expression of that gene, or some environmental influence, switches on the next gene in the programme. |
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| The exact mechanism of differentiation is not fully clear, but certainly the embryonic cells themselves cause differentiation of one another by passing chemical substances between them. The fate of a developing cell, thus, depends on the influence of its neighbouring cell, or simply on its position in the embryo. The cells that induce or control the development of other cells in an embryo are called “inducers” or “organisers”, and the interactions of embryonic cells are termed “inductions”. |
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| Experiments of Robert Briggs and Thomas King throw some light on certain aspects of differentiation. They removed or destroyed the nuclei of frog or toad eggs and introduced into them new nuclei from the cells of an embryo, which developed into tadpoles. But the eggs that received nuclei from the fully differentiated cells, say intestinal cells of tadpole, did not produce tadpoles. These results indicate that: |
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 Some changes occurs in the nuclei of cells during differentiation. |
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 Differentiation does not result in the loss of, or permanent inactivation of, the genes; in fact it “turns off” the genes in a reversible manner. This means that a differentiated cell can de-differentiate itself and become a totipotent embryonic cell capable of expressing all of its genes. However, dedifferentiation is often limited in animals to early stages of differentiation whereas in plants it can take place even in mature cells. |
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 It is the cytoplasm of the differentiated cell that determines the expression of the genetic information, and prevents the cells from acting as a fertilized egg and forming a new individual. The nucleus of an embryonic cell contains the full genetic information but it expresses only that information which is needed for the embryo while in the cytoplasm of an embryonic cell, though it expresses the entire information to form a new offspring when introduced into the cytoplasm of an egg. |
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| Thus, the change in gene activity is brought about by its interaction with the immediate environment, the cytoplasm. The cytoplasm is modulated by factors such as nutrition, temperature, light, between cell interaction, medium, hormones, etc. These factors may exert influence by modulating gene activity. Hence, the modulation of the gene and its cytoplasm are interdependent. |
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| It is now clearly established that alteration in the expression of specific genes brings about differentiation, and change in the gene expression results from the interaction of three factors nucleus, cytoplasm and environment. |
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Developoment and Differentiation
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