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How do some cells become brain cells and others become skin cells, when the DNA in all cells is exactly the same?

How is cell specialization achieved?
The genetic makeup or DNA of all cells in an adult body is the same. How do some cells become brain cells and others become skin cells, when the DNA in all cells is exactly the same? How this happens?
The reason is differential expression of genes in cells.
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            During the process of cell specialization, in each cell types; only specific genes are “turned on” and transcribed to RNA and translated to proteins. Rest of the genes remain inactive. That is, genes active in the neurons may not be active in skin cells.
For instance, genes for actin and myosin filaments are present in all animal cells, but these genes are primarily active in muscle cells. Cell specialization involves the preferential or selective activation of specific genes leading to the synthesis of some specific proteins like antibodies in plasma cells or Hb in erythrocytes.

House keeping genes vs Luxury genes
Remember, there are some genes that are expressed in all types of cells or genes essential for survival  of all cell types like genes making membranes or genes of major metabolic pathways like glycolysis.. These genes are called ‘house keeping’ genes.

But genes that are expressed in only certain types of cells or expressed differentially are called ‘luxury genes’ or smart genes’. Eg: IgG genes in plasma cells; opsin genes for vision (in eyes). This differential expression leads to cell specialization.

After fertilization, the first cell the zygote has nucleus of both the gametes where the cytoplasm is entirely provided by egg. Thus the zygote has only maternal effect genes contributed by egg cytoplasm only. This is conducive for the zygote development. On first division, zygotic gene are expressed that will trigger further development and differentiation.

In short: Zygote-> determination ->differentiation (to specialized cells and tissues).
How do some cells become brain cells and others become skin cells, when the DNA in all cells is exactly the same?
Characteristics of Cell Differentiation:
  • Cell differentiation is a stable phenomenon. Take the example of neurons, once differentiated persist throughout lifetime.
  • Differentiation is induced by specific stimuli.
  • But once differentiated or established, these cells are stimulus independent.
  • Differentiation is accomplished by morphological difference of that fated or determined cell. Look at the figure different shapes or morphology of various types of cells in our body.
  • Ultimately differentiation is controlled by genetic factors.

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