if all the body cells of an animal are genetically identical,how do they become functionally different from one another?
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They become different by the process of cell differentiation. When the organism is an early embryo there is a rough differentiation determined by the position of the cells. If the cells are in the exterior they differentiate into exoderme, if they are inside they form the endoderme , and if they are situated in the middle it results the mesoderme. The cells know their position in the embryo (chemical semnals between cells) and they are programmed to make some genetic material at chromosomal level unusable and to leave active only the specific and the common information. For example a nervous cell will have the cell replication information inactivated or an epitelial cell will not be able to decode the information needed to produce hemoglobin.
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Every cell in the body has the same 20,000 or so genes. What makes cells, and tissues, different, is that the cells of different tissues express different subsets of those 20,000 genes. Which genes the cell expresses determines what type of cell it will be. For example, every cell has the genes for hemoglobin, but the only cells that express those hemoglobin genes are those cells in the bone marrow that are to become red blood cells. This is because only these cells express the receptor for EPO, the hormone that tells cells to make hemoglobin.
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because they change into different types of cell so i think there dna will still be the same but they are still different types of cell but the dna might be slightly different i dont think not all cells will be identical