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This process is called differentiation. It's quite a big subject but I'll try to put it into a few words: the gene expression pattern of a cell is defined by chromatin structure - some DNA is available for RNA polymerases to transcribe whereas other regions are folded away in a repressed conformation. Stem cells have less repressed chromatin in general terms. I suppose the question is how the plastic chromatin structure in stem cells is altered to give the more repressed chromatin structure found in differentiated cell. Although the structure is more repressed in general, some genes will be available in an open chromatin structure - these will include the genes characteristic for a particular cell type. I hope that's clearer - if not let me know.
this is what I understood:DNA code says to this stem cell,for example a cell in brain,''be a neuron'' and it becomes a neuron?
Are you asloing that as a question?
Hi, the question of how the stem cell knows what differentiation pathway to go down ie what type of cell to become is a good one. It depends on which type of stem cell it is and where it is: receptors on the stem cell surface get this information which has an effect on the genes expressed. Some proteins (called transcription factors) can have an effect on whole sets of genes and so act as master regulators. To summarize the stem cells are affected by their environment and the gene expression is controlled by signals transduced through receptors on the cell surface.
thank you! you've helped a lot!
For interest and completeness, chromatin modifying enzymes have very recently been found to also mediate the backward transition, from differentiated cells to induced pluripotent stem cells. Remarkably without help from KLF4 and c-Myc. http://www.nature.com/nature/journal/v483/n7391/full/nature10953.html