Cell differentiation, the process by which a cell is changed from a stem cell to a specialized cell, is controlled by relatively few regulatory proteins working in groups, called combinatorial control.
Although all cells in an organism contain the same DNA, they all have different structures and functions. Cell differentiation is the process by which a cell becomes specialized to perform a certain function.In order to produce the correct type of cell, the cell must regulate gene expression. Eukaryotic cells regulate gene expression by organizing groups of transcription regulators to control expression of a single gene, a process called combinatorial control. The combinatorial control process typically employs both repressors that decrease gene transcription and activators that increase gene transcription. By employing repressors and activators simultaneously, it is ensured that exactly the right portions of the right genes are transcribed at exactly the right time. Often the regulatory DNA sequences, specific regions in DNA that control gene expression, lie very far apart on the DNA strand, with long stretches between them called spacers. The proteins that regulate transcription include, along with repressors and activators, chromatin-remodeling complexes (which make the gene more or less accessible to transcription proteins), histone-modifying complexes (which modify histones to make DNA more or less accessible for transcription), transcription factors (which recruit transcription proteins to the promoter and help initiate transcription), promoter regions (such as the TATA box, which is a DNA sequence that indicates where a genetic sequence should be read), and mediators (which tie multiple proteins of the other types together to ensure they function as a unit).
Although the complex of proteins and DNA involved in combinatorial control is very large, often the process of beginning transcription is set in motion by just one transcription factor. In this way, cells can rapidly respond to signals, allowing them to quickly produce the proteins required. During cell differentiation, a small number of transcription factors can activate multiple complexes of combinatorial controls, causing many genes to be transcribed in large numbers at once. Furthermore, a relatively small number of regulatory proteins can combine to regulate a large number of variations in proteins produced in a cell. This large variation in proteins is the reason cells are different from one another. This process is how stem cells differentiate to yield specialized cells: bone cells, muscle cells, nerve cells, and so on.
Once differentiated, cells generally remain that way. However, cells have been induced to change their differentiation through the application of target cell transcription factors, which are proteins that help regulate and control gene transcription. In fact, cells have been induced to revert to pluripotent stem cells, cells that can divide into other types of stem cells, by altering the expression of genes within the cell.