Cells differ from each other in morphology (structure), and this difference is a reflection of physiological activities and biochemical functions that are ultimately under the control of genes. The differences that can be seen grossly, observed in the microscope, and detected by biochemical and molecular procedures together comprise what is known as differentiation.
Differentiation resulting from selective gene action of a genome (the entire genetic complement of an organism) held in common by all cells has been a tenet of modern genetic biology. It is the business of a cell to produce all of the proteins and enzymes held in common by most cells. The commonly produced gene products are sometimes referred to as housekeeping proteins. However, the adult fly, frog or human are comprised of a great diversity of differentiated cells. The differentiated cells produce, in addition to the housekeeping gene products, tissue-specific proteins. A unique portion of the genome of differentiated cells is activated and this accounts for differentiation. While all other cells have these gene sequences, they are silent except in the specific cell type under consideration. Thus, cell types differ from each other not because of genomes, nor because of the activity of housekeeping genes, but by the activation of tissue-specific genes which convey cell specificity.
Gene regulation that permits differentiation is the result of promoters and enhancers (which occur in DNA on either side of specific genes) and regulatory proteins which bind to the promoters and enhancers and which in turn enhance or inhibit gene expression.
Differentiation is associated with embryology. The undifferentiated cells of a zygote, morula, and blastula give rise to progressively more differentiated cells until the adult forms, which is a mosaic of many highly differentiated cells. Ordinarily, differentiated cells have lost the competence to give rise any other kind of cell. For example, muscle cells never give rise to organ cells, and vise versa. Moreover, contained within the mosaic of terminally differentiated cells are a number of stem cells. A stem cell is a less than fully differentiated cell that has retained the competence to give rise to another stem cell and a cell that will become fully differentiated. Consider the skin--it would rapidly be lost because of abrasion and the wear and tear of use were it not for replacement by cells from the basal layer of epidermis. Differentiation of the skin stem cell progeny gives rise to post-mitotic keratinized protective cells. Blood is a tissue type that must be continually replaced. It is not surprising, therefore, to note that new blood cells develop from stem cells, which like skin stem cells, at division give rise to both more stem cells and cells which will differentiate as blood.
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