Makers and maintainers of protoplasm; the basic living unit of all organisms except viruses. The cells of organisms other than bacteria are eukaryotes those containing a defined nucleus in which chromosomes contain the DNA recipes from which cells synthesize protein. Cells know what to do and which genes to turn on because of what surrounding cells do in reference to a chemical-directional gradient. In organisms of greater complexity cells specialize into a variety of tissues.

       
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