Cells can be separated into two major groups—prokaryotes, cells whose DNA is not segregated within a well-defined nucleus surrounded by a membranous nuclear envelope, and eukaryotes, those with a membrane-enveloped nucleus. The cyanobacteria and bacteria (kingdom Monera) are prokaryotes. They are smaller in size and simpler in internal structure than eukaryotes and are believed to have evolved much earlier (see evolution). All organisms other than cyanobacteria and bacteria consist of one or more eukaryotic cells.
All cells share a number of common properties; they store information in genes made of DNA (see nucleic acid); they use proteins as their main structural material; they synthesize proteins in the cell's ribosomes using the information encoded in the DNA and mobilized by means of RNA; they use adenosine triphosphate as the means of transferring energy for the cell's internal processes; and they are enclosed by a cell membrane, composed of proteins and a double layer of lipid molecules, that controls the flow of materials into and out of the cell.
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