DNA also directs the building of proteins according to a code. Every human cell has about 6 feet of very tightly wound DNA strands contained within its nucleus, and every adult carries billions of miles of ultrathin DNA strands in his or her body. Each cell is separated from the rest of the world by a membrane so thin that it cannot be seen under a light microscope. Despite its thinness, the surface membrane is exceedingly sturdy, controlling everything that goes into and out of the cell and relaying vital messages.
Similar membranes enclose or make up a large number of the cell's organelles.
There is a fundamental distinction between two of the major categories of cells. Eukaryotic cells , in contrast, have two membranes separating the nucleus from the cytoplasm, as well as many other internal membranes to segregate their organelles. The cells of all animals and plants are eukaryotic. And while, in general, prokaryotic organisms produce only exact duplicates of themselves, higher eukaryotic organisms are capable of differentiation into many kinds of cells. This gives eukaryotes certain obvious advantages.