2.9 DNA


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    All the layers we just discussed collectively make up the container, or envelope, of a cell. As you have seen, different species use different combinations of these components to form their envelopes; the only constant is the cytoplasmic (or inner, for diderms) membrane.

    Now consider what these envelopes contain. In addition to water and small molecules, you have already seen some large protein complexes like motility machines. You have also seen many ribosomes–the protein/RNA complexes responsible for translating RNA into proteins. But you might have been surprised not to see something else: deoxyribonucleic acid, or DNA. The molecule containing the instructions for the life of the cell is of paramount importance, yet often invisible by microscopy. But not always. Thin filaments of DNA, only about 2 nm (two billionths of a meter) wide, blend in with the dense cytoplasm of the cell. When a cell lyses, though, its cytoplasm diffuses into the environment and the DNA filaments stand out against the now-much-reduced background. You can get an idea of the sheer abundance of DNA inside a cell from this Haloarcula argentinensis whose envelope has ruptured and contents are spilling out.