dna is the storage form of cellular information. it is more stable than rna for chemical reasons. however, it is like a complete instruction manual, containing (almost) all the information necessary for making an organism. the cell, however, only needs the instructions for itself, or its own cell type. the mrnas are the specific instructions copied off the dna, but the cell only copies (or transcribes) the ones that it needs. the mrna instructions are then translated into protein molecules, which do things like move your muscles or help you see. this is called the central dogma of molecular biology : dna -> rna -> protein. the mrna is the intermediary between the dna and the protein, and only the ones that are useful to that particular cell are copied.