It is often emphasised how closely the structure and function of biological materials are related to each other. This is certainly true for all biological macromolecules, but to some extent it is also true for any chemical substance: whether it takes part in a simple chemical reaction or in a complex biological process, the function of the molecule is determined by its structure. In the case of nucleic acids, however, the relationship of structure and function is unique: they are two aspects of the same basic property of the molecule; structure and function are, in fact, identical.