The understanding of biology has been transformed from being at a gross anatomical level to a molecular level. A major contribution to this has come from the application of physical techniques, especially X-ray diffraction, for the determination of structures which have provided, therefore, explanations for many key functions of organisms. The molecular basis of heredity followed from the discovery of the double-helix structure of deoxyribonucleic acid (DNA). The mechanism of action of many different protein molecules can be explained on the basis of their three-dimensional structures, for example, oxygen transport and storage, enzymes, membrane proteins and the immune response. Also the means by which viral infection takes place is currently being unravelled. The application of all this structural information has started with the engineering of new proteins with enhanced or modified functions and also the rational design of drugs. On the horizon is the detailed structure determination of the ribosome, which is the cell organelle involved in protein synthesis. A list of books dealing with the structure and function of macromolecules in detail is given in the bibliography, section 5.

The scale of atoms, molecules and macromolecules is illustrated in figure 3.1. The dimensions of bond lengths are of the order of 10-10m or 1 Å and determine, therefore, the resolving power and the wavelength required in the technique of X-ray diffraction applied to determining molecular structure.