Our understanding of the mechanisms regulating gene expression, which determine the patterns of growth and development in all living organisms, ultimately involves the elucidation of the detailed and dy­ namic interactions of proteins with nucleic acids -both DNA and RNA. Until recently the commonly presented view of the DNA double helix­ as visualized on the covers of many textbooks and journals - was as a monotonous static straight rod incapable in its own right of directing the processes necessary for the conservation and selective reading of genetic information. This view, although perhaps extreme, was reinforced by the necessary linearity of genetic maps. The reality is that the biological functions of both DNA and RNA are dependent on complex, and sometimes transient, three-dimensional nucleoprotein structures in which genetically distant elements are brought into close spatial proximity. It is in such structures that the enzymatic manipulation of DNA in the essential biological processes as DNA replication, transcription and recombination are effected - the complexes are the mediators of the 'DNA transactions' of Hatch Echols.