ABSTRACT
The biological functions of DNA and RNA were initially assumed to involve only their primary sequence as required for storage of the genetic code. Consistent with this view was the helical B structure of DNA initially proposed by Watson and Crick [1]. While initial experimental work based on fiber diffraction indicated heterogeneity in DNA structure, such as the A and B forms of DNA, it propagated the idea that the structure of DNA was that of a regular helix [2]. This view started to change when the first structures of DNA based on single-molecule X-ray crystallography were obtained, which showed local conformational heterogeneity to be present in DNA while the overall structures still assumed canonical forms. Later, structural studies of RNA, particularly transfer RNA (tRNA), revealed the structure of RNA to have significant tertiary characteristics beyond the helical structures dominating DNA. More recently, X-ray crystallographic studies of DNA-protein complexes revealed DNA structures that are significantly distorted from the helical conformations traditionally envisioned for DNA. Furthermore, it has become evident that the structural distortion of DNA and the wide variety of tertiary structures of RNA are essential for their biological activity [2,3].
