DNA structure: revisiting the Watson-Crick double helix

Abstract

Watson and Crick's postulation in 1953, exactly 50 years ago, of a double helical structure for DNA, heralded a revolution in our understanding of biology at the molecular level. The fact that it immediately suggested a possible copying mechanism for the genetic material aroused the maximum interest, but the structure itself (often referred to as the B-DNA structure, by association with the corresponding X-ray fibre pattern) has also attained an almost iconic status. It was for a long time regarded as being the only biologically relevant structure, even though Watson and Crick had themselves pointed out that the structure could readily undergo changes, depending on the environment. Subsequent studies on synthetic poly-nucleotides as well as naturally occurring DNA sequences with certain repeat patterns, have established that the DNA molecule could have intrinsic, as well as environment-induced, structural polymorphism. Some of the structures show only minor differences, from the canonical structure, while a few are completely different, even in their essential features, such as handedness, base-pairing scheme or number of strands. The various DNA structures have been characterized as A, B, C, etc. and there is a DNA structure associated with 18 other letters of the English alphabet. Only the letters F, Q, U, V and Y are left to choose from, to describe any new forms of DNA structure that may appear in future! Apart from these structures, with a one-letter 'name', there are several other generic descriptions of DNA structure and many such structures have been characterized in recent years. It is reasonable to expect that the 3 billion base pairs in the human genome (leading to a 2 m long molecule which is packaged in a microscopic size nucleus) will exhibit a variety of polymorphic forms, which could be important for the biological packaging as well as function of DNA. It is also clear that the ideal and canonical Watson-Crick DNA structure is indeed just that-an ideal and aesthetic representation of the DNA molecule, which actually possesses a chameleonlike property of adapting itself to its environment and function by twisting, turning and stretching. Some of these structures are discussed here as also the lacunae in our current understanding of the dynamics of DNA structure.