DNA structure: Revisiting the Watson–Crickdouble helix

Watson and Crick’s postulated a double helical structure for DNA, heralded a revolution in our understanding of biology at

the molecular level.

 

THE righthanded doubl helical structure proposed In 1953 by WatsonCrick for deoxyribos nucleic acid is the most wellrecognized structure for this polymeric molecule1. WhileWatsonCrick were undoubtedly the first to propose an essentially correct model for DNA structure, a wide varietyof available data was used by them to arrive at this ‘canonical’ model for DNA, in particular the nucleotidebase composition data of Chargaff and informationfrom the X-ray fibre diffraction pattern  ofB-form DNA, as recorded by Rosalind Franklin2. It wascommonly believed for several decades, that this B-formis the only structure of DNA that has biological relevance,even though Rosalind Franklin’s fibre diffractiondata2,3 for A and B forms had clearly shownthat the DNA molecule could readily undergo structuraltransitions depending on the environment, viz. variationin relative humidity in this case. Fibre diffraction studiesin the sixties and seventies also revealed several otherforms of DNA structure for synthetic oligo- and polynucleotides,depending on the base sequence and environment.Subsequent biochemical and structural studiesshowed that regions of genomic DNA, under variousphysiological conditions, can assume different structures,particularly when some well-defined sequence motifs orrepeats occur. It was probably the characterization of suchsequence repeats as ‘junk DNA’ by Francis Crick4 thatput a damper on the study of such sequences till recently.