The nucleotides in the nucleic acids arecovalently linked by a secondphosphoester bond that joins the5’phosphate of one nucleotide and3’ –OH group of the adjacent nucleotideTwo polynucleotides interact with one anotherand produce DNA double helix structure.
James Watson and Francis Crick (1953)
1. X –ray diffraction studies: M. Wilkins and R. Franklin on DNA fiberspolynucleotide is helical and the bases of the nucleotides are stackedwith their planes separated by a spacing of 3.4 A
2. Chargraff’s rule the amounts of purine and pyrimidines present in the organism are equal, i.e. A+T = G+C
Comparison of morphological features and helical parameters of the three major types of DNA helix.Morphological ConformationCharacteristics A B ZHelical sense Right Right LeftPitch (base pairs per 11 10 12turn)Major groove Deep, Wide Flat narrowMinor groove Broad, Narrow Narrow and shallow very deepHelix diameter 2.3 mm 1.9 mm 1.8 mm