1. The nucleotides in the nucleic acids are
covalently linked by a second
phosphoester bond that joins the
5’phosphate of one nucleotide and
3’ –OH group of the adjacent nucleotide
Two polynucleotides interact with one another
and produce DNA double helix structure.

2. James Watson and Francis Crick (1953)

3. 1. X –ray diffraction studies: M. Wilkins and R. Franklin on DNA fibers
polynucleotide is helical and the bases of the nucleotides are stacked
with their planes separated by a spacing of 3.4 A

4. 2. Chargraff’s rule
the amounts of purine and pyrimidines present in the
organism are equal, i.e. A+T = G+C

7. Comparison of morphological features and helical parameters of the
three major types of DNA helix.
Morphological Conformation
Characteristics
A B Z
Helical sense Right Right Left
Pitch (base pairs per 11 10 12
turn)
Major groove Deep, Wide Flat
narrow
Minor groove Broad, Narrow Narrow and
shallow very deep
Helix diameter 2.3 mm 1.9 mm 1.8 mm