Question

Watson and Crick used models / modeling to determine the structure of DNA. What pieces of scientific evidence led them to determine each the following structures:

a) That DNA is double stranded.

    

b) The two strands of nucleotides in DNA run antiparallel.

c) The nucleotides in the two antiparallel strands are arranged such that a purine is always paired with a pyrimidine.

Answer 1

a. Franklin and Wilikins used X ray diffraction to understand physical structure of DNA. It produced a pattern indicating double stranded helical structure. Erwin Chargaff's base pairing rule was a strong evidence for double stranded nature as A-T bond was exactly same as C-G bond.

b. Antiparallel strands of DNA is because of their hydrogen bonding between complementary base pairs. One evidence is that if DNA structure were parallel, base pairing would not be possible and hence no hydrogen bonding. The complementary base pairing alongwith X ray diffraction proved that nucleotides run in opposite direction.

c. The radius of double helix known from X ray data was explained if a purine base pairs with a pyrimidine. Watson and Crick assumed that A-T and G-C pairing have complementary "lock and key" shapes to permit efficient H-bonding and accounts exactly for the diameter of DNA.