Question

In the Watson and Crick model of DNA, the ladder are composed of
(a) Sugar
(b) a purine and pyrimidine
(c) two purines
(d) a sugar and phosphate molecule

Answer 1

Hint: The Polynucleotide chain of DNA has a regular helix. Every DNA molecule contains two polymer strands coiling around one another. The sides of the ladder are formed by alternating sugar (deoxyribose) and phosphate molecules while the steps of the ladder are formed by a pair of nitrogen bases.

Complete answer:
On the basis of the Watson and Crick model of DNA, the "steps" of the ladder are made out of purine and pyrimidine and the “backbone” consists of sugar and a phosphate molecule. This model is known as a double helix because of two long strands of DNA twisting around one another like a twisted ladder.

Additional Information:
Based on this model, the DNA molecule comprises two strands that are associated together by hydrogen bonds and helically twisted.
Each step on one strand comprises a nucleotide of purine base which substitutes with that of pyrimidine base. Accordingly, a strand of DNA molecule is a polymer of four nucleotides i.e. A, G, T, C. The two strands join together and produce a double helix. Bases of two nucleotides form hydrogen bonds i.e. A joins with T by two hydrogen bonds (A = T) and G joins with C by three hydrogen bonds (G = C).
So the correct answer is ‘a purine and pyrimidine’.

Note: The hydrogen bonds between the two strands are such that they maintain a distance of 20 ${ A }^{ o }$. The double helix coils in the right-hand direction i.e. clockwise direction and finishes a turn at each 34 ${ A }^{ o }$ distance. A major groove, the turning of the double helix brings about the appearance of a deep and wide groove. The major groove is the site of the binding of a specific protein.