Question

The width of DNA is;
(a) 18$\overset { \circ }{ A } $
(b) 20$\overset { \circ }{ A } $
(c) 34$\overset { \circ }{ A } $
(d) 35$\overset { \circ }{ A } $

Answer 1

Hint: According to Watson and Crick, DNA is a double helix model where DNA type B is the most stable right - handed helix. The diameter of this stable DNA is 2 more than that of Z - DNA if measured in angstrom.

Complete answer:
James Watson and Francis Crick based on information proposed that DNA is a double helix model. One of the hallmarks of their proposition was the base pairing between the two strands of polynucleotide chains. The width/helical diameter of the base pair is 20$\overset { \circ }{ A } $ (2nm).
Additional Information: The salient features of the double helix DNA structure are: - DNA consists of two polynucleotide chains. The backbone is constituted by sugar- phosphate and bases projects inside. - The two chains of DNA run in anti- parallel fashion with 5’ - 3’ polarity in one and 3’- 5’ polarity in other chains. - The bases in two strands are paired through hydrogen bonds forming base pairs (bp). Adenine forms two H-bonds with thymine from the opposite strand and vice versa. Guanine is bonded with cytosine with three H-bonds. Purine always comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of helix. - A double chain of DNA is helically twisted in a right- handed fashion. - Each turn of a double helix or the pitch of the helix is 3.4nm. It has roughly 10 base pairs in each turn. The distance between two adjacent base pairs is approximately equal to 0.34nm. - The pane of one base pair stack over the other in a double helix. This, in addition to H-bonds, confers stability to the helical structure.
So, the answer is, ‘20A°.’

Note:
- Type A DNA has a helical diameter of 23 A°. - The double helix structure of DNA contains a major groove and a minor groove and in type B DNA, the major groove is wider than the minor groove. - Since there is a difference in the width of the major and minor groove, many proteins bind to a wider major groove.