Question

The structures of an A-form and a B-form double helix are different in several respects. Use a drawing to describe how is the position of the base pair within the helix related to the size of the minor groove and major groove. Which of the two forms is favored by RNA, and which is favored by DNA? In which are the base pairs perpendicular to the axis of the helix, and in which are they tilted? Which of the two helices is thermodynamically more stable?

Answer 1

Answer :)

The difference between B-form and A-form DNA is the location of base-pairs within the duplex. It is showing by oval shapes in the image. In A-form, the base pairs are shifted away from the central axis and nearer to the major groove. It makes a ribbon-like helix having a more open cylindrical core in A-form. On the other hand, in B-form, the base pairs are nearly centered over the helical axis. That is why bases are perpendicular to the axis of B-form helix. B-form DNA is favored more in physiological conditions. B-form DNA is more thermodynamically stable because lack of 2’OH in the deoxyribose sugar, it prevents the steric hindrance. RNA favors A-form helix because of the presence of ribose sugar that contains 2’ OH groups.

Representing distance between base pair and groove Minor groove Minor groove Major Major groove groove B DNA A DNA

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