Q.62 Which of the following statements with respect to the orientation of the nitrogenous bases to the
pentose sugars, and the puckering of the sugar, is correct?
(A) Anti, and 2’-endo in A form DNA (B) Anti, and 2’-endo in B form DNA
(C) Syn, and 3’-endo in A form DNA (D) Syn, and 3’ -endo in B form DNA

DNA Forms: Base Orientation and Sugar Puckering Explained

B-form DNA features anti orientation of nitrogenous bases relative to pentose sugars and C2′-endo sugar puckering, making option (B) the correct choice in this multiple-choice question.​

Correct Answer

Option (B) Anti, and 2’-endo in B form DNA accurately describes the standard conformation of B-DNA, the predominant form in cells under physiological conditions. Nitrogenous bases adopt an anti glycosidic bond orientation, positioning them away from the sugar, while the deoxyribose sugar puckers in the C2′-endo conformation. This configuration contributes to B-DNA’s smooth right-handed helix with 10.5 base pairs per turn.​

Option Analysis

• (A) Anti, and 2’-endo in A form DNA: Incorrect, as A-DNA uses C3′-endo sugar puckering despite anti base orientation, resulting in a shorter, wider helix seen in low-humidity conditions.​

• (B) Anti, and 2’-endo in B form DNA: Correct for hydrated DNA, where anti conformation allows proper base stacking and C2′-endo puckering supports the narrow minor groove.​

• (C) Syn, and 3’-endo in A form DNA: Incorrect; syn orientation is rare and linked to Z-DNA, while A-DNA maintains anti glycosidic bonds with C3′-endo sugars.​

• (D) Syn, and 3’-endo in B form DNA: Incorrect, as B-DNA exclusively features anti orientation and C2′-endo puckering.​

Structural Differences

These conformations influence DNA-protein interactions and stability in biological processes.​