Nucleotides and Nucleic Acids (5D) Practice Test

Investigators compare two 30-bp nucleic acid duplexes at 25°C in 100 mM monovalent salt. Duplex X is DNA:DNA; duplex Y is RNA:RNA with the same base sequence (with U in place of T). Circular dichroism indicates duplex Y adopts an A-form helix under these conditions, whereas duplex X adopts a B-form helix. The core concept is how sugar structure influences nucleic acid helical geometry. Which statement is most consistent with these observations?

Assume both duplexes are perfectly complementary and of equal length; no chemical modifications are present.

To validate strand orientation in an in vitro transcription product, researchers labeled the RNA with $\gamma$-$^{32}$P-ATP using polynucleotide kinase (PNK), which transfers the terminal phosphate from ATP to a free 5′-OH. The RNA was efficiently labeled only after treatment with alkaline phosphatase; untreated RNA showed minimal labeling. Which conclusion about nucleic acid function is best supported by the labeling behavior?

Assume: alkaline phosphatase removes terminal phosphates; PNK requires a 5′-OH acceptor.