Nucleic Acids Practice Test

An enzyme binds double-stranded DNA by interacting with the major groove, where chemical groups from base pairs are exposed. In a particular 12-bp region, the enzyme recognizes the pattern of hydrogen-bond donors and acceptors presented in the groove. A lab-made duplex has the same base sequence but is synthesized with the complementary strand reversed so that the phosphodiester backbone runs 5'→3' in the same direction as the original strand (parallel rather than antiparallel). The bases are still capable of hydrogen bonding, but the geometry of the grooves changes. Which statement best predicts the consequence for enzyme binding?

A DNA fragment is rich in G-C base pairs, which form three hydrogen bonds per pair, while A-T pairs form two; both use a sugar-phosphate backbone linked by phosphodiester bonds. In a lab, two equal-length double-stranded DNA fragments are heated under identical salt conditions. Fragment 1 contains 70% G-C pairs, and Fragment 2 contains 30% G-C pairs. The strands separate when enough hydrogen bonds between paired bases are disrupted by thermal motion. Which outcome is most consistent with these molecular interactions? Which fragment will require a higher temperature to separate into single strands?