Correct option is B
Explanation-
Option B - "Sugar puckering on the DNA strand that pairs with RNA will change."
Normal B-DNA has C2'-endo sugar puckering. A-form helices (which RNA-DNA hybrids adopt) are associated with C3'-endo sugar puckering. Although RNA naturally prefers C3'-endo, the DNA strand also adjusts its sugar puckering toward C3'-endo to accommodate the A-form geometry.
Hence, in the RNA–DNA hybrid, the DNA strand does undergo a conformational change, especially in sugar puckering from C2′-endo to more A-like C3′-endo.Conformational adaptation is necessary for hybrid stability.
This is correct because the RNA–DNA hybrid adopts A-form helical geometry, and the DNA strand undergoes a change in sugar puckering (from typical C2′-endo in B-DNA to a more C3′-endo A-form conformation).
Incorrect options-
Option A - "Sugar puckering on the DNA strand that pairs with RNA will remain unchanged."
This assumes the DNA strand retains its C2'-endo (B-form) puckering. However, in RNA-DNA hybrids, the structure becomes A-form, which requires the DNA strand to change its puckering toward C3'-endo. So, sugar puckering changes, and this option is factually incorrect.
Option C - "Sugar puckering on the DNA strand that pairs with RNA will remain unchanged but the number of base pairs per turn in the RNA-DNA hybrid will increase."
Although number of base pairs per turn does increase in RNA–DNA hybrids (from ~10.5 in B-DNA to ~11 in A-form) but sugar puckering does not remain unchanged. As explained, DNA strand shifts from C2′-endo to C3′-endo.
Hence, this option is only half right, and so cannot be accepted as correct.
Option D - "Sugar puckering on the DNA strand that pairs with RNA will change but the number of base pairs per turn in the RNA-DNA hybrid will remain unchanged."
Sugar puckering does change (C2′-endo → C3′-endo) but the number of base pairs per turn does not remain the same.
In B-DNA: ~10.5 bp/turn
In RNA–DNA hybrids (A-form): ~11 bp/turn
So, both parts must be correct, but this one fails on the second half.
So, the correct answer is option B
