Hydrogen Bond Dynamics in Transcription

Steps:

• DNA unwinds at the promoter: complementary base bonds break (event 1).
• Incoming nucleotides pair with the DNA template, forming hydrogen bonds (event 2) and sugar-phosphate bonds (event 3).
• As RNA polymerase advances, the RNA-DNA hybrid separates: hydrogen bonds break (event 1 again).
• DNA strands reanneal behind the polymerase: complementary base bonds reform (event 2 again).
• Events 3 and 4 repeat for each nucleotide added to the growing mRNA chain.

Why D is correct:

• Events 1 and 2 each occur twice—breaking and forming hydrogen bonds happen once for DNA unwinding/separation and once for hybrid formation/rewinding—per the core mechanism of transcription elongation.

Why the others are wrong:

• A: Event 3 occurs multiple times (once per phosphodiester linkage in mRNA).
• B: Events 3 and 4 occur repeatedly (for every nucleotide incorporated).
• C: Event 4 occurs multiple times (pairing for each added nucleotide).

Final answer: D