Which enzyme synthesizes RNA during transcription?

• A. DNA polymerase
• B. Ligase
• C. Helicase
• D. RNA polymerase

Answer: (D)

RNA polymerase is the enzyme that makes RNA during transcription. It binds to the DNA promoter, unwinds a small region, and uses the DNA template strand to assemble a complementary RNA strand in the 5' to 3' direction, adding RNA nucleotides (A pairs with U in RNA, C with G, G with C). It does not require a primer to start. In bacteria, initiation involves a sigma factor guiding the RNA polymerase to promoters; in eukaryotes, transcription of mRNA mainly involves RNA polymerase II with various transcription factors. The other enzymes mentioned have different roles: DNA polymerase copies DNA, needing a primer; helicase unwinds DNA but does not synthesize RNA; ligase seals DNA ends. The key idea is that RNA polymerase is the enzyme responsible for synthesizing the RNA transcript during transcription.