Which enzyme synthesizes RNA during transcription?

• A. RNA polymerase
• B. DNA polymerase
• C. Reverse transcriptase
• D. Ligase

Answer: (A)

During transcription, RNA is built from a DNA template by RNA polymerase. This enzyme reads the DNA sequence and adds ribonucleotides complementary to the template, extending the RNA strand in the 5' to 3' direction without needing a primer. In bacteria, a single RNA polymerase carries out transcription with a sigma factor guiding initiation; in eukaryotes, RNA polymerase II performs mRNA synthesis with various transcription factors helping initiate. Other enzymes have different roles: DNA polymerase copies DNA during replication and requires a primer, reverse transcriptase makes DNA from an RNA template, and ligase seals breaks in the DNA backbone. Thus RNA polymerase is the enzyme that synthesizes RNA during transcription.