Synthesis of a new RNA strand occurs in which direction?

• A. 3' to 5'
• B. 5' to 3'
• C. Both
• D. None

Answer: (B)

RNA synthesis happens in the 5' to 3' direction. RNA polymerase adds each new nucleotide to the 3' end of the growing strand, using an incoming ribonucleoside triphosphate as the substrate. This chemical step—the 3' hydroxyl attacking the incoming phosphate—builds the chain away from the 5' end, so the RNA strand extends from 5' to 3'. The template DNA is read in the opposite direction, 3' to 5', to supply the correct bases. Choosing the 3' to 5' direction would require adding nucleotides to the 5' end, which the polymerization chemistry does not support, and thus is not how RNA synthesis occurs.