Optical tweezers were used to track single molecules of two different classes of RNA polymerase as they transcribe DNA into RNA. She shows that RNAP I (which transcribes ribosomal RNA) and RNAP II (which transcribes messenger RNA) both exhibit backtracking dependent pauses and are able to recover from short backtracks via one-dimensional diffusion. Furthermore, they both are able to utilize RNA cleavage reactions to recover from intermediate backtracks. However, the presence of an extra subunit (A12.2) in RNAP I and the association of the cleavage stimulatory factor TFIIS to RNAP II lead to distinct mechanisms to prevent nonrecoverable backtracking.