Nick translation by Pol I. DNA polymerase I (Pol I) is organized into three major domains: DNA polymerase, 3′→5′ proofreading exonuclease, and 5′→3′ exonuclease. At a nick—here, the gap between lagging-strand fragments—Pol I degrades the RNA primer in the 5′→3′ direction, releasing rNMPs, and simultaneously extends the 3′ terminus with dNTPs in the same direction. The net result is movement of the nick in the 5′→3′ direction along the DNA until all RNA is removed. DNA ligase can then seal the fragments (not shown here).